Coues' Rice Rat
Encyclopedia
Oryzomys couesi, also known as Coues' Rice Rat, is a semiaquatic rodent
in the family Cricetidae
occurring from southernmost Texas through Mexico and Central America into northwestern Colombia. It is usually found in wet habitats, such as marshes, but also lives in drier forests and shrubland
s. Weighing about 43 to 82 g (1.5 to 2.9 oz), O. couesi is a medium-sized to large rat. The coarse fur is buff to reddish above and white to buff below. The hindfeet show some specializations for life in the water, such as reduced ungual tufts of hair around the digits. It has 56 chromosome
s. There is much geographic variation in size, proportions, color, and skull features. Oryzomys couesi is active during the night and builds nests of vegetation that are suspended among reeds about 1 m (3 ft) above the ground. It is an excellent swimmer and dives well, but can also climb in vegetation. An omnivore
, it eats both plant and animal food, including seeds and insects. It breeds throughout the year; females give birth to about four young after a pregnancy of 21 to 28 days. The species may be infected by several different parasites and by two hantavirus
es.
The species was first described in 1877, the first of many related species from the region described until the 1910s. In 1918, Edward Alphonso Goldman
consolidated most into the single species Oryzomys couesi and in 1960 Raymond Hall united this taxon
with its United States relative, the marsh rice rat
(O. palustris), into a single widespread species; subsequently, many related, localized species retained by Goldman were also included in this taxon. After studies of the contact zone in Texas, where O. couesi and the marsh rice rat meet, were published in 1979 and underscored the distinctness of the two, they were again regarded as separate. Since then, some of the peripheral forms of the group, such as Oryzomys antillarum from Jamaica and Oryzomys peninsulae
from the Baja California Peninsula
, have been reinstated as species. Nevertheless, O. couesi as currently constituted is likely a composite of several species; a 2010 study, using DNA sequence
data, found evidence to recognize separate species from the Pacific and eastern sides of the distribution of O. couesi and two additional species from Panama
and Costa Rica
. Generally, Oryzomys couesi is common and of no conservation concern, and it is even considered a plague species in places, but some populations are threatened.
. The eighth species of the genus, the marsh rice rat
(O. palustris) is the only member of its own group (unless western populations are classified
as a separate species, O. texensis). Oryzomys previously included many other species, which were reclassified in various studies culminating in contributions by Marcelo Weksler and coworkers in 2006 that removed more than forty species from the genus. All are placed in the tribe Oryzomyini
("rice rats"), a diverse assemblage of over a hundred species, and on higher taxonomic levels in the subfamily Sigmodontinae
of the family Cricetidae
, along with hundreds of other species of mainly small rodents.
first described Oryzomys couesi in 1877, using three specimens from Mexico and Guatemala. He named the animal Hesperomys couesi, placing it in the now-defunct genus Hesperomys
, and noted similarities to the marsh rice rat
(then called Hesperomys palustris) and two species now placed in Tylomys
. The specific name, couesi, honors American naturalist Elliott Coues
, who had done much work on North American rodents. In 1893, Oldfield Thomas
wrote that the species, by then placed in the genus Oryzomys
as Oryzomys couesi, had caused much confusion about its identity, because the three specimens (one from Cobán
, Guatemala, and two from Mexico) used by Alston in fact belonged to two or three different species. He restricted the name couesi to the animal from Guatemala, and introduced the new name Oryzomys fulgens for one of the Mexican animals. Several other related species were described from the early 1890s onwards and in 1901 Clinton Hart Merriam
united many of those into a palustris-mexicanus group of species, which also included the marsh rice rat.
Edward Alphonso Goldman
revised North American Oryzomys in 1918 and consolidated many forms into a single species Oryzomys couesi, with ten subspecies
distributed from southern Texas
and western Mexico south to Costa Rica. He placed it in an Oryzomys palustris group with the marsh rice rat and several species with more limited distributions, which he regarded as related to O. couesi but distinctive enough to be classified as separate species. In the 1930s, a few more forms related to O. couesi were described. As then recognized, the ranges of the marsh rice rat, a United States species, and Oryzomys couesi meet in southern Texas. In 1960, Raymond Hall reviewed specimens from this contact zone and found no grounds on which to separate the two species; thus, he reduced O. couesi to a subspecies of the marsh rice rat. Other workers continued this lumping and by 1971 all other species Goldman had placed in the O. palustris group were classified under the marsh rice rat, together with Oryzomys azuerensis from Panama, described as a species in 1937.
Additional studies of the palustris–couesi contact zone in Texas published in 1979, using more specimens and characters, indicated that the two species are in fact easily distinguishable there; therefore, O. couesi has since been regarded as a species distinct from the marsh rice rat. Afterward, some of the other forms synonymized under O. couesi or O. palustris were resurrected as separate species—Oryzomys nelsoni from the Marías Islands, western Mexico, and Oryzomys antillarum from Jamaica. In 2009, Michael Carleton and Joaquin Arroyo-Cabrales reviewed western Mexican Oryzomys, reaffirmed the distinctness of O. nelsoni, and reinstated O. peninsulae
from the tip of the Baja California Peninsula
and O. albiventer
from interior Mexico as species. Still, O. couesi included 22 synonyms
, and Carleton and Arroyo-Cabrales wrote that further research on O. couesi and related species would certainly result in the recognition of additional species.
A 2010 study by Delton Hanson and colleagues used DNA sequence data from the mitochondrial
gene cytochrome b
(Cytb) and two nuclear
markers, exon
1 of the interphotoreceptor retinoid-binding protein gene (Rbp3) and intron
2 of alcohol dehydrogenase
gene 1 (Adh1-I2) to study relationships among populations of the marsh rice rat and O. couesi. The Cytb data placed all studied specimens of O. couesi in a clade
sister to the marsh rice rat; the mean genetic distance
between the two groups was 11.30%, much larger than the distance between sister species in the related genera Melanomys
and Nectomys
(7.48% and 7.52%, respectively). Within the O. couesi clade, two populations from Panama and Costa Rica were successively basal to the other specimens, which fell into two large subclades—one containing animals from the Pacific seaboard from western Mexico to El Salvador and the other containing rats from the eastern seaboard from Texas to Nicaragua. The Panamanian and Costa Rican populations differed by 6.53% to 11.93% from the others and the western and eastern subclades differed by 4.41% on average. Data from both of the slower-evolving nuclear markers Rbp3 and Adh1-I2 also placed examples of Oryzomys in two main clades, but did not recover the western and eastern groups of O. couesi as separate clades. In addition, Adh1-I2 placed the Costa Rican population within the marsh rice rat clade and placed some western O. couesi specimens closer to the marsh rice rat than to the O. couesi group. The combined dataset supported the western and eastern clades within O. couesi and placed the Costa Rican population marginally closer to the marsh rice rat than to O. couesi. Using the genetic species concept, the authors suggested that the four groups they found within O. couesi should be recognized as distinct species. If this suggestion is followed, the eastern subclade would retain the name Oryzomys couesi, the western group would be named Oryzomys mexicanus, and the appropriate names for the Panamanian and Costa Rican species remain unclear.
Populations of Oryzomys couesi from Jalisco
, western Mexico, east to El Salvador form a single Cytb clade, which Hanson and colleagues proposed to recognize as the species Oryzomys mexicanus. These animals differ by 4.4% from Oryzomys couesi in the strict sense, which occurs to the north and east, are separated by mountain ranges from the latter, harbor different species of hantavirus
, and according to Merriam (1901) have more robust skulls, with larger molars
, stronger zygomatic arch
es (cheekbones), and better developed ridges along the margins of the interorbital region
of the skull (between the eyes). Within the "Oryzomys mexicanus" clade, Cytb sequence differences average 2.06% and western (Jalisco to Oaxaca
) and eastern (Chiapas
and El Salvador) groups form distinct subclades; Hanson and colleagues recognized these as different subspecies, mexicanus in the west and zygomaticus in the east.
As defined by Carleton and Arroyo-Cabrales in 2009, the subspecies Oryzomys couesi mexicanus occurs along the Pacific coast from central Sonora to southeastern Oaxaca
and inland along rivers into central Michoacán
, southern Morelos
, southern Puebla
, and northwestern Oaxaca. It usually lives below 1000 m (3300 ft) altitude, but has been found at 1525 m (5000 ft) in Jalisco
. This distributional pattern is similar to that of other western Mexican rodents such as Sigmodon mascotensis, Hodomys alleni, Peromyscus perfulvus, and Osgoodomys banderanus and has been recognized as a distinct biogeographic
zone in some reviews. O. c. mexicanus occurs close to three other Oryzomys species—O. albiventer, O. peninsulae, and O. nelsoni—which are larger and different in some proportions and details of coloration.
Joel Asaph Allen
first described Oryzomys mexicanus as a full species in 1897 from specimens from Jalisco. In the same publication, he also described Oryzomys bulleri from nearby Nayarit
, but he did not compare the two with each other. Merriam added a second species from Nayarit, Oryzomys rufus, in 1901, noting that it was smaller and more reddish than mexicanus. Goldman synonymized the three as O. couesi mexicanus in 1918 and in 2009 Carleton and Arroyo-Cabrales concurred, arguing that the differences between rufus and mexicanus were age-related and within the normal range of variation of the animal. Another subspecies, Oryzomys couesi lambi, was described by Burt in 1934 from central coastal Sonora, which extended the range of the species by 400 mi (640 km) at the time. This form is dark gray-brown, much darker than mexicanus, and has a shorter tail and weaker jugals. Carleton and Arroyo-Cabrales wrote that it is similar to mexicanus, but that further research is needed to determine whether it should be recognized as a subspecies. Large O. couesi from northern Sinaloa
may also belong to this form. Goldman wrote that mexicanus was very similar to nominate couesi, but usually with paler fur; the upperparts are more buffy than in couesi and the underparts are usually white, but may be buffy, the normal color in couesi.
Oryzomys zygomaticus was first described by Merriam in 1901 as a separate species similar to mexicanus, but with the zygomatic arches broadly spreading and curved downward. Goldman, who reduced it to a subspecies of couesi, recorded it from southwestern Guatemala and nearby Chiapas and described it as slightly paler than O. c. couesi but darker than O. c. mexicanus. Three specimens from central El Salvador have Cytb sequences similar to those of zygomaticus, but in The Mammals of El Salvador (1961), Burt and Stirton recorded only the subspecies couesi from the country, while noting that specimens from some localities were slightly paler than others.
Goldman grouped four subspecies of couesi from the interior plateaus of central Mexico together—albiventer, crinitus, aztecus, and regillus. Three of those (albiventer from Jalisco, crinitus from the Distrito Federal
, and aztecus from Morelos
) were described by Merriam in 1901, and Goldman had himself described regillus from Michoacán
in 1915. According to Goldman, aztecus is pale and large-toothed, crinitus is large, dark and large-toothed, regillus is large and dark, and albiventer is large and relatively pale.
In their 2009 review of western Mexican Oryzomys, Carleton and Arroyo-Cabrales classified Oryzomys albiventer
as a separate species from lowland mexicanus on the basis of clear morphometric
differentiation and offered some comments on the status of crinitus, regillus, and aztecus, including the holotype
s of the three forms in their morphometric analyses. The holotypes of regillus and aztecus were at the upper end of the range of variation in their large series of mexicanus from the western lowlands, and crinitus clustered with specimens of O. peninsulae
from the tip of the Baja California Peninsula
. They suggested that regillus and aztecus may represent no more than robust upland populations of mexicanus, but could not exclude the possibility that they represent a different species. That crinitus, which occurs at over 2000 m (6600 ft) altitude in the Valley of Mexico
, was the same species as peninsulae from the lowlands of the Baja California Peninsula they could not accept and they recommended further research to determine the relationships of crinitus. A specimen from inland Michoacán has Cytb data characteristic of mexicanus, but Hanson and colleagues did not have data for other interior Mexican Oryzomys.
The holotype
of the species Oryzomys fulgens, which Thomas had described in 1893, has no more precise locality than "Mexico", but the Valley of Mexico has been suggested as its origin. It is a large, coarse-furred, bright reddish, long-tailed species with a broad skull with widely spreading zygomatic arches. Goldman wrote that it was similar to crinitus, but more intensely colored, and differed in the form of the interorbital region; he retained it as a separate species pending further investigations. Carleton and Arroyo-Cabrales noted that archival research may yet uncover the precise origin of O. fulgens, which could establish it as an older name for one of the other central Mexican Oryzomys.
Oryzomys populations from Texas to Nicaragua form a single Cytb clade, within which the average sequence divergence is 1.28%, and Hanson and colleagues proposed that the name Oryzomys couesi be restricted to this clade. These populations correspond to two subspecies recognized by Goldman (O. c. aquaticus and O. c. couesi) and an island form he retained as a species (O. cozumelae). Two other subspecies Goldman recognized, O. c. richmondi and O. c. peragrus, and a third, O. c. pinicola, that was described after Goldman's paper occur in the same region, but have not been studied genetically.
The northernmost populations of Oryzomys couesi, those in southernmost Texas and nearby Tamaulipas
, Mexico, are classified as the subspecies aquaticus, which was described as a separate species, Oryzomys aquaticus, in 1891. Here the range of O. couesi meets that of the marsh rice rat; in parts of Kenedy, Willacy and Cameron
counties, Texas, and in far northeastern Tamaulipas, the two are sympatric (occur in the same places). In the contact zone, couesi occurs further inland, while the marsh rice rat lives along the coast. In experimental conditions, the two fail to interbreed and genetic analysis yields no evidence of gene flow
or hybridization in the wild. Compared to populations further to the south, aquaticus is larger and paler and has a more robust skull. Specimens from Tamaulipas are slightly darker than those from Texas. The Cytb sequences of specimens of aquaticus form a separate group, but cluster among specimens of O. c. couesi from further south.
The form peragrus is known from further south in Mexico, in the Río Verde
basin of San Luis Potosi
, the state of Hidalgo, and far northern Veracruz
. Late Pleistocene
fossils of this form have been found in Cueva de Abre, Tamaulipas. According to Goldman, it is intermediate in color between O. c. aquaticus and O. c. couesi, but has a skull similar to that of aquaticus.
Goldman united populations ranging from northern Veracruz through eastern Mexico, Guatemala, Honduras, and Nicaragua south to far northwestern Costa Rica in the nominate subspecies, Oryzomys couesi couesi. He placed six other names as full synonyms of this form, which has its type locality in Guatemala—Oryzomys jalapae Allen and Chapman, 1897, from Veracruz; Oryzomys jalapae rufinus Merriam, 1901, from Veracruz; Oryzomys teapensis Merriam, 1901, from Tabasco
; Oryzomys goldmani Merriam, 1901, from Veracruz; Oryzomys jalapae apatelius Eliot, 1904, from Veracruz; and Oryzomys richardsoni Allen, 1910, from Nicaragua. According to Goldman, individual variation within the subspecies is large, which has led to the large number of published synonyms, but populations from all parts of its range are essentially similar.
The subspecies Oryzomys couesi pinicola was described in 1932 from a pine ridge in western British Honduras (now Belize
); it is smaller and darker than nominate couesi, which also occurs in Belize, and has a more delicate skull. In 1901, Merriam described the Oryzomys of the island of Cozumel
as a separate species, Oryzomys cozumelae, and Goldman kept it as such because of its large size, dark fur, and long tail. In 1965, however, Knox Jones and Timothy Lawlor judged the differences between cozumelae and mainland couesi trivial and found that cozumelae was inside the range of variation of mainland Oryzomys populations; accordingly, they demoted the island form to a subspecies. Mark Engstrom and colleagues, writing in 1989, reaffirmed this conclusion. For an island form, this population is highly genetically variable. In its Cytb sequence data, it falls among populations of nominate couesi. Oryzomys couesi is also found on Turneffe Atoll off the coast of Belize and Roatán
off Honduras.
The Oryzomys of the eastern lowlands of Nicaragua was described as a separate species, Oryzomys richmondi, by Merriam in 1901, and Goldman retained it as a subspecies of O. couesi on the basis of its distinctly dark fur. In reviewing Nicaraguan Oryzomys in 1986, Jones and Engstrom did not keep richmondi as separate, because they thought the difference in color too small for the recognition of subspecies. Oryzomys dimidiatus, a small, dark Oryzomys with gray underparts, occurs with O. couesi in southeastern Nicaragua. According to Jones and Engstrom, rice rats from the island of Ometepe
in Lake Nicaragua
are distinctive in their large skull and small external measurements, with an especially short tail, soft fur that is orange-brown above and buffish below, and lack of sphenopalatine vacuities
(openings in the roof of the mesopterygoid fossa, the gap behind the end of the bony palate
). They considered that this population probably represented a separate subspecies, but declined to propose a new name because they had only one adult specimen. In Nicaragua, O. couesi occurs up to an altitude of 1250 m (4100 ft).
Oryzomys from Costa Rica have historically been referred to O. c. couesi, but Hanson and colleagues found that two specimens from Refugio Nacional de Vida Silvestre Mixto Maquenque, northeastern Costa Rica, differed as much from other O. couesi (11.93% Cytb sequence divergence) as O. couesi differed from the marsh rice rat (11.30%). They suggested that these animals represented a species distinct from O. couesi, but were unable to resolve the correct name for the species because they could not examine samples of dimidiatus or richmondi.
Oryzomys is rare in Panama. Panamanian Oryzomys were first described by Goldman in 1912, who introduced the name Oryzomys gatunensis for a specimen from Gatún
in the Canal Zone
. In 1918, Goldman kept the animal as a separate species, remarking that it was similar to richmondi, but distinctive in the well-developed ridges along the margins of the interorbital region, the short interparietal bone (part of the roof of the braincase), and the long nasal bone
s. In 1937, Bole described another species of Panamanian Oryzomys, Oryzomys azuerensis from Paracoté, Veraguas Province
. It is a brown form, lacking the reddish tones of nearby populations, and has a broad skull with a short rostrum (front part) and ridges on the interorbital region like those of gatunensis. Although Goldman recommended to him that gatunensis and azuerensis both be treated as subspecies of couesi, Bole described azuerensis as a species because it did not seem intermediate between the geographically closest forms, gatunensis and couesi, and was separated by a large gap from the nearest known populations of O. couesi in northwestern Costa Rica and southeastern Nicaragua. In a 1966 review of Panamanian mammals, Charles Handley reduced both gatunensis and azuerensis to subspecies of the marsh rice rat (in which O. couesi was included at the time), and when O. couesi was reinstated as a separate species these forms went with it. Specimens from near the type locality of azuerensis differ by about 7% in their Cytb sequences from other O. couesi, which suggests that they may represent a separate species. However, Hanson and colleagues did not reinstate azuerensis as a species, because they could not examine samples of gatunensis.
Oryzomys couesi was first reported from Colombia in 1987, when Philip Hershkovitz
reported on its occurrence at Montería
in Córdoba Department
, northwestern Colombia. The Colombian specimen is ochraceous in color throughout and according to Hershkovitz almost identical to specimens from Guatemala, but distinctive in that the upper lip is white. He suggested that O. couesi may also be discovered in the Pacific lowlands of the Chocó in western Colombia.
s have been proposed for Oryzomys couesi and the synonyms currently associated with it. Eliot in 1905 and Goldman in 1918 gave separate common names for each of the species and subspecies they recognized. Many authors have used "Coues' Rice Rat" or some variation thereof for O. couesi, but "Coues' Oryzomys" has also been used.
Oryzomys couesi is a medium-sized to large rat with coarse fur that is buff to reddish above, becoming paler towards the sides and cheeks and darker on the rump and face. The underparts are white to buff. The fur is shorter, brighter, and more intense in color than in the marsh rice rat. The snout ends bluntly and the moderately large eyes show reddish eyeshine. The small ears are black on the outside and the inside is covered with short, gray to buff or red hairs. The long tail is dark brown above and white to light brown below. The feet are long and stout. On the forefeet, the ungual tuft
s (tufts of hair on the digits) are present. Many of the pads on the hindfeet are reduced, as are the ungual tufts, and small interdigital webs
may be present in at least some specimens. Some of these traits are common adaptations to life in the water in oryzomyines. As in most other oryzomyines, females have eight mammae. Head and body length is 98 to 142 mm (3.9 to 5.6 in), tail length is 107 to 152 mm (4.2 to 6.0 in), hindfoot length is 27 to 33 mm (1.1 to 1.3 in), ear length is 13 to 18 mm (0.51 to 0.71 in), and body mass is 43 to 82 grams (1.5 to 2.9 oz). Studies in Texas and El Salvador found that males are slightly larger than females.
The stomach
has the characteristic pattern of sigmodontines (unilocular-hemiglandular): it is not split in two chambers by an incisura angularis and the front part (antrum
) is covered by a glandular epithelium. The gall bladder is absent, a synapomorphy
(shared-derived character) of Oryzomyini. The karyotype
includes 56 chromosome
s and a fundamental number of 56 autosomal
arms (2n = 56, FNa = 56). The autosomes include 26 pairs of acrocentric chromosomes, with a long and a very short arm, and one medium-sized submetacentric pair, with one arm shorter than the other. The X chromosome
is either acrocentric, with a long and a short arm, or subtelocentric, with a long and a vestigial arm. The form of the sex chromosomes has been used to distinguish the marsh rice rat from Oryzomys couesi, but there are no consistent differences between the two.
As is characteristic of Sigmodontinae, Oryzomys couesi has a complex penis, with the baculum
(penis bone) ending in three cartilaginous
digits at its tip. The outer surface of the penis is mostly covered by small spines, but there is a broad band of nonspinous tissue. The papilla (nipple-like projection) on the dorsal (upper) side of the penis is covered with small spines, a character Oryzomys couesi shares only with Oligoryzomys
and the marsh rice rat among oryzomyines examined. On the urethral process, located in the crater at the end of the penis, a fleshy process
(the subapical lobule) is present; it is absent in all other oryzomyines with studied penes except the marsh rice rat and Holochilus brasiliensis.
, frontal
, and maxillary bones meet. The zygomatic plate
, the flattened front part of the zygomatic arch, is broad and develops a notch at its front end. The plate's back margin is located before the first upper molar. The jugal bone, part of the zygomatic arch, is reduced, as usual in oryzomyines. The sphenopalatine foramen
, a foramen
(opening) at the side of the skull above the molars, is small; it is much larger in the marsh rice rat. The narrowest part of the interorbital region is towards the front and the edges are lined by prominent shelves. The parietal bone
s extend to the sides of the braincase. The interparietal bone is narrow and wedge-shaped, so that the parietal and squamosal bones meet extensively.
The incisive foramina
, openings in the front part of the palate, reach backward between the molars. The palate is long, extending substantially beyond the third molars, the usual condition in oryzomyines. The back part, near the third molars, is usually perforated by prominent posterolateral palatal pits
, which are recessed into fossae (depressions). Sphenopalatine vacuities are usually absent, but have been reported in some populations. There is no alisphenoid strut
, an extension of the alisphenoid bone that in some oryzomyines separates two foramina in the skull. The condition of the arteries
in the head is highly derived
. The subsquamosal fenestra
, an opening in the back part of the skull determined by the shape of the squamosal bone, is present. The squamosal lacks a suspensory process that contacts the tegmen tympani, the roof of the tympanic cavity
, a defining character of oryzomyines. There are some openings in the mastoid bone.
In the mandible
(lower jaw), the mental foramen
, an opening just before the first molar, opens sidewards, not upwards as in a few other oryzomyines. The upper and lower masseteric ridges, which anchor some of the chewing muscles, join at a point below the first molar and do not extend forward beyond that point. The capsular process
, a raising of the bone of the back of the mandible that houses the back end of the incisor
, is large.
on each side of the jaws), as usual in muroid rodents
. The upper incisors are opisthodont, with the chewing edge located behind the vertical plane of the teeth. The molars are bunodont, with the cusps higher than the connecting crests, and brachydont
, low-crowned, as in most other oryzomyines. Many accessory crests, including the mesoloph on the upper molars and the mesolophid on the lower molars, are present, another trait O. couesi shares with most but not all other oryzomyines. The flexi and flexids (valleys between the cusps and crests) at the labial (outer) side of the molars are closed by cingula
(ridges).
On the first and second upper molars, the flexi do not extend to the midline of the molars. The anterocone, the front cusp of the upper first molar, is not divided in two by an indentation at its front (anteromedian flexus). A crest, the anteroloph, is present behind the labial cuspule. As in most oryzomyines, the upper molars all have one root on the inner (lingual) side and two on the outer (labial) side; in addition, the first upper molar usually has another small labial root.
On the first lower molar, the labial and lingual conules of the anteroconid, the frontmost cusp, are separated by an anteromedian fossette. The second lower molar bears a crest, the anterolophid, before the two cusps, the protoconid and metaconid, that form the front edge of the molar in some other oryzomyines. There is a distinct ridge (anterolabial cingulum) at the outer front (anterolabial) edge of the molar, before the protoconid. The third lower molar also bears an anterolophid and an anterolabial cingulum. The first lower molar has large roots at the front and back of the tooth and two smaller ones in between, at the labial and lingual side. The second and third lowers molars have two large roots, one at the front and one at the back.
is absent, as in all members of the Sigmodontinae; if present, as in some other rodents, this foramen perforates the distal (far) end of the humerus
(upper arm bone).
, but not the central plateau of Mexico, through Central America south and east to northwestern Colombia; see under "Taxonomy" for details. The species has also been found in late Pleistocene
cave deposits in Mexico and Honduras. It is common in watery habitats, such as marshes and small streams, but also occurs in forests and shrubland
s with sufficient cover. In addition, it is found in sugarcane and rice fields. In Texas, it occurs in marsh vegetation along resacas (oxbow lake
s) and in Veracruz, it has even been found on the dry coastal plain
among shrubs. It occurs from 2300 m (7500 ft) altitude down to sea level. On Cozumel, the proportion of juveniles and females is higher near roads that function as habitat edges. Cozumel rice rats rarely cross roads, which may isolate subpopulations on the island.
Oryzomys couesi lives on the ground and is semiaquatic, spending much time in the water, as Alston in his original description already recognized, but is also a good climber. A study in Costa Rica found that O. couesi is an excellent swimmer, diving well and using its tail to propel itself. It is probably able to forage underwater, which may help differentiate its niche
from that of the ecologically similar cotton rat Sigmodon hirsutus, which also swims well, but does not dive. When disturbed, O. couesi will enter the water and swim away. It is primarily active during the night. Oryzomys couesi builds globular nests of woven vegetation suspended among reeds, about 1 m (3 ft) above the water or the ground; in Texas, larger individuals make larger nests. It does not usually make its own runways in vegetation, but may use those of other rodents, such as cotton rats.
Population densities
range from 5 to 30 per ha (2 to 12 per acre). On Cozumel, density is around 14.5 to 16.5 per ha (5.9 to 6.7 per acre), but shows large seasonal variation. In western Mexico, one study found densities of 3 per ha (1.2 per acre) in cloud forest
and 1 per ha (0.4 per acre) in a disturbed area. In 24 hours, male Texas O. couesi move up to 153 m (502 ft) and females up to 126 m (413 ft). The diet includes both plant material, including seeds and green parts, and animals, including small fish, crustaceans, snails, insects like ants and beetles, and other invertebrates. It probably breeds around the year and after a pregnancy of 21 to 28 days, the female produces litters of two to seven young, with an average of 3.8, according to Reid's Mammals of Central America & Southeast Mexico. In 28 pregnant females from Nicaragua, litter size varied from one to eight, averaging 4.4. The young become reproductively active when seven weeks old and the life cycle
is short.
The introduced
snake Boa constrictor
preys on O. couesi on Cozumel. Parasites recorded on O. couesi in Veracruz include unidentified ticks, mites, fleas, and fly larvae. The flea Polygenis odiosus was found on an Oryzomys couesi from Cozumel. Out of ten O. couesi in San Luis Potosí, five each were infected by the nematode
worms Hassalstrongylus musculi
and H. bocqueti, with about 25 worms per rat, and two were infected by one or two cestodes of the genus Raillietina
. The mite
s Eubrachylaelaps circularis and Gigantolaelaps boneti have been found on Oryzomys couesi in Oaxaca, the sucking louse
Hoplopleura oryzomydis
in Nicaragua, the mites Laelaps oryzomydis, Echinonyssus microchelae, Ornithonyssus bacoti, Prolistrophorus frontalis, and Prolistrophorus bakeri
in Colima, and the apicomplexa
n Eimeria couesii
in Mexico. The species is infected by two hantavirus
es—Catacamas virus in Honduras and Playa de Oro virus
in western Mexico—which are related to the Bayou virus infecting the marsh rice rat, a common cause of hantavirus infections in the United States. No hantavirus infections in humans have been linked to O. couesi hantaviruses, however. Chiapas O. couesi easily survive experimental infection with several arbovirus
es, including the Venezuelan equine encephalitis virus
, suggesting that the species may serve as a reservoir
for that virus.
", because it is a widely distributed, common species with broad habitat tolerance that occurs in many protected area
s. Habitat destruction
, such as drainage of wetlands, may threaten some populations. In many areas, it is so common that it is considered a plague species. Populations even persist in the Valley of Mexico, as evidenced by a photograph published in 2006. However, it is listed as threatened in Texas, where its distribution is very limited, because of habitat loss. In 1979, Benson and Gehlbach estimated the size of the Texas population to be about 15,000. A 2001 study predicted that climate change
would drive the Texas population to extinction, because no suitable habitats would continue to exist. The Cozumel population has declined substantially since the mid 1980s, perhaps due to habitat disturbance and predation by introduced species.
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
in the family Cricetidae
Cricetidae
The Cricetidae are a family of rodents in the large and complex superfamily Muroidea. It includes true hamsters, voles, lemmings, and New World rats and mice...
occurring from southernmost Texas through Mexico and Central America into northwestern Colombia. It is usually found in wet habitats, such as marshes, but also lives in drier forests and shrubland
Shrubland
Shrubland, scrubland, scrub or brush is a plant community characterized by vegetation dominated by shrubs, often also including grasses, herbs, and geophytes. Shrubland may either occur naturally or be the result of human activity...
s. Weighing about 43 to 82 g (1.5 to 2.9 oz), O. couesi is a medium-sized to large rat. The coarse fur is buff to reddish above and white to buff below. The hindfeet show some specializations for life in the water, such as reduced ungual tufts of hair around the digits. It has 56 chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s. There is much geographic variation in size, proportions, color, and skull features. Oryzomys couesi is active during the night and builds nests of vegetation that are suspended among reeds about 1 m (3 ft) above the ground. It is an excellent swimmer and dives well, but can also climb in vegetation. An omnivore
Omnivore
Omnivores are species that eat both plants and animals as their primary food source...
, it eats both plant and animal food, including seeds and insects. It breeds throughout the year; females give birth to about four young after a pregnancy of 21 to 28 days. The species may be infected by several different parasites and by two hantavirus
Hantavirus
Hantaviruses are negative sense RNA viruses in the Bunyaviridae family. Humans may be infected with hantaviruses through rodent bites, urine, saliva or contact with rodent waste products...
es.
The species was first described in 1877, the first of many related species from the region described until the 1910s. In 1918, Edward Alphonso Goldman
Edward Alphonso Goldman
Edward Alphonso Goldman was an American zoologist. He worked extensively in Mexico with Edward William Nelson and described and revised many groups of mammals....
consolidated most into the single species Oryzomys couesi and in 1960 Raymond Hall united this taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
with its United States relative, the marsh rice rat
Marsh Rice Rat
The marsh rice rat is a semiaquatic North American rodent in the family Cricetidae. It is found mostly in the eastern and southern United States, from New Jersey and Kansas south to Florida and northeasternmost Tamaulipas, Mexico; its range previously extended further west and north, where it may...
(O. palustris), into a single widespread species; subsequently, many related, localized species retained by Goldman were also included in this taxon. After studies of the contact zone in Texas, where O. couesi and the marsh rice rat meet, were published in 1979 and underscored the distinctness of the two, they were again regarded as separate. Since then, some of the peripheral forms of the group, such as Oryzomys antillarum from Jamaica and Oryzomys peninsulae
Oryzomys peninsulae
Oryzomys peninsulae, also known as the Lower California Rice Rat, is a species of rodent from western Mexico. Restricted to the southern tip of the Baja California peninsula, it is a member of the genus Oryzomys of family Cricetidae...
from the Baja California Peninsula
Baja California Peninsula
The Baja California peninsula , is a peninsula in northwestern Mexico. Its land mass separates the Pacific Ocean from the Gulf of California. The Peninsula extends from Mexicali, Baja California in the north to Cabo San Lucas, Baja California Sur in the south.The total area of the Baja California...
, have been reinstated as species. Nevertheless, O. couesi as currently constituted is likely a composite of several species; a 2010 study, using DNA sequence
DNA sequence
The sequence or primary structure of a nucleic acid is the composition of atoms that make up the nucleic acid and the chemical bonds that bond those atoms. Because nucleic acids, such as DNA and RNA, are unbranched polymers, this specification is equivalent to specifying the sequence of...
data, found evidence to recognize separate species from the Pacific and eastern sides of the distribution of O. couesi and two additional species from Panama
Panama
Panama , officially the Republic of Panama , is the southernmost country of Central America. Situated on the isthmus connecting North and South America, it is bordered by Costa Rica to the northwest, Colombia to the southeast, the Caribbean Sea to the north and the Pacific Ocean to the south. The...
and Costa Rica
Costa Rica
Costa Rica , officially the Republic of Costa Rica is a multilingual, multiethnic and multicultural country in Central America, bordered by Nicaragua to the north, Panama to the southeast, the Pacific Ocean to the west and the Caribbean Sea to the east....
. Generally, Oryzomys couesi is common and of no conservation concern, and it is even considered a plague species in places, but some populations are threatened.
Taxonomy
Oryzomys couesi and at least six more narrowly distributed species with peripheral distributions together form the O. couesi group within the genus OryzomysOryzomys
Oryzomys is a genus of semiaquatic rodents in the tribe Oryzomyini living in southern North America and far northern South America. It includes eight species, two of which—the marsh rice rat of the United States and O. couesi of Mexico and Central America—are widespread; the six others have...
. The eighth species of the genus, the marsh rice rat
Marsh Rice Rat
The marsh rice rat is a semiaquatic North American rodent in the family Cricetidae. It is found mostly in the eastern and southern United States, from New Jersey and Kansas south to Florida and northeasternmost Tamaulipas, Mexico; its range previously extended further west and north, where it may...
(O. palustris) is the only member of its own group (unless western populations are classified
Biological classification
Biological classification, or scientific classification in biology, is a method to group and categorize organisms by biological type, such as genus or species. Biological classification is part of scientific taxonomy....
as a separate species, O. texensis). Oryzomys previously included many other species, which were reclassified in various studies culminating in contributions by Marcelo Weksler and coworkers in 2006 that removed more than forty species from the genus. All are placed in the tribe Oryzomyini
Oryzomyini
Oryzomyini is a tribe of rodents in the subfamily Sigmodontinae of family Cricetidae. It includes about 120 species in about thirty genera, distributed from the eastern United States to the southernmost parts of South America, including many offshore islands...
("rice rats"), a diverse assemblage of over a hundred species, and on higher taxonomic levels in the subfamily Sigmodontinae
Sigmodontinae
The subfamily Sigmodontinae is one of the most diverse groups of mammals. It includes New World rats and mice, with at least 376 species. Many authorities include the Neotominae and Tylomyinae as part of a larger definition of Sigmodontinae. When those genera are included, the species count...
of the family Cricetidae
Cricetidae
The Cricetidae are a family of rodents in the large and complex superfamily Muroidea. It includes true hamsters, voles, lemmings, and New World rats and mice...
, along with hundreds of other species of mainly small rodents.
History
Edward AlstonEdward Alston
Sir Edward Alston , was the president of the College of Physicians.Alston was born in Suffolk, and was educated at St. John's College, Cambridge. He graduated B.A. 1615, M.D. 1626. In 1631 he was elected a fellow of the College of Physicians, and was president from 1655 until 1666. At the...
first described Oryzomys couesi in 1877, using three specimens from Mexico and Guatemala. He named the animal Hesperomys couesi, placing it in the now-defunct genus Hesperomys
Hesperomys
Hesperomys is an obsolete genus of American rodents. It was initially broadly defined to include most of the cricetid rodents of the Americas, except the Arvicolinae, but later became restricted to the members of the modern genus Calomys; the latter name has been in use for this genus instead of...
, and noted similarities to the marsh rice rat
Marsh Rice Rat
The marsh rice rat is a semiaquatic North American rodent in the family Cricetidae. It is found mostly in the eastern and southern United States, from New Jersey and Kansas south to Florida and northeasternmost Tamaulipas, Mexico; its range previously extended further west and north, where it may...
(then called Hesperomys palustris) and two species now placed in Tylomys
Tylomys
Tylomys is a genus of rodent in the family Cricetidae.It contains the following species:* Chiapan Climbing Rat * Fulvous-bellied Climbing Rat * Mira Climbing Rat...
. The specific name, couesi, honors American naturalist Elliott Coues
Elliott Coues
Elliott Coues was an American army surgeon, historian, ornithologist and author.Coues was born in Portsmouth, New Hampshire. He graduated at Columbian University, Washington, D.C., in 1861, and at the Medical school of that institution in 1863...
, who had done much work on North American rodents. In 1893, Oldfield Thomas
Oldfield Thomas
Oldfield Thomas FRS was a British zoologist.Thomas worked at the Natural History Museum on mammals, describing about 2,000 new species and sub-species for the first time. He was appointed to the Museum Secretary's office in 1876, transferring to the Zoological Department in 1878...
wrote that the species, by then placed in the genus Oryzomys
Oryzomys
Oryzomys is a genus of semiaquatic rodents in the tribe Oryzomyini living in southern North America and far northern South America. It includes eight species, two of which—the marsh rice rat of the United States and O. couesi of Mexico and Central America—are widespread; the six others have...
as Oryzomys couesi, had caused much confusion about its identity, because the three specimens (one from Cobán
Cobán
The city of Cobán is the capital of the department of Alta Verapaz in central Guatemala. It also serves as the administrative center for the surrounding Cobán municipality. It is located 219 km from Guatemala City....
, Guatemala, and two from Mexico) used by Alston in fact belonged to two or three different species. He restricted the name couesi to the animal from Guatemala, and introduced the new name Oryzomys fulgens for one of the Mexican animals. Several other related species were described from the early 1890s onwards and in 1901 Clinton Hart Merriam
Clinton Hart Merriam
Clinton Hart Merriam was an American zoologist, ornithologist, entomologist and ethnographer.Known as "Hart" to his friends, Dr. Clinton Hart Merriam was born in New York City in 1855. His father, Clinton Levi Merriam, was a U.S. congressman. He studied biology and anatomy at Yale University and...
united many of those into a palustris-mexicanus group of species, which also included the marsh rice rat.
Edward Alphonso Goldman
Edward Alphonso Goldman
Edward Alphonso Goldman was an American zoologist. He worked extensively in Mexico with Edward William Nelson and described and revised many groups of mammals....
revised North American Oryzomys in 1918 and consolidated many forms into a single species Oryzomys couesi, with ten subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
distributed from southern Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
and western Mexico south to Costa Rica. He placed it in an Oryzomys palustris group with the marsh rice rat and several species with more limited distributions, which he regarded as related to O. couesi but distinctive enough to be classified as separate species. In the 1930s, a few more forms related to O. couesi were described. As then recognized, the ranges of the marsh rice rat, a United States species, and Oryzomys couesi meet in southern Texas. In 1960, Raymond Hall reviewed specimens from this contact zone and found no grounds on which to separate the two species; thus, he reduced O. couesi to a subspecies of the marsh rice rat. Other workers continued this lumping and by 1971 all other species Goldman had placed in the O. palustris group were classified under the marsh rice rat, together with Oryzomys azuerensis from Panama, described as a species in 1937.
Additional studies of the palustris–couesi contact zone in Texas published in 1979, using more specimens and characters, indicated that the two species are in fact easily distinguishable there; therefore, O. couesi has since been regarded as a species distinct from the marsh rice rat. Afterward, some of the other forms synonymized under O. couesi or O. palustris were resurrected as separate species—Oryzomys nelsoni from the Marías Islands, western Mexico, and Oryzomys antillarum from Jamaica. In 2009, Michael Carleton and Joaquin Arroyo-Cabrales reviewed western Mexican Oryzomys, reaffirmed the distinctness of O. nelsoni, and reinstated O. peninsulae
Oryzomys peninsulae
Oryzomys peninsulae, also known as the Lower California Rice Rat, is a species of rodent from western Mexico. Restricted to the southern tip of the Baja California peninsula, it is a member of the genus Oryzomys of family Cricetidae...
from the tip of the Baja California Peninsula
Baja California Peninsula
The Baja California peninsula , is a peninsula in northwestern Mexico. Its land mass separates the Pacific Ocean from the Gulf of California. The Peninsula extends from Mexicali, Baja California in the north to Cabo San Lucas, Baja California Sur in the south.The total area of the Baja California...
and O. albiventer
Oryzomys albiventer
Oryzomys albiventer is a rodent in the genus Oryzomys of family Cricetidae from interior western Mexico, in the states of Jalisco, Guanajuato, and Michoacán. First described in 1901 as a separate species, it was later lumped under O. couesi and the marsh rice rat until it was reinstated as a...
from interior Mexico as species. Still, O. couesi included 22 synonyms
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...
, and Carleton and Arroyo-Cabrales wrote that further research on O. couesi and related species would certainly result in the recognition of additional species.
A 2010 study by Delton Hanson and colleagues used DNA sequence data from the mitochondrial
Mitochondrial DNA
Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...
gene cytochrome b
Cytochrome b
Cytochrome b/b6 is the main subunit of transmembrane cytochrome bc1 and b6f complexes. In addition, it commonly refers to a region of mtDNA used for population genetics and phylogenetics.- Function :...
(Cytb) and two nuclear
Nuclear DNA
Nuclear DNA, nuclear deoxyribonucleic acid , is DNA contained within a nucleus of eukaryotic organisms. In mammals and vertebrates, nuclear DNA encodes more of the genome than the mitochondrial DNA and is composed of information inherited from two parents, one male, and one female, rather than...
markers, exon
Exon
An exon is a nucleic acid sequence that is represented in the mature form of an RNA molecule either after portions of a precursor RNA have been removed by cis-splicing or when two or more precursor RNA molecules have been ligated by trans-splicing. The mature RNA molecule can be a messenger RNA...
1 of the interphotoreceptor retinoid-binding protein gene (Rbp3) and intron
Intron
An intron is any nucleotide sequence within a gene that is removed by RNA splicing to generate the final mature RNA product of a gene. The term intron refers to both the DNA sequence within a gene, and the corresponding sequence in RNA transcripts. Sequences that are joined together in the final...
2 of alcohol dehydrogenase
Alcohol dehydrogenase
Alcohol dehydrogenases are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones with the reduction of nicotinamide adenine dinucleotide...
gene 1 (Adh1-I2) to study relationships among populations of the marsh rice rat and O. couesi. The Cytb data placed all studied specimens of O. couesi in a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
sister to the marsh rice rat; the mean genetic distance
Genetic distance
Genetic distance refers to the genetic divergence between species or between populations within a species. It is measured by a variety of parameters. Smaller genetic distances indicate a close genetic relationship whereas large genetic distances indicate a more distant genetic relationship...
between the two groups was 11.30%, much larger than the distance between sister species in the related genera Melanomys
Melanomys
Melanomys is a genus of rodent in the tribe Oryzomyini of family Cricetidae, which is distributed in northern South America and adjacent Central America. It contains three species, two of which—Melanomys robustulus and Melanomys zunigae—have limited distributions...
and Nectomys
Nectomys
Nectomys is a genus of rodent in the tribe Oryzomyini of family Cricetidae. It is closely related to Amphinectomys and was formerly considered congeneric with Sigmodontomys...
(7.48% and 7.52%, respectively). Within the O. couesi clade, two populations from Panama and Costa Rica were successively basal to the other specimens, which fell into two large subclades—one containing animals from the Pacific seaboard from western Mexico to El Salvador and the other containing rats from the eastern seaboard from Texas to Nicaragua. The Panamanian and Costa Rican populations differed by 6.53% to 11.93% from the others and the western and eastern subclades differed by 4.41% on average. Data from both of the slower-evolving nuclear markers Rbp3 and Adh1-I2 also placed examples of Oryzomys in two main clades, but did not recover the western and eastern groups of O. couesi as separate clades. In addition, Adh1-I2 placed the Costa Rican population within the marsh rice rat clade and placed some western O. couesi specimens closer to the marsh rice rat than to the O. couesi group. The combined dataset supported the western and eastern clades within O. couesi and placed the Costa Rican population marginally closer to the marsh rice rat than to O. couesi. Using the genetic species concept, the authors suggested that the four groups they found within O. couesi should be recognized as distinct species. If this suggestion is followed, the eastern subclade would retain the name Oryzomys couesi, the western group would be named Oryzomys mexicanus, and the appropriate names for the Panamanian and Costa Rican species remain unclear.
Western Mexico to El Salvador
|
Populations of Oryzomys couesi from Jalisco
Jalisco
Jalisco officially Estado Libre y Soberano de Jalisco is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is located in Western Mexico and divided in 125 municipalities and its capital city is Guadalajara.It is one of the more important states...
, western Mexico, east to El Salvador form a single Cytb clade, which Hanson and colleagues proposed to recognize as the species Oryzomys mexicanus. These animals differ by 4.4% from Oryzomys couesi in the strict sense, which occurs to the north and east, are separated by mountain ranges from the latter, harbor different species of hantavirus
Hantavirus
Hantaviruses are negative sense RNA viruses in the Bunyaviridae family. Humans may be infected with hantaviruses through rodent bites, urine, saliva or contact with rodent waste products...
, and according to Merriam (1901) have more robust skulls, with larger molars
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
, stronger zygomatic arch
Zygomatic arch
The zygomatic arch or cheek bone is formed by the zygomatic process of temporal bone and the temporal process of the zygomatic bone , the two being united by an oblique suture; the tendon of the Temporalis passes medial to the arch to gain insertion into the coronoid process...
es (cheekbones), and better developed ridges along the margins of the interorbital region
Interorbital region
The interorbital region of the skull is located between the eyes, anterior to the braincase. The form of the interorbital region may exhibit significant variation between taxonomic groups....
of the skull (between the eyes). Within the "Oryzomys mexicanus" clade, Cytb sequence differences average 2.06% and western (Jalisco to Oaxaca
Oaxaca
Oaxaca , , officially Estado Libre y Soberano de Oaxaca is one of the 31 states which, along with the Federal District, comprise the 32 federative entities of Mexico. It is divided into 571 municipalities; of which 418 are governed by the system of customs and traditions...
) and eastern (Chiapas
Chiapas
Chiapas officially Estado Libre y Soberano de Chiapas is one of the 31 states that, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 118 municipalities and its capital city is Tuxtla Gutierrez. Other important cites in Chiapas include San Cristóbal de las...
and El Salvador) groups form distinct subclades; Hanson and colleagues recognized these as different subspecies, mexicanus in the west and zygomaticus in the east.
As defined by Carleton and Arroyo-Cabrales in 2009, the subspecies Oryzomys couesi mexicanus occurs along the Pacific coast from central Sonora to southeastern Oaxaca
Oaxaca
Oaxaca , , officially Estado Libre y Soberano de Oaxaca is one of the 31 states which, along with the Federal District, comprise the 32 federative entities of Mexico. It is divided into 571 municipalities; of which 418 are governed by the system of customs and traditions...
and inland along rivers into central Michoacán
Michoacán
Michoacán officially Estado Libre y Soberano de Michoacán de Ocampo is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 113 municipalities and its capital city is Morelia...
, southern Morelos
Morelos
Morelos officially Estado Libre y Soberano de Morelos is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 33 municipalities and its capital city is Cuernavaca....
, southern Puebla
Puebla
Puebla officially Estado Libre y Soberano de Puebla is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 217 municipalities and its capital city is Puebla....
, and northwestern Oaxaca. It usually lives below 1000 m (3300 ft) altitude, but has been found at 1525 m (5000 ft) in Jalisco
Jalisco
Jalisco officially Estado Libre y Soberano de Jalisco is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is located in Western Mexico and divided in 125 municipalities and its capital city is Guadalajara.It is one of the more important states...
. This distributional pattern is similar to that of other western Mexican rodents such as Sigmodon mascotensis, Hodomys alleni, Peromyscus perfulvus, and Osgoodomys banderanus and has been recognized as a distinct biogeographic
Biogeography
Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...
zone in some reviews. O. c. mexicanus occurs close to three other Oryzomys species—O. albiventer, O. peninsulae, and O. nelsoni—which are larger and different in some proportions and details of coloration.
Joel Asaph Allen
Joel Asaph Allen
Joel Asaph Allen was an American zoologist and ornithologist, born in Springfield, Massachusetts.He studied at Harvard University under Louis Agassiz...
first described Oryzomys mexicanus as a full species in 1897 from specimens from Jalisco. In the same publication, he also described Oryzomys bulleri from nearby Nayarit
Nayarit
Nayarit officially Estado Libre y Soberano de Nayarit is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 20 municipalities and its capital city is Tepic.It is located in Western Mexico...
, but he did not compare the two with each other. Merriam added a second species from Nayarit, Oryzomys rufus, in 1901, noting that it was smaller and more reddish than mexicanus. Goldman synonymized the three as O. couesi mexicanus in 1918 and in 2009 Carleton and Arroyo-Cabrales concurred, arguing that the differences between rufus and mexicanus were age-related and within the normal range of variation of the animal. Another subspecies, Oryzomys couesi lambi, was described by Burt in 1934 from central coastal Sonora, which extended the range of the species by 400 mi (640 km) at the time. This form is dark gray-brown, much darker than mexicanus, and has a shorter tail and weaker jugals. Carleton and Arroyo-Cabrales wrote that it is similar to mexicanus, but that further research is needed to determine whether it should be recognized as a subspecies. Large O. couesi from northern Sinaloa
Sinaloa
Sinaloa officially Estado Libre y Soberano de Sinaloa is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 18 municipalities and its capital city is Culiacán Rosales....
may also belong to this form. Goldman wrote that mexicanus was very similar to nominate couesi, but usually with paler fur; the upperparts are more buffy than in couesi and the underparts are usually white, but may be buffy, the normal color in couesi.
Oryzomys zygomaticus was first described by Merriam in 1901 as a separate species similar to mexicanus, but with the zygomatic arches broadly spreading and curved downward. Goldman, who reduced it to a subspecies of couesi, recorded it from southwestern Guatemala and nearby Chiapas and described it as slightly paler than O. c. couesi but darker than O. c. mexicanus. Three specimens from central El Salvador have Cytb sequences similar to those of zygomaticus, but in The Mammals of El Salvador (1961), Burt and Stirton recorded only the subspecies couesi from the country, while noting that specimens from some localities were slightly paler than others.
Interior Mexico
|
Goldman grouped four subspecies of couesi from the interior plateaus of central Mexico together—albiventer, crinitus, aztecus, and regillus. Three of those (albiventer from Jalisco, crinitus from the Distrito Federal
Mexico City
Mexico City is the Federal District , capital of Mexico and seat of the federal powers of the Mexican Union. It is a federal entity within Mexico which is not part of any one of the 31 Mexican states but belongs to the federation as a whole...
, and aztecus from Morelos
Morelos
Morelos officially Estado Libre y Soberano de Morelos is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 33 municipalities and its capital city is Cuernavaca....
) were described by Merriam in 1901, and Goldman had himself described regillus from Michoacán
Michoacán
Michoacán officially Estado Libre y Soberano de Michoacán de Ocampo is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 113 municipalities and its capital city is Morelia...
in 1915. According to Goldman, aztecus is pale and large-toothed, crinitus is large, dark and large-toothed, regillus is large and dark, and albiventer is large and relatively pale.
In their 2009 review of western Mexican Oryzomys, Carleton and Arroyo-Cabrales classified Oryzomys albiventer
Oryzomys albiventer
Oryzomys albiventer is a rodent in the genus Oryzomys of family Cricetidae from interior western Mexico, in the states of Jalisco, Guanajuato, and Michoacán. First described in 1901 as a separate species, it was later lumped under O. couesi and the marsh rice rat until it was reinstated as a...
as a separate species from lowland mexicanus on the basis of clear morphometric
Morphometrics
Morphometrics refers to the quantitative analysis of form, a concept that encompasses size and shape. Morphometric analyses are commonly performed on organisms, and are useful in analyzing their fossil record, the impact of mutations on shape, developmental changes in form, covariances between...
differentiation and offered some comments on the status of crinitus, regillus, and aztecus, including the holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
s of the three forms in their morphometric analyses. The holotypes of regillus and aztecus were at the upper end of the range of variation in their large series of mexicanus from the western lowlands, and crinitus clustered with specimens of O. peninsulae
Oryzomys peninsulae
Oryzomys peninsulae, also known as the Lower California Rice Rat, is a species of rodent from western Mexico. Restricted to the southern tip of the Baja California peninsula, it is a member of the genus Oryzomys of family Cricetidae...
from the tip of the Baja California Peninsula
Baja California Peninsula
The Baja California peninsula , is a peninsula in northwestern Mexico. Its land mass separates the Pacific Ocean from the Gulf of California. The Peninsula extends from Mexicali, Baja California in the north to Cabo San Lucas, Baja California Sur in the south.The total area of the Baja California...
. They suggested that regillus and aztecus may represent no more than robust upland populations of mexicanus, but could not exclude the possibility that they represent a different species. That crinitus, which occurs at over 2000 m (6600 ft) altitude in the Valley of Mexico
Valley of Mexico
The Valley of Mexico is a highlands plateau in central Mexico roughly coterminous with the present-day Distrito Federal and the eastern half of the State of Mexico. Surrounded by mountains and volcanoes, the Valley of Mexico was a centre for several pre-Columbian civilizations, including...
, was the same species as peninsulae from the lowlands of the Baja California Peninsula they could not accept and they recommended further research to determine the relationships of crinitus. A specimen from inland Michoacán has Cytb data characteristic of mexicanus, but Hanson and colleagues did not have data for other interior Mexican Oryzomys.
The holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
of the species Oryzomys fulgens, which Thomas had described in 1893, has no more precise locality than "Mexico", but the Valley of Mexico has been suggested as its origin. It is a large, coarse-furred, bright reddish, long-tailed species with a broad skull with widely spreading zygomatic arches. Goldman wrote that it was similar to crinitus, but more intensely colored, and differed in the form of the interorbital region; he retained it as a separate species pending further investigations. Carleton and Arroyo-Cabrales noted that archival research may yet uncover the precise origin of O. fulgens, which could establish it as an older name for one of the other central Mexican Oryzomys.
Texas to Nicaragua
|
Oryzomys populations from Texas to Nicaragua form a single Cytb clade, within which the average sequence divergence is 1.28%, and Hanson and colleagues proposed that the name Oryzomys couesi be restricted to this clade. These populations correspond to two subspecies recognized by Goldman (O. c. aquaticus and O. c. couesi) and an island form he retained as a species (O. cozumelae). Two other subspecies Goldman recognized, O. c. richmondi and O. c. peragrus, and a third, O. c. pinicola, that was described after Goldman's paper occur in the same region, but have not been studied genetically.
The northernmost populations of Oryzomys couesi, those in southernmost Texas and nearby Tamaulipas
Tamaulipas
Tamaulipas officially Estado Libre y Soberano de Tamaulipas is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 43 municipalities and its capital city is Ciudad Victoria. The capital city was named after Guadalupe Victoria, the...
, Mexico, are classified as the subspecies aquaticus, which was described as a separate species, Oryzomys aquaticus, in 1891. Here the range of O. couesi meets that of the marsh rice rat; in parts of Kenedy, Willacy and Cameron
Cameron County, Texas
Cameron County is the southernmost county located in the U.S. state of Texas. In 2010, its population was 406,220. Its county seat is Brownsville. Cameron was founded in 1848...
counties, Texas, and in far northeastern Tamaulipas, the two are sympatric (occur in the same places). In the contact zone, couesi occurs further inland, while the marsh rice rat lives along the coast. In experimental conditions, the two fail to interbreed and genetic analysis yields no evidence of gene flow
Gene flow
In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...
or hybridization in the wild. Compared to populations further to the south, aquaticus is larger and paler and has a more robust skull. Specimens from Tamaulipas are slightly darker than those from Texas. The Cytb sequences of specimens of aquaticus form a separate group, but cluster among specimens of O. c. couesi from further south.
The form peragrus is known from further south in Mexico, in the Río Verde
Río Verde (San Luis Potosi)
-References:*Atlas of Mexico, 1975 .*The Prentice Hall American World Atlas, 1984.*Rand McNally, The New International Atlas, 1993....
basin of San Luis Potosi
San Luis Potosí
San Luis Potosí officially Estado Libre y Soberano de San Luis Potosí is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 58 municipalities and its capital city is San Luis Potosí....
, the state of Hidalgo, and far northern Veracruz
Veracruz
Veracruz, formally Veracruz de Ignacio de la Llave officially Estado Libre y Soberano de Veracruz de Ignacio de la Llave , is one of the 31 states that, along with the Federal District, comprise the 32 federative entities of Mexico. It is divided in 212 municipalities and its capital city is...
. Late Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
fossils of this form have been found in Cueva de Abre, Tamaulipas. According to Goldman, it is intermediate in color between O. c. aquaticus and O. c. couesi, but has a skull similar to that of aquaticus.
Goldman united populations ranging from northern Veracruz through eastern Mexico, Guatemala, Honduras, and Nicaragua south to far northwestern Costa Rica in the nominate subspecies, Oryzomys couesi couesi. He placed six other names as full synonyms of this form, which has its type locality in Guatemala—Oryzomys jalapae Allen and Chapman, 1897, from Veracruz; Oryzomys jalapae rufinus Merriam, 1901, from Veracruz; Oryzomys teapensis Merriam, 1901, from Tabasco
Tabasco
Tabasco officially Estado Libre y Soberano de Tabasco is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 17 municipalities and its capital city is Villahermosa....
; Oryzomys goldmani Merriam, 1901, from Veracruz; Oryzomys jalapae apatelius Eliot, 1904, from Veracruz; and Oryzomys richardsoni Allen, 1910, from Nicaragua. According to Goldman, individual variation within the subspecies is large, which has led to the large number of published synonyms, but populations from all parts of its range are essentially similar.
The subspecies Oryzomys couesi pinicola was described in 1932 from a pine ridge in western British Honduras (now Belize
Belize
Belize is a constitutional monarchy and the northernmost country in Central America. Belize has a diverse society, comprising many cultures and languages. Even though Kriol and Spanish are spoken among the population, Belize is the only country in Central America where English is the official...
); it is smaller and darker than nominate couesi, which also occurs in Belize, and has a more delicate skull. In 1901, Merriam described the Oryzomys of the island of Cozumel
Cozumel
Cozumel is an island in the Caribbean Sea off the eastern coast of Mexico's Yucatan Peninsula, opposite Playa del Carmen, and close to the Yucatan Channel. Cozumel is one of the ten municipalities of the state of Quintana Roo...
as a separate species, Oryzomys cozumelae, and Goldman kept it as such because of its large size, dark fur, and long tail. In 1965, however, Knox Jones and Timothy Lawlor judged the differences between cozumelae and mainland couesi trivial and found that cozumelae was inside the range of variation of mainland Oryzomys populations; accordingly, they demoted the island form to a subspecies. Mark Engstrom and colleagues, writing in 1989, reaffirmed this conclusion. For an island form, this population is highly genetically variable. In its Cytb sequence data, it falls among populations of nominate couesi. Oryzomys couesi is also found on Turneffe Atoll off the coast of Belize and Roatán
Roatán
Roatán, located between the islands of Útila and Guanaja, is the largest of Honduras' Bay Islands. The island was formerly known as Ruatan and Rattan...
off Honduras.
The Oryzomys of the eastern lowlands of Nicaragua was described as a separate species, Oryzomys richmondi, by Merriam in 1901, and Goldman retained it as a subspecies of O. couesi on the basis of its distinctly dark fur. In reviewing Nicaraguan Oryzomys in 1986, Jones and Engstrom did not keep richmondi as separate, because they thought the difference in color too small for the recognition of subspecies. Oryzomys dimidiatus, a small, dark Oryzomys with gray underparts, occurs with O. couesi in southeastern Nicaragua. According to Jones and Engstrom, rice rats from the island of Ometepe
Ometepe
Ometepe is an island formed by two volcanoes rising from Lake Nicaragua in the Republic of Nicaragua. Its name derives from the Nahuatl words ome and tepetl , meaning two mountains...
in Lake Nicaragua
Lake Nicaragua
Lake Nicaragua or Cocibolca or Granada or is a vast freshwater lake in Nicaragua of tectonic origin. With an area of , it is the largest lake in Central America, the 19th largest lake in the world and the 9th largest in the Americas. It is slightly smaller than Lake Titicaca. With an elevation...
are distinctive in their large skull and small external measurements, with an especially short tail, soft fur that is orange-brown above and buffish below, and lack of sphenopalatine vacuities
Sphenopalatine vacuities
In rodents, sphenopalatine vacuities are perforations of the roof of the mesopterygoid fossa, the open space behind the palate, in between the parapterygoid fossae. They may perforate the presphenoid or basisphenoid bone...
(openings in the roof of the mesopterygoid fossa, the gap behind the end of the bony palate
Palate
The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...
). They considered that this population probably represented a separate subspecies, but declined to propose a new name because they had only one adult specimen. In Nicaragua, O. couesi occurs up to an altitude of 1250 m (4100 ft).
Costa Rica, Panama, and Colombia
|
Oryzomys from Costa Rica have historically been referred to O. c. couesi, but Hanson and colleagues found that two specimens from Refugio Nacional de Vida Silvestre Mixto Maquenque, northeastern Costa Rica, differed as much from other O. couesi (11.93% Cytb sequence divergence) as O. couesi differed from the marsh rice rat (11.30%). They suggested that these animals represented a species distinct from O. couesi, but were unable to resolve the correct name for the species because they could not examine samples of dimidiatus or richmondi.
Oryzomys is rare in Panama. Panamanian Oryzomys were first described by Goldman in 1912, who introduced the name Oryzomys gatunensis for a specimen from Gatún
Gatún
Gatun is a small town on the Atlantic Side of the Panama Canal, located south of the city of Colón at the point in which Gatun Lake meets the channel to the Caribbean Sea...
in the Canal Zone
Panama Canal Zone
The Panama Canal Zone was a unorganized U.S. territory located within the Republic of Panama, consisting of the Panama Canal and an area generally extending 5 miles on each side of the centerline, but excluding Panama City and Colón, which otherwise would have been partly within the limits of...
. In 1918, Goldman kept the animal as a separate species, remarking that it was similar to richmondi, but distinctive in the well-developed ridges along the margins of the interorbital region, the short interparietal bone (part of the roof of the braincase), and the long nasal bone
Nasal bone
The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face, and form, by their junction, "the bridge" of the nose.Each has two surfaces and four borders....
s. In 1937, Bole described another species of Panamanian Oryzomys, Oryzomys azuerensis from Paracoté, Veraguas Province
Veraguas Province
Veraguas is a province of Panama, located in the centre-west of the country. The capital is the city of Santiago de Veraguas. The province covers 10,677.2 km² and is divided into twelve districts.-History:...
. It is a brown form, lacking the reddish tones of nearby populations, and has a broad skull with a short rostrum (front part) and ridges on the interorbital region like those of gatunensis. Although Goldman recommended to him that gatunensis and azuerensis both be treated as subspecies of couesi, Bole described azuerensis as a species because it did not seem intermediate between the geographically closest forms, gatunensis and couesi, and was separated by a large gap from the nearest known populations of O. couesi in northwestern Costa Rica and southeastern Nicaragua. In a 1966 review of Panamanian mammals, Charles Handley reduced both gatunensis and azuerensis to subspecies of the marsh rice rat (in which O. couesi was included at the time), and when O. couesi was reinstated as a separate species these forms went with it. Specimens from near the type locality of azuerensis differ by about 7% in their Cytb sequences from other O. couesi, which suggests that they may represent a separate species. However, Hanson and colleagues did not reinstate azuerensis as a species, because they could not examine samples of gatunensis.
Oryzomys couesi was first reported from Colombia in 1987, when Philip Hershkovitz
Philip Hershkovitz
Philip Hershkovitz was an American mammalogist. Born in Pittsburgh, he attended the Universities of Pittsburgh and Michigan and lived in South America collecting mammals. In 1947, he was appointed a curator at the Field Museum of Natural History in Chicago and he continued to work there until his...
reported on its occurrence at Montería
Montería
Montería is a municipality and city located in Northern Colombia and is the capital of the Department of Córdoba. The city is located away from the Caribbean, by the Sinú River. The city and region are famous for their distinct cultural heritages, which include a blend of indigenous Zenú Indians,...
in Córdoba Department
Córdoba Department
Córdoba is a Department of the Republic of Colombia located to the north of this country in the Colombian Caribbean Region. Córdoba faces to the north with the Caribbean sea, to the northeast with the Sucre Department, east with the Bolívar Department and south with the Antioquia Department...
, northwestern Colombia. The Colombian specimen is ochraceous in color throughout and according to Hershkovitz almost identical to specimens from Guatemala, but distinctive in that the upper lip is white. He suggested that O. couesi may also be discovered in the Pacific lowlands of the Chocó in western Colombia.
Common names
Several common nameCommon name
A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...
s have been proposed for Oryzomys couesi and the synonyms currently associated with it. Eliot in 1905 and Goldman in 1918 gave separate common names for each of the species and subspecies they recognized. Many authors have used "Coues' Rice Rat" or some variation thereof for O. couesi, but "Coues' Oryzomys" has also been used.
Description
| Population | nNumber of specimens measured. | Total length | Tail | Hindfoot |
|---|---|---|---|---|
| Western Mexico to El Salvador | ||||
| San José de Guaymas, Sonora (lambi)Only averages available. | 4 | 227 | 113 | 29 |
| Escuinapa, Sinaloa (mexicanus) | 10 | 251.4 (239–273) | 137.4 (127–165) | 28.9 (27–35) |
| Jalisco (mexicanus) | 58 | 245.7 (195–301) | 132.3 (102–160) | 30.2 (26–34) |
| Nayarit (mexicanus) | 62 | 244.8 (210–288) | 125.1 (105–150) | 30.5 (27–33) |
| Nenton, Guatemala (zygomaticus) | 1 | 290 | 152 | 33 |
| El Salvador | 87 | 190–304 | 109–194 | 25–33 |
| Interior Mexico | ||||
| Morelos (aztecus) | 3 | 307 (297–318) | 161 (154–170) | 34 (33–35) |
| Mexico City (crinitus) | 2 | 307, 280 | 161, 148 | 37, 35 |
| Michoacán (regillus) | 3 | 308 (285–320) | 168 (155–180) | 35 (34–36) |
| Unknown (fulgens) | 1 | 311 | 151 | 37.5With claws. |
| Texas to Nicaragua | ||||
| Brownsville, Texas (aquaticus) | 5 | 297 (283–310) | 161 (138–180) | 34.5 (32–38) |
| Rio Verde, San Luis Potosí (peragrus) | 3 | 281 (265–294) | 157 (143–167) | 34 (33–35) |
| Orizaba, Veracruz (couesi) | 7 | 263 (248–294) | 148 (139–174) | 33.1 (32–34.5) |
| Tumbala, Chiapas (couesi) | 4 | 252 (242–265) | 130 (127–135) | 30.7 (30–31) |
| El Cayo, Belize (pinicola) | 18 | 108 (96–128)Head and body length instead of total length. | 122 (107–146) | 27 (24.6–29) |
| Cozumel (cozumelae) | 6 | 306 (285–327) | 172 (163–177) | 34.3 (33–35.5) |
| Yaruca, Honduras (couesi) | 10 | 267.5 (255–280) | 138 (130–145) | 29.1 (28–32) |
| San Antonio, Nicaragua (couesi) | 25 | 264.9 (242–292) | 135.6 (127–150) | 28.8 (27–31) |
| Southeastern Nicaragua (richmondi) | 10 | 275.8 (255–295) | 137 (124–151) | 30.9 (29–33.5) |
| Isla de Omotepe, Nicaragua | 1 | 260 | 121 | 30 |
| Costa Rica, Panama, and Colombia | ||||
| Azuero, Panama (azuerensis) | 1 | 203 | 107.5 | 30 |
| Gatún, Panama (gatunensis) | 1 | 224 | 115 | 31.5 |
| Measurements are in millimeters and are in the form "average (minimum–maximum)", except where there are only one or two measured specimens. | ||||
Oryzomys couesi is a medium-sized to large rat with coarse fur that is buff to reddish above, becoming paler towards the sides and cheeks and darker on the rump and face. The underparts are white to buff. The fur is shorter, brighter, and more intense in color than in the marsh rice rat. The snout ends bluntly and the moderately large eyes show reddish eyeshine. The small ears are black on the outside and the inside is covered with short, gray to buff or red hairs. The long tail is dark brown above and white to light brown below. The feet are long and stout. On the forefeet, the ungual tuft
Ungual tuft
In mammals, ungual tufts are tufts of hairs at the base of claws of the fore- and hindfeet. Their presence has been used as a character in cladistic studies of Cricetidae....
s (tufts of hair on the digits) are present. Many of the pads on the hindfeet are reduced, as are the ungual tufts, and small interdigital webs
Interdigital webbing
Interdigital webbing is the presence of membranes of skin between the digits. Normally in mammals, webbing is present in the embryo but resorbed later in development, but in various mammal species it occasionally persists in adulthood...
may be present in at least some specimens. Some of these traits are common adaptations to life in the water in oryzomyines. As in most other oryzomyines, females have eight mammae. Head and body length is 98 to 142 mm (3.9 to 5.6 in), tail length is 107 to 152 mm (4.2 to 6.0 in), hindfoot length is 27 to 33 mm (1.1 to 1.3 in), ear length is 13 to 18 mm (0.51 to 0.71 in), and body mass is 43 to 82 grams (1.5 to 2.9 oz). Studies in Texas and El Salvador found that males are slightly larger than females.
The stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
has the characteristic pattern of sigmodontines (unilocular-hemiglandular): it is not split in two chambers by an incisura angularis and the front part (antrum
Antrum
In biology, antrum is a general term for a cavity or chamber which may have specific meaning in reference to certain organs or sites in the body.In vertebrates, it may mean for example:...
) is covered by a glandular epithelium. The gall bladder is absent, a synapomorphy
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
(shared-derived character) of Oryzomyini. The karyotype
Karyotype
A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...
includes 56 chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s and a fundamental number of 56 autosomal
Autosome
An autosome is a chromosome that is not a sex chromosome, or allosome; that is to say, there is an equal number of copies of the chromosome in males and females. For example, in humans, there are 22 pairs of autosomes. In addition to autosomes, there are sex chromosomes, to be specific: X and Y...
arms (2n = 56, FNa = 56). The autosomes include 26 pairs of acrocentric chromosomes, with a long and a very short arm, and one medium-sized submetacentric pair, with one arm shorter than the other. The X chromosome
X chromosome
The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals and is common in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system...
is either acrocentric, with a long and a short arm, or subtelocentric, with a long and a vestigial arm. The form of the sex chromosomes has been used to distinguish the marsh rice rat from Oryzomys couesi, but there are no consistent differences between the two.
As is characteristic of Sigmodontinae, Oryzomys couesi has a complex penis, with the baculum
Baculum
The baculum is a bone found in the penis of most mammals. It is absent in humans, but present in other primates, such as the gorilla and chimpanzee.The bone aids in sexual intercourse.-Purpose:...
(penis bone) ending in three cartilaginous
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
digits at its tip. The outer surface of the penis is mostly covered by small spines, but there is a broad band of nonspinous tissue. The papilla (nipple-like projection) on the dorsal (upper) side of the penis is covered with small spines, a character Oryzomys couesi shares only with Oligoryzomys
Oligoryzomys
Oligoryzomys is a genus of rodents in the tribe Oryzomyini of family Cricetidae. Many species are known as pygmy rice rats or colilargos...
and the marsh rice rat among oryzomyines examined. On the urethral process, located in the crater at the end of the penis, a fleshy process
Process (anatomy)
In anatomy, a process is a projection or outgrowth of tissue from a larger body. The vertebra has several kinds of processes,such as: transverse process, prezygapophysis, postzygapophysis.-Examples:Examples of processes include:...
(the subapical lobule) is present; it is absent in all other oryzomyines with studied penes except the marsh rice rat and Holochilus brasiliensis.
Skull
The nasal and premaxillary bones do not extend back beyond the point where the lacrimalLacrimal bone
The lacrimal bone, the smallest and most fragile bone of the face, is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders.-Lateral or orbital surface:...
, frontal
Frontal bone
The frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....
, and maxillary bones meet. The zygomatic plate
Zygomatic plate
In rodent anatomy, the zygomatic plate is a bony plate derived from the flattened front part of the zygomatic arch . At the back, it connects to the front root of the zygomatic arch, and at the top it is connected to the rest of the skull via the antorbital bridge. It is part of the maxillary...
, the flattened front part of the zygomatic arch, is broad and develops a notch at its front end. The plate's back margin is located before the first upper molar. The jugal bone, part of the zygomatic arch, is reduced, as usual in oryzomyines. The sphenopalatine foramen
Sphenopalatine foramen
The sphenopalatine foramen is a foramen in the skull that connects the nasal cavity with the pterygopalatine fossa.-Structure:The processes of the superior border of the palatine bone are separated by the sphenopalatine notch, which is converted into the sphenopalatine foramen by the under surface...
, a foramen
Foramen
In anatomy, a foramen is any opening. Foramina inside the body of humans and other animals typically allow muscles, nerves, arteries, veins, or other structures to connect one part of the body with another.-Skull:...
(opening) at the side of the skull above the molars, is small; it is much larger in the marsh rice rat. The narrowest part of the interorbital region is towards the front and the edges are lined by prominent shelves. The parietal bone
Parietal bone
The parietal bones are bones in the human skull which, when joined together, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall....
s extend to the sides of the braincase. The interparietal bone is narrow and wedge-shaped, so that the parietal and squamosal bones meet extensively.
The incisive foramina
Incisive foramen
The fossa incisiva is an opening in the bone of the oral hard palate where blood vessels and nerves may pass. There are four of these openings in the incisive fossa.-Formation:...
, openings in the front part of the palate, reach backward between the molars. The palate is long, extending substantially beyond the third molars, the usual condition in oryzomyines. The back part, near the third molars, is usually perforated by prominent posterolateral palatal pits
Posterolateral palatal pits
In anatomy, posterolateral palatal pits are gaps at the sides of the back of the bony palate, near the last molars. Posterolateral palatal pits are present, in various degrees of development, in several members of the rodent family Cricetidae...
, which are recessed into fossae (depressions). Sphenopalatine vacuities are usually absent, but have been reported in some populations. There is no alisphenoid strut
Alisphenoid strut
In some rodents, the alisphenoid strut is an extension of the alisphenoid bone that separates two foramina in the skull, the masticatory–buccinator foramen and the foramen ovale accessorius...
, an extension of the alisphenoid bone that in some oryzomyines separates two foramina in the skull. The condition of the arteries
Artery
Arteries are blood vessels that carry blood away from the heart. This blood is normally oxygenated, exceptions made for the pulmonary and umbilical arteries....
in the head is highly derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
. The subsquamosal fenestra
Subsquamosal fenestra
In some rodents, the subsquamosal fenestra is an opening between two parts of the squamosal bone, at the back of the skull. It can be seen in lateral view. Most Oryzomyini have the fenestra, but some species, including those in the genera Nectomys, Sigmodontomys, and Melanomys among others, lack...
, an opening in the back part of the skull determined by the shape of the squamosal bone, is present. The squamosal lacks a suspensory process that contacts the tegmen tympani, the roof of the tympanic cavity
Tympanic cavity
The tympanic cavity is a small cavity surrounding the bones of the middle ear.It is formed from the tubotympanic recess, an expansion of the first pharyngeal pouch....
, a defining character of oryzomyines. There are some openings in the mastoid bone.
In the mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
(lower jaw), the mental foramen
Mental foramen
The mental foramen is one of two holes located on the anterior surface of the mandible. It permits passage of the mental nerve and vessels. The mental foramen descends slightly in edentulous individuals.- Variations :...
, an opening just before the first molar, opens sidewards, not upwards as in a few other oryzomyines. The upper and lower masseteric ridges, which anchor some of the chewing muscles, join at a point below the first molar and do not extend forward beyond that point. The capsular process
Capsular process
In rodents, the capsular process or projection is a bony capsule that contains the root of the lower incisor. It is visible on the labial side of the mandible as a raising in the bone...
, a raising of the bone of the back of the mandible that houses the back end of the incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
, is large.
Teeth
The dental formula is (one upper and one lower incisor and three upper and three lower molarsMolar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
on each side of the jaws), as usual in muroid rodents
Muroid molar
Rodents of the superfamily Muroidea, which includes mice, rats, voles, hamsters, bamboo rats, and many other species, generally have three molars in each quadrant of the jaws. A few of the oldest species retain the fourth upper premolar, and some living species have lost the third and even the...
. The upper incisors are opisthodont, with the chewing edge located behind the vertical plane of the teeth. The molars are bunodont, with the cusps higher than the connecting crests, and brachydont
Brachydont
Brachydont is a type of dentition characterized by low-crowned teeth, as opposed to high-crowned, hypsodont teeth. Human teeth are brachydont.-External links:*http://animaldiversity.ummz.umich.edu/site/topics/mammal_anatomy/tooth_diversity.html...
, low-crowned, as in most other oryzomyines. Many accessory crests, including the mesoloph on the upper molars and the mesolophid on the lower molars, are present, another trait O. couesi shares with most but not all other oryzomyines. The flexi and flexids (valleys between the cusps and crests) at the labial (outer) side of the molars are closed by cingula
Cingulum (tooth)
In dentistry, cingulum refers to an anatomical feature of the anterior teeth . It refers to the portion of the teeth, occurring on the lingual or palatal aspects, that forms a convex protuberance at the cervical third of the anatomic crown. It represents the lingual or palatal developmental lobe...
(ridges).
On the first and second upper molars, the flexi do not extend to the midline of the molars. The anterocone, the front cusp of the upper first molar, is not divided in two by an indentation at its front (anteromedian flexus). A crest, the anteroloph, is present behind the labial cuspule. As in most oryzomyines, the upper molars all have one root on the inner (lingual) side and two on the outer (labial) side; in addition, the first upper molar usually has another small labial root.
On the first lower molar, the labial and lingual conules of the anteroconid, the frontmost cusp, are separated by an anteromedian fossette. The second lower molar bears a crest, the anterolophid, before the two cusps, the protoconid and metaconid, that form the front edge of the molar in some other oryzomyines. There is a distinct ridge (anterolabial cingulum) at the outer front (anterolabial) edge of the molar, before the protoconid. The third lower molar also bears an anterolophid and an anterolabial cingulum. The first lower molar has large roots at the front and back of the tooth and two smaller ones in between, at the labial and lingual side. The second and third lowers molars have two large roots, one at the front and one at the back.
Postcranial skeleton
As usual in oryzomyines, there are twelve ribs. The first rib articulates with both the last cervical (neck) and first thoracic (chest) vertebrae, a synapomorphy of the Sigmodontinae. Anapophyses, processes at the back of a vertebra, are absent from the fifth lumbar. Between the second and third caudal vertebrae, hemal arches (small bones) are present with a spinous back border. The entepicondylar foramenEntepicondylar foramen
The entepicondylar foramen is an opening in the distal end of the humerus present in some mammals. It is often present in primitive placentals, such as the enigmatic Madagascan Plesiorycteropus...
is absent, as in all members of the Sigmodontinae; if present, as in some other rodents, this foramen perforates the distal (far) end of the humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
(upper arm bone).
Ecology and behavior
The distribution of Oryzomys couesi extends from southern Texas and central SonoraSonora
Sonora officially Estado Libre y Soberano de Sonora is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided into 72 municipalities; the capital city is Hermosillo....
, but not the central plateau of Mexico, through Central America south and east to northwestern Colombia; see under "Taxonomy" for details. The species has also been found in late Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
cave deposits in Mexico and Honduras. It is common in watery habitats, such as marshes and small streams, but also occurs in forests and shrubland
Shrubland
Shrubland, scrubland, scrub or brush is a plant community characterized by vegetation dominated by shrubs, often also including grasses, herbs, and geophytes. Shrubland may either occur naturally or be the result of human activity...
s with sufficient cover. In addition, it is found in sugarcane and rice fields. In Texas, it occurs in marsh vegetation along resacas (oxbow lake
Oxbow lake
An oxbow lake is a U-shaped body of water formed when a wide meander from the main stem of a river is cut off to create a lake. This landform is called an oxbow lake for the distinctive curved shape, named after part of a yoke for oxen. In Australia, an oxbow lake is called a billabong, derived...
s) and in Veracruz, it has even been found on the dry coastal plain
Coastal plain
A coastal plain is an area of flat, low-lying land adjacent to a seacoast and separated from the interior by other features. One of the world's longest coastal plains is located in eastern South America. The southwestern coastal plain of North America is notable for its species diversity...
among shrubs. It occurs from 2300 m (7500 ft) altitude down to sea level. On Cozumel, the proportion of juveniles and females is higher near roads that function as habitat edges. Cozumel rice rats rarely cross roads, which may isolate subpopulations on the island.
Oryzomys couesi lives on the ground and is semiaquatic, spending much time in the water, as Alston in his original description already recognized, but is also a good climber. A study in Costa Rica found that O. couesi is an excellent swimmer, diving well and using its tail to propel itself. It is probably able to forage underwater, which may help differentiate its niche
Niche differentiation
The term niche differentiation , as it applies to the field of ecology, refers to the process by which natural selection drives competing species into different patterns of resource use or different niches...
from that of the ecologically similar cotton rat Sigmodon hirsutus, which also swims well, but does not dive. When disturbed, O. couesi will enter the water and swim away. It is primarily active during the night. Oryzomys couesi builds globular nests of woven vegetation suspended among reeds, about 1 m (3 ft) above the water or the ground; in Texas, larger individuals make larger nests. It does not usually make its own runways in vegetation, but may use those of other rodents, such as cotton rats.
Population densities
Population density
Population density is a measurement of population per unit area or unit volume. It is frequently applied to living organisms, and particularly to humans...
range from 5 to 30 per ha (2 to 12 per acre). On Cozumel, density is around 14.5 to 16.5 per ha (5.9 to 6.7 per acre), but shows large seasonal variation. In western Mexico, one study found densities of 3 per ha (1.2 per acre) in cloud forest
Cloud forest
A cloud forest, also called a fog forest, is a generally tropical or subtropical evergreen montane moist forest characterized by a persistent, frequent or seasonal low-level cloud cover, usually at the canopy level. Cloud forests often exhibit an abundance of mosses covering the ground and...
and 1 per ha (0.4 per acre) in a disturbed area. In 24 hours, male Texas O. couesi move up to 153 m (502 ft) and females up to 126 m (413 ft). The diet includes both plant material, including seeds and green parts, and animals, including small fish, crustaceans, snails, insects like ants and beetles, and other invertebrates. It probably breeds around the year and after a pregnancy of 21 to 28 days, the female produces litters of two to seven young, with an average of 3.8, according to Reid's Mammals of Central America & Southeast Mexico. In 28 pregnant females from Nicaragua, litter size varied from one to eight, averaging 4.4. The young become reproductively active when seven weeks old and the life cycle
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
is short.
The introduced
Introduced species
An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...
snake Boa constrictor
Boa constrictor
The Boa constrictor is a large, heavy-bodied species of snake. It is a member of the family Boidae found in North, Central, and South America, as well as some islands in the Caribbean. A staple of private collections and public displays, its color pattern is highly variable yet distinctive...
preys on O. couesi on Cozumel. Parasites recorded on O. couesi in Veracruz include unidentified ticks, mites, fleas, and fly larvae. The flea Polygenis odiosus was found on an Oryzomys couesi from Cozumel. Out of ten O. couesi in San Luis Potosí, five each were infected by the nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
worms Hassalstrongylus musculi
Hassalstrongylus musculi
Hassalstrongylus musculi is a nematode worm of the genus Hassalstrongylus that infects the marsh rice rat and house mouse in the United States and Oryzomys couesi, Oligoryzomys fulvescens, and Handleyomys melanotis in San Luis Potosí, Mexico...
and H. bocqueti, with about 25 worms per rat, and two were infected by one or two cestodes of the genus Raillietina
Raillietina
Raillietina is the name a genus of tapeworms that includes helminth parasites of vertebrates, and mostly of birds. The genus was named in 1920 in honour of a French veterinarian and helminthologist, Louis-Joseph Alcide Railliet. Of the 37 species recorded under the genus, Raillietina demerariensis,...
. The mite
Mite
Mites, along with ticks, are small arthropods belonging to the subclass Acari and the class Arachnida. The scientific discipline devoted to the study of ticks and mites is called acarology.-Diversity and systematics:...
s Eubrachylaelaps circularis and Gigantolaelaps boneti have been found on Oryzomys couesi in Oaxaca, the sucking louse
Sucking louse
Sucking lice have around 500 species and represent the smaller of the two traditional suborders of lice. As opposed to the paraphyletic chewing lice, which are now divided among three suborders, the sucking lice are monophyletic.The Anoplura are all blood-feeding ectoparasites of mammals...
Hoplopleura oryzomydis
Hoplopleura oryzomydis
Hoplopleura oryzomydis is a sucking louse that is known from the southern United States, Nicaragua, Panama, and Venezuela. It is known from several oryzomyine rodents: the marsh rice rat , O. couesi, Handleyomys alfaroi, Transandinomys talamancae, Melanomys caliginosus, and Sigmodontomys...
in Nicaragua, the mites Laelaps oryzomydis, Echinonyssus microchelae, Ornithonyssus bacoti, Prolistrophorus frontalis, and Prolistrophorus bakeri
Prolistrophorus bakeri
Prolistrophorus bakeri is a parasitic mite in the genus Prolistrophorus. Together with the Argentinean P. hirstianus, it forms the subgenus Beprolistrophorus...
in Colima, and the apicomplexa
Apicomplexa
The Apicomplexa are a large group of protists, most of which possess a unique organelle called apicoplast and an apical complex structure involved in penetrating a host's cell. They are unicellular, spore-forming, and exclusively parasites of animals. Motile structures such as flagella or...
n Eimeria couesii
Eimeria couesii
Eimeria couesii is an apicomplexan parasite of the genus Eimeria that infects the intestine of the rice rat Oryzomys couesi in Mexico. It has an oocyst residuum and Stieda bodies, structures that are absent in some other Eimeria....
in Mexico. The species is infected by two hantavirus
Hantavirus
Hantaviruses are negative sense RNA viruses in the Bunyaviridae family. Humans may be infected with hantaviruses through rodent bites, urine, saliva or contact with rodent waste products...
es—Catacamas virus in Honduras and Playa de Oro virus
Playa de Oro virus
Playa de Oro virus is a probable species of hantavirus found in the rodents Oryzomys couesi and Sigmodon mascotensis in the Mexican state of Colima. The former is thought to be the main host...
in western Mexico—which are related to the Bayou virus infecting the marsh rice rat, a common cause of hantavirus infections in the United States. No hantavirus infections in humans have been linked to O. couesi hantaviruses, however. Chiapas O. couesi easily survive experimental infection with several arbovirus
Arbovirus
Arbovirus is a term used to refer to a group of viruses that are transmitted by arthropod vectors. The word arbovirus is an acronym . Some arboviruses are able to cause emergent disease.-Transmission:...
es, including the Venezuelan equine encephalitis virus
Venezuelan equine encephalitis virus
Venezuelan equine encephalitis virus is a mosquito-borne viral pathogen that causes Venezuelan equine encephalitis or encephalomyelitis . VEE can affect all equine species, such as horses, donkeys, and zebras. After infection, equines may suddenly die or show progressive central nervous system...
, suggesting that the species may serve as a reservoir
Natural reservoir
Natural reservoir or nidus, refers to the long-term host of the pathogen of an infectious disease. It is often the case that hosts do not get the disease carried by the pathogen or it is carried as a subclinical infection and so asymptomatic and non-lethal...
for that virus.
Conservation status
The IUCN lists Oryzomys couesi as "Least ConcernLeast Concern
Least Concern is an IUCN category assigned to extant taxon or lower taxa which have been evaluated but do not qualify for any other category. As such they do not qualify as threatened, Near Threatened, or Conservation Dependent...
", because it is a widely distributed, common species with broad habitat tolerance that occurs in many protected area
Protected area
Protected areas are locations which receive protection because of their recognised natural, ecological and/or cultural values. There are several kinds of protected areas, which vary by level of protection depending on the enabling laws of each country or the regulations of the international...
s. Habitat destruction
Habitat destruction
Habitat destruction is the process in which natural habitat is rendered functionally unable to support the species present. In this process, the organisms that previously used the site are displaced or destroyed, reducing biodiversity. Habitat destruction by human activity mainly for the purpose of...
, such as drainage of wetlands, may threaten some populations. In many areas, it is so common that it is considered a plague species. Populations even persist in the Valley of Mexico, as evidenced by a photograph published in 2006. However, it is listed as threatened in Texas, where its distribution is very limited, because of habitat loss. In 1979, Benson and Gehlbach estimated the size of the Texas population to be about 15,000. A 2001 study predicted that climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...
would drive the Texas population to extinction, because no suitable habitats would continue to exist. The Cozumel population has declined substantially since the mid 1980s, perhaps due to habitat disturbance and predation by introduced species.
Literature cited
- Allen, J.A. 1891. Notes on new or little-known North American mammals, based on recent additions to the collection of mammals in the American Museum of Natural History. Bulletin of the American Museum of Natural History 3:263–310.
- Allen, J.A. 1897. Further notes on mammals collected in Mexico by Dr. Audley C. Buller, with descriptions of new species. Bulletin of the American Museum of Natural History 9:47–57.
- Allen, J.A. 1910. Additional mammals from Nicaragua. Bulletin of the American Museum of Natural History 28:87–115.
- Allen, J.A. and Chapman, F.M. 1897. On a collection of mammals from Jalapa and Las Vigas, state of Vera Cruz, Mexico. Bulletin of the American Museum of Natural History 9:197–208.
- Alston, E.R. 1877 ["1876"]. On two new species of Hesperomys. Proceedings of the Zoological Society of London 1876(4):755–757.
- Alston, E.R. 1882. Biologia centrali-americana. Mammalia. R.H. Porter, 220 pp.
- Baker, R.H. 1951. Mammals from Tamaulipas, Mexico. University of Kansas Publications, Museum of Natural History 5:207–215.
- Barnard, W.P., Ernst, J.V. and Stevens, R.O. 1971. Eimeria palustris sp. n. and Isospora hammondi sp. n. (Coccidia: Eimeriidae) from the marsh rice rat, Oryzomys palustris (Harlan) (subscription required). The Journal of Parasitology 57(6):1293–1296.
- Benson, D.E. and Gehlbach, F.R. 1979. Ecological and taxonomic notes on the rice rat (Oryzomys couesi) in Texas (subscription required). Journal of Mammalogy 60(1):225–228.
- Bole, B.J., Jr. 1937. Annotated list of the mammals of the Mariato River District of the Azuero Peninsula. Scientific Publications of the Cleveland Museum of Natural History 7:140–196.
- Burt, W.H. 1934. A new rice rat (Oryzomys) from Sonora, Mexico. Proceedings of the Biological Society of Washington 47:107–108.
- Burt, W.H. and Stirton, R.A. 1961. The mammals of El Salvador. Miscellaneous Publications of the Museum of Zoology, University of Michigan 117:1–69.
- Cameron, G.N. and Scheel, D. Getting warmer: Effect of global climate change on distribution of rodents in Texas (subscription required). Journal of Mammalogy 82(3):652–680.
- Carleton, M.D. and Arroyo-Cabrales, J. 2009. Review of the Oryzomys couesi complex (Rodentia: Cricetidae: Sigmodontinae) in Western Mexico. Bulletin of the American Museum of Natural History 331:94–127.
- Carleton, M.D. and Musser, G.G. 1984. Muroid rodents. Pp. 289–379 in Anderson. S. and Jones, J.K., Jr. (eds.). Orders and families of Recent mammals of the world. John Wiley and Sons, New York, 686 pp.
- Chu, Y.-K., Owen, R.D., Sánchez-Hernández, C., Romero-Almarez, M. de L. and Jonsson, C.B. 2008. Genetic characterization and phylogeny of a hantavirus from Western Mexico (subscription required). Virus Research 131:180–188.
- Cook, W.M., Timm, R.M. and Hyman, D.E. 2001. Swimming ability in three Costa Rican dry forest rodents. Revista de Biologia Tropical 49(3–4):1177–1181.
- Dalquest, W.W. and Roth, E. 1970. Late Pleistocene mammals from a cave in Tamaulipas, Mexico (subscription required). The Southwestern Naturalist 15(2):217–230.
- Deardorff, E.R., Forrester, N.L., Travassos da Rosa, A.P., Estrada-Franco, J.G., Navarro-Lopez, R., Tesh, R.B. and Weaver, S.C. 2009. Experimental infection of potential reservoir hosts with Venezuelan equine encephalitis virus, Mexico. Emerging Infectious Diseases 15(4):519–525.
- Deardorff, E.R., Forrester, N.L., Travassos da Rosa, A.P., Estrada-Franco, J.G., Navarro-Lopez, R., Tesh, R.B. and Weaver, S.C. 2010. Short report: Experimental infections of Oryzomys couesi with sympatric arboviruses from Mexico. American Journal of Tropical Medicine and Hygiene 82(2):350–353.
- Dickinson, E.C. 2005. The Proceedings of the Zoological Society of London, 1859–1900: an exploration of breaks between calendar years of publication (subscription required). Journal of Zoology 266(4):427–430.
- Duff, A. and Lawson, A. 2004. Mammals of the World: A checklist. New Haven: A & C Black, 312 pp. ISBN 0-7136-6021-X
- Eckerlin, R.P. 2005. Fleas (Siphonaptera) of the Yucatan Peninsula (Campeche, Quintana Roo, and Yucatan), Mexico. Caribbean Journal of Science 41(1):152–157.
- Eliot, D.G. 1904. Descriptions of apparently new species and subspecies of mammals and a new generic name proposed. Field Columbian Museum Zoological Series 3:263–270.
- Eliot, D.G. 1905. A checklist of mammals of the North American continent, the West Indies and the neighboring seas. Field Columbian Museum Zoological Series 6:1–761.
- Emerson, K.C. 1971. Records of a collection of Mallophaga and Anoplura from Nicaraguan mammals (subscription required). Journal of the Kansas Entomological Society 44(3):332–334.
- Engstrom, M.D., Schmidt, C.A., Morales, J.C. and Dowler, R.C. 1989. Records of mammals from Isla Cozumel, Quintana Roo, Mexico (subscription required). The Southwestern Naturalist 34(3):413–415.
- Estébanes-González, M.L. and Cervantes, F.A. 2005. Mites and ticks associated with some small mammals in Mexico (subscription required). International Journal of Acarology 31(1):23–37.
- Estébanes-González, M.L., Sánchez-Hernández, C., Romero-Almaraz, M. de L. and Schnell, G.D. 2011. Ácaros parásitos de roedores de Playa de Oro, Colima, México. Acta Zoológica Mexicana 27(1):169–176 (in Spanish).
- Fuentes-Montemayor, E., Cuarón, A.D., Vázquez-Domínguez, E., Benítez-Malvido, J., Valenzuela-Galván, D. and Andresen, E. 2009. Living on the edge: roads and edge effects on small mammal populations (subscription required). Journal of Animal Ecology 78:857–865.
- Genoways, H.H. and Jones, J.K., Jr. 1971. Second specimen of Oryzomys dimidiatus. Journal of Mammalogy 52:833–834.
- Goldman, E.A. 1912. Descriptions of twelve new species and subspecies of mammals from Panama. Smithsonian Miscellaneous Collections 56(36):1–11.
- Goldman, E.A. 1915. Five new rice rats of the genus Oryzomys from Middle America. Proceedings of the Biological Society of Washington 27:127–130.
- Goldman, E.A. 1918. The rice rats of North America. North American Fauna 43:1–100.
- Haiduk, M.W., Bickham, J.W. and Schmidly, D.J. 1979. Karyotypes of six species of Oryzomys from Mexico and Central America (subscription required). Journal of Mammalogy 60(3):610–615.
- Hall, E.R. 1960. Oryzomys couesi only subspecifically different from the marsh rice rat, Oryzomys palustris (subscription required). The Southwestern Naturalist 5(3):171–173.
- Hall, E.R. and Dalquest, W.W. 1963. The mammals of Veracruz. University of Kansas Publications, Museum of Natural History 14:165–362.
- Handley, C.O., Jr. 1966. Checklist of the mammals of Panama. Pp. 753–795 in Wenzel, R.L. and Tipton, V.J. (eds.). Ectoparasites of Panama. Chicago: Field Museum of Natural History, xii + 861 pp.
- Hanson, J.D., Indorf, J.L., Swier, V.J. and Bradley, R.D. 2010. Molecular divergence within the Oryzomys palustris complex: evidence for multiple species (abstract only). Journal of Mammalogy 91(2):336–347.
- Harris, W.P., Jr. 1943. A list of mammals from Costa Rica. Occasional Papers of the Museum of Zoology, University of Michigan 476:1–15.
- Hershkovitz, P. 1987. First South American record of Coues' marsh rice rat, Oryzomys couesi (subscription required). Journal of Mammalogy 68(1):152–154.
- Hooper, E.T. and Musser, G.G. 1964. The glans penis in Neotropical cricetines (Family Muridae) with comments on classification of muroid rodents. Miscellaneous Publications of the University of Michigan Museum of Zoology 123:1–57.
- Jones, J.K., Jr. and Engstrom, M.D. 1986. Synopsis of the rice rats (genus Oryzomys) of Nicaragua. Occasional Papers, The Museum, Texas Tech University 103:1–23.
- Jones, J.K., Jr. and Lawlor, T.E. 1965. Mammals from Isla Cozumel, México, with description of a new species of harvest mouse. University of Kansas Publications, Museum of Natural History 16:409–419.
- Jones, J.K., Jr., Carter, D.C. and Webster, W.D. 1983. Records of mammals from Hidalgo, Mexico (subscription required). The Southwestern Naturalist 28(3):378–380.
- Kays, R.W. and Wilson, D.E. 2000. Mammals of North America. Princeton and Oxford: Princeton University Press, 240 pp. ISBN 0-691-07012-1
- Koopman, K.F. 1959. The zoogeographical limits of the West Indies (subscription required). Journal of Mammalogy 40(2):236–240.
- Linzey, A.V., Timm, R., Woodman, N., Matson, J. and Samudio, R. 2008. . In IUCN. IUCN Red List of Threatened Species. Version 2009.2. <www.iucnredlist.org>. Downloaded on December 8, 2009.
- Medellín, X.L. and Medellín, R.A. 2006. Oryzomys couesi (Alston, 1877). Pp. 709–710 in Ceballos, G. and Oliva, G. (eds.). Los mamíferos silvestres de México. Mexico City: Comisión Nacional para el Conocimiento y Uso de la Biodiversidad and Fondo de Cultura Económica, 986 pp. ISBN 978-970-9000-30-6
- Merriam, C.H. 1901a. Six new mammals from Cozumel Island, Yucatan. Proceedings of the Biological Society of Washington 14:99–104.
- Merriam, C.H. 1901b. Synopsis of the rice rats (genus Oryzomys) of the United States and Mexico. Proceedings of the Washington Academy of Sciences 3:273–295.
- Milazzo, M.L., Cajimat, M.N., Hanson, J.D., Bradley, R.D., Quintana, M., Sherman, C., Velásquez, R.T. and Fulhorst, C.F. 2006. Catacamas virus, a hantaviral species naturally associated with Oryzomys couesi (Coues' oryzomys) in Honduras. American Journal of Tropical Medicine and Hygiene 75(5):1003–1010.
- Murie, A. 1932. A new Oryzomys from a pine ridge in British Honduras. Occasional Papers of the Museum of Zoology, University of Michigan 245:1–3.
- Murie, A. 1935. Mammals from Guatemala and British Honduras. Miscellaneous Publications of the Museum of Zoology, University of Michigan 26:1–30.
- Musser, G.G. and Carleton, M.D. 2005. Superfamily Muroidea. Pp. 894–1531 in Wilson, D.E. and Reeder, D.M. (eds.). Mammal Species of the World: a taxonomic and geographic reference. 3rd ed. Baltimore: The Johns Hopkins University Press, 2 vols., 2142 pp. ISBN 978-0-8018-8221-0
- Platt, S.G., Rainwater, T.R., Miller, B.W. and Miller, C.M. 2000. Notes on the mammals of Turneffe Atoll, Belize. Caribbean Journal of Science 36(1–2):166–168.
- Poole, A.J. and Schantz, V.S. 1942. Catalog of the type specimens of mammals in the United States National Museum, including the Biological Surveys collection. Bulletin of the United States National Museum 178:1–705.
- Reid, F. 2006. A Field Guide to Mammals of North America, North of Mexico. 4th ed. Houghton Mifflin Harcourt, 579 pp. ISBN 978-0-395-93596-5
- Reid, F. 2009. A Field Guide to the Mammals of Central America and Southeast Mexico. 2nd edition. Oxford University Press US, 346 pp. ISBN 978-0-19-534322-9
- Sánchez H., J., Ochoa G., J. and Voss, R.S. 2001. Rediscovery of Oryzomys gorgasi (Rodentia: Muridae) with notes on taxonomy and natural history (subscription required). Mammalia 65:205–214.
- Schmidly, D.J. and Davis, W.B. 2004. The mammals of Texas. 2nd edition. University of Texas Press, 501 pp. ISBN 978-0-292-70241-7
- Schmidt, C.A. and Engstrom, M.D. 1994. Genic variation and systematics of rice rats (Oryzomys palustris species group) in southern Texas and northeastern Tamaulipas, Mexico (subscription required). Journal of Mammalogy 75(4):914–928.
- Thomas, O. 1893. Notes on some Mexican Oryzomys. Annals and Magazine of Natural History (6)11:402–405.
- Underwood, H.T., Owen, J.G. and Engstrom, M.D. 1986. Endohelminths of three species of Oryzomys (Rodentia: Cricetidae) from San Luis Potosi, Mexico (subscription required). The Southwestern Naturalist 31(3):410–411.
- Vázquez, L.B., Medellín, R.A. and Cameron, G.N. 2000. Population and community ecology of small rodents in montane forest of western Mexico (subscription required). Journal of Mammalogy 81(1):77–85.
- Vega, R., Vázquez-Domínguez, E., Mejía-Puente, A. and Cuaro, A.D. 2004. Unexpected high levels of genetic variability and the population structure of an island endemic rodent (Oryzomys couesi cozumelae) (subscription required). Biological Conservation 137:210–222.
- Weksler, M. 2006. Phylogenetic relationships of oryzomyine rodents (Muroidea: Sigmodontinae): separate and combined analyses of morphological and molecular data. Bulletin of the American Museum of Natural History 296:1–149.
- Weksler, M., Percequillo, A.R. and Voss, R.S. 2006. Ten new genera of oryzomyine rodents (Cricetidae: Sigmodontinae). American Museum Novitates 3537:1–29.
- Woodman, N. 1995. Morphological variation between Pleistocene and Recent samples of Cryptotis (Insectivora: Soricidae) from the Yucatan Peninsula, Mexico (subscription required). Journal of Mammalogy 76(1):223–231.
- Woodman, N. and Croft, D. 2005. Fossil shrews from Honduras and their significance for late glacial evolution in body size (Mammalia: Soricidae: Cryptotis). Fieldiana Geology 51:1–30.