Oryzomys
Encyclopedia
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
(O. palustris) of the United States and O. couesi of Mexico and Central America—are widespread; the six others have more restricted distributions. The species have had eventful taxonomic
histories, and most species were at one time included in the marsh rice rat; additional species may be recognized in the future. The name Oryzomys was established in 1857 by Spencer Fullerton Baird
for the marsh rice rat and was soon applied to over a hundred species of American rodents. Subsequently, the genus gradually became more narrowly defined until its current contents were established in 2006, when ten new genera were established for species previously placed in Oryzomys.
Species of Oryzomys are medium-sized rats with long, coarse fur. The upperparts are gray to reddish and the underparts white to buff. The animals have broad feet without complete ungual tufts of hair around the claws and, in at least some species, with webbing between the toes
. The rostrum (front part of the skull) is broad and the braincase is high. Both the marsh rice rat and O. couesi have 56 chromosome
s, lack a gall bladder, and have a complex penis (as is characteristic of the Sigmodontinae) with some traits that are rare among oryzomyines; these characteristics are unknown in the other species of this genus.
The habitat includes various kinds of wetlands, such as lakes, marshes, and rivers. Oryzomys species swim well, are active during the night, and eat both plant and animal food. They build woven nests of vegetation. After a gestation period
of 21 to 28 days, about four young are born. Species of Oryzomys are infected by numerous parasites and carry at least three hantavirus
es, one of which (Bayou virus) also infects humans. Two, perhaps three, species have gone extinct over the last two centuries and at least one other is endangered, but the widespread marsh rice rat and O. couesi are not threatened.
, a diverse group of well over a hundred species, many of which were formerly also included in Oryzomys. Oryzomyini is one of several tribes within the subfamily Sigmodontinae
of the family Cricetidae
, which includes hundreds of other species of mainly small rodents, distributed mainly in the Americas and Eurasia.
Within Oryzomyini, a 2006 phylogenetic
analysis by Marcelo Weksler which used both morphological and DNA sequence
data found some evidence that Oryzomys is most closely related to a group including Holochilus
, Lundomys, and Pseudoryzomys. Although analyses based on morphological and combined data supported this relationship, sequences of the Rbp3
gene alone instead placed Oryzomys among a group that included Nectomys
, Sigmodontomys
, and a few other genera. In all analyses, Oryzomys appeared within clade D of Oryzomyini. The relationship between Oryzomys and the Holochilus group was supported by five synapomorphies
(shared derived characters)—absence or reduction of both the hypothenar and interdigital pads; reduction of ungual tuft
s of hairs surrounding the claws; having the back margin of the zygomatic plate
of the skull at the same level as the front of the first upper molar
; and the anterocone (front cusp) of the first upper molar divided by an anteromedian fossette. The first three are adaptations to the semiaquatic lifestyle that Oryzomys and the members of the Holochilus group share, and may thus be examples of convergent evolution
.
for the marsh rice rat
(now Oryzomys palustris) of the eastern United States, which had been first described twenty years earlier by Richard Harlan
. The name combines the Greek oryza "rice" and mys "mouse" and refers to the feeding habits of the marsh rice rat. Baird placed Oryzomys as a subgenus
of the now-defunct genus Hesperomys
and included only the marsh rice rat in it, a classification which was followed by Elliott Coues
in 1874 and 1877. In 1890, Oryzomys was raised to generic rank, and in subsequent years numerous additional species were ascribed to it, many of which were soon moved to separate genera. In the 1898 Catalogus Mammalium, Édouard Louis Trouessart
listed 67 species of Oryzomys, including some that are now placed in Calomys, Necromys
, Thomasomys
, and other genera unrelated to Oryzomys. Some of the new genera proposed were soon subsumed in Oryzomys again, and in The Families and Genera of Living Rodents (1941), John Ellerman
listed Microryzomys
, Oligoryzomys
, Melanomys
, Nesoryzomys
, and Oecomys
as synonym
s of Oryzomys and included about 127 species in it. In 1948, Philip Hershkovitz
suggested that other oryzomyines like Nectomys
and Megalomys
could as well be included in Oryzomys, and Clayton Ray followed this suggestion in 1962.
Hershkovitz and Ray's classification was never widely followed, and from 1976 on authors started to reinstate some of the other groups lumped in Oryzomys as separate genera. The genus was reduced to 43 species (out of 110 in Oryzomyini) in the third edition (2005) of Mammal Species of the World, but it was still not a natural, monophyletic
group; rather, it mostly united those oryzomyines that lacked the conspicuous specializations of other genera. In 2006, Marcelo Weksler's comprehensive phylogenetic analysis produced further evidence that the genus was polyphyletic
, as species of Oryzomys were dispersed all over the oryzomyine tree. He proposed that eleven new genera should be created to accommodate those species that were not closely related to the type species
of Oryzomys, the marsh rice rat; he considered other options that would require fewer new genera, but argued that that would result in less meaningful genus-level groups in Oryzomyini. Later in the same year, Weksler, Percequillo, and Voss created ten new genera—Aegialomys
, Cerradomys
, Eremoryzomys, Euryoryzomys
, Hylaeamys
, Mindomys, Nephelomys
, Oreoryzomys, Sooretamys, and Transandinomys
—for species formerly placed in Oryzomys and placed six more species related to "Oryzomys" alfaroi in Handleyomys
pending the description of more new genera for them. They left only five species in Oryzomys, which was now finally a natural, monophyletic group. Because of subsequent taxonomic work, the number of species has since increased to at least eight.
Some problems remain: ?Oryzomys pliocaenicus
, a Miocene
fossil from Kansas, is of uncertain identity but may belong in Bensonomys, and the identity of Oryzomys anoblepas
, a Brazilian subfossil, is yet to be determined. Fossils from the Miocene of Oregon and Pliocene
of New Mexico have also been ascribed to Oryzomys, but probably incorrectly. A possible Oryzomys has been recorded from the Irvingtonian
(Pleistocene) of Saskatchewan
.
from Colombia, in 1970 and the next year he noted that O. dimidiatus, previously classified as a Nectomys, was similar to O. palustris. After 1979, the marsh rice rat and O. couesi were again regarded as separate as a result of further work in Texas, where their ranges meet. While reviewing O. gorgasi in 2001, J. Sánchez H. and colleagues redefined and characterized the O. palustris group and listed O. couesi, O. dimidiatus, O. gorgasi, and the marsh rice rat as its members; Guy Musser
and Michael Carleton in the 2005 third edition of Mammal Species of the World additionally listed O. nelsoni from María Madre Island in western Mexico.
In 2006, Weksler and colleagues followed the 2001 definition by Sánchez and others for the restricted genus Oryzomys, but added O. antillarum from Jamaica
as a species. Carleton and Joaquin Arroyo-Cabrales reviewed Oryzomys from western Mexico in 2009 and in this context provided an extended diagnosis of Oryzomys. They recognized eight species: the six previously mentioned plus O. albiventer
and O. peninsulae
. Also in 2009, Robert Voss and Weksler identified the subfossil
Oryzomys curasoae from Curaçao
as an island population of O. gorgasi. The next year, Delton Hanson and colleagues published a study using DNA sequence data from the cytochrome b
, interphotoreceptor retinoid-binding protein, and alcohol dehydrogenase
1 genes to assess relationships within Oryzomys. They recommended that the marsh rice rat be split into two species and that O. couesi be split into four species on the basis of the observed sequence divergence and other data.
Merriam divided his palustris-mexicanus group in two "series" according to the color of the underparts (white or fulvous). Goldman divided his palustris group in two "sections"—a couesi section with O. couesi and six related species, and a palustris section with O. palustris only. He noted that the latter differed from the former in the generally darker, more brownish, longer fur, and larger sphenopalatine vacuities
(openings in the mesopterygoid fossa, the gap behind the end of the palate
). As Weksler's 2006 analysis included only O. couesi and the marsh rice rat among species of Oryzomys in the strict sense, he could not test those groups. Carleton and Arroyo-Cabrales concurred with Goldman's division, listing additional characters, and noted that the palustris group may be more semiaquatically adapted than the members of the couesi group are. In the latter, the fur is usually reddish-brown, as opposed to grayish-brown in the palustris group. Members of the couesi group have smaller sphenopalatine vacuities and a smaller sphenopalatine foramen
, a foramen
(opening) in the side of the skull above the molars, and a more highly developed anterolabial cingulum on the third lower molar (a crest at the front of the tooth). The hypothenar pad of the hindfoot, located on the sole far from the fingers, is present in the couesi group, but absent in the palustris group. Interdigital webbing
may be more highly developed in the palustris group. Using morphological data, Voss and Weksler found a closer relationship between O. couesi and O. gorgasi to the exclusion of O. palustris, but with low confidence. The DNA sequence data of Hanson and colleagues supported a deep separation between the palustris and couesi groups, but a Costa Rican sample (assigned to O. couesi) was about as distant from the two groups as they were from each other.
The genus currently includes the following species:
Oryzomys contains medium-sized, semi-aquatically specialized oryzomyine rodents. They have long, coarse fur that is grayish to reddish on the upperparts and white to buff on the underparts. The marsh rice rat superficially resembles the introduced species
black rat
and brown rat
, but has larger differences in color between the upper- and underparts. The vibrissae (whiskers) are short and the ears are small and well-haired. The tail is usually as long as or longer than the head and body and is sparsely haired, but the hairs on the lower side are longer than those above. Females have eight mammae, as in most oryzomyines. The hindfeet are broad and have the first and fifth digits notably shorter than the middle three. The upper surface is hairy, but the underside is naked and covered with small irregularities (squamae
). The pads are generally poorly developed, as are the ungual tufts. Interdigital webbing
may be present, but its development is variable within the genus.
The karyotype
has been recorded in various populations of the marsh rice rat and O. couesi and is apparently stable within the genus at 56 chromosome
s, with the fundamental number of chromosomal arms ranging from 56 to 60 (2n = 56, FN = 56–60). In both species, 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. Furthermore, the gall bladder is absent, a synapomorphy of Oryzomyini.
Oryzomys species have a large skull with a short rostrum and high braincase. The interorbital region
, located between the eyes, is narrowest to the front and is flanked by well-developed beads at its margins. The zygomatic plate
is broad and has a well-developed zygomatic notch at its front. The zygomatic arch is robust and contains a small but distinct jugal bone. The interparietal bone, part of the roof of the braincase, is narrow and short; its narrowness is a synapomorphy for O. couesi plus the marsh rice rat according to Weksler's analysis. The incisive foramina are long, with their back margin at the front of the first molars or further back. The palate is also long, extending beyond the back margin of the maxilla
ry bone, and is perforated near the third molars by well-developed posterolateral palatal pits
. There is no alisphenoid strut
, an extension of the alisphenoid bone that in some other oryzomyines separates two foramina in the skull. The auditory bulla
e are large. The condition of the arteries
in the head is highly derived
. In the mandible
(lower jaw), the coronoid process
, a process
at the back, is well developed and the capsular process
, a raising of the mandibular bone housing the root of the lower incisor, is conspicuous.
As usual in oryzomyines, the molars are pentalophodont (have the mesolophs and mesolophids, accessory crests, well developed) and bunodont, with the cusps higher than the connecting crests. The cusps on the upper molars are arranged in two longitudinal series, not three as in the black and brown rats. The front cusps of the first upper and lower molar (anterocone and anteroconid, respectively) are broad and not divided completely by an anteromedian flexus or flexid. Behind the anterocone, the anteroloph (a smaller crest) is complete and separated from the anterocone. On both the second and third lower molars, the anterolophid (a crest on the inner front corner) is present, a putative synapomorphy of the genus. The first molars have additional small roots in addition to the main ones, so that the upper first molar has four and the lower has three or four roots.
As is characteristic of Sigmodontinae, the marsh rice rat and O. couesi have a complex penis, with the baculum
(penis bone) displaying large protuberances at the sides. 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 these two species share only with Oligoryzomys
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 Holochilus brasiliensis. Both traits are recovered as synapomorphies of O. couesi plus the marsh rice rat in Weksler's analysis.
in the eastern United States through Mexico and Central America south to northwestern Colombia and east to northwestern Venezuela and Curaçao
. Species of Oryzomys usually live in wet habitats such as marshes, streams, and mangrove
s, but both the marsh rice rat and O. couesi are also occasionally encountered in drier habitats. They occur or occurred on many continental-shelf
islands and one oceanic island, Jamaica
; their adeptness at colonizing islands may be caused by their close association with water and frequent occurrence in coastal wetlands. The oldest fossils date to the Rancholabrean
of the United States, about 300,000 years ago; although there have been some earlier North American records, those are not in fact referable to Oryzomys or even Oryzomyini. Oryzomyines likely evolved in South America east of the Andes; the presence of Oryzomys in Central America and other trans-Andean regions is thought to be the result of one of several independent invasions of this region by oryzomyines. Alternatively, Oryzomys may have evolved from the Pliocene
North American Jacobsomys. O. antillarum may have reached Jamaica during the last glacial period while sea levels were low.
Behavior is known mainly from the marsh rice rat and O. couesi, with some scattered data from the other species. Oryzomys are semiaquatic, spending much time in the water, and otherwise mainly live on the ground; both the marsh rice rat and O. couesi are known to be excellent swimmers and will flee into the water when disturbed. Both are also active during the night and build nests of interwoven vegetation, which may be suspended above the water. Breeding may occur throughout the year in both species, but is known to be seasonally variable in the marsh rice rat. In both, gestation takes about 21 to 28 days and litter size is usually one to seven, averaging three to five. Young marsh rice rats and O. couesi become reproductively active when about 50 days old.
The marsh rice rat, O. couesi, and O. gorgasi are known to be omnivore
s, eating both plant and animal material. They eat both seeds and green plant parts and consume a variety of animals, including insect
s, crustacean
s, and many others. The barn owl
(Tyto alba) is a major predator on the marsh rice rat and remains of O. antillarum, O. couesi, and O. gorgasi have been found in owl pellet deposits. Several other animals are known to prey on Oryzomys. A variety of parasites are known from O. couesi and the marsh rice rat and two parasitic nematode
s have been found in O. gorgasi.
such as the small Asian mongoose and the brown
and black rat
. These same causes may threaten O. gorgasi, which the IUCN Red List
assesses as "Endangered". O. albiventer has been affected by human alteration of its habitat, but likely still survives. In contrast, the widespread species, the marsh rice rat and O. couesi, are common and of no conservation concern—indeed, both have been considered a pest—but some populations are threatened. Like these two species, O. dimidiatus is assessed as "Least Concern
" by the Red List.
The marsh rice rat is the natural reservoir
of the Bayou virus, the second most common cause of hantavirus pulmonary syndrome in the United States. Two other hantaviruses, Catacamas virus and Playa de Oro virus
, occur in O. couesi in Honduras and western Mexico, respectively, but are not known to infect humans.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of semiaquatic rodents 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...
living in southern North America and far northern South America. It includes eight species, two of which—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) of the United States and O. couesi of Mexico and Central America—are widespread; the six others have more restricted distributions. The species have had eventful taxonomic
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
histories, and most species were at one time included in the marsh rice rat; additional species may be recognized in the future. The name Oryzomys was established in 1857 by Spencer Fullerton Baird
Spencer Fullerton Baird
Spencer Fullerton Baird was an American ornithologist, ichthyologist and herpetologist. Starting in 1850 he was assistant-secretary and later secretary of the Smithsonian Institution in Washington, D.C...
for the marsh rice rat and was soon applied to over a hundred species of American rodents. Subsequently, the genus gradually became more narrowly defined until its current contents were established in 2006, when ten new genera were established for species previously placed in Oryzomys.
Species of Oryzomys are medium-sized rats with long, coarse fur. The upperparts are gray to reddish and the underparts white to buff. The animals have broad feet without complete ungual tufts of hair around the claws and, in at least some species, with webbing between the toes
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...
. The rostrum (front part of the skull) is broad and the braincase is high. Both the marsh rice rat and O. couesi have 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, lack a gall bladder, and have a complex penis (as is characteristic of the Sigmodontinae) with some traits that are rare among oryzomyines; these characteristics are unknown in the other species of this genus.
The habitat includes various kinds of wetlands, such as lakes, marshes, and rivers. Oryzomys species swim well, are active during the night, and eat both plant and animal food. They build woven nests of vegetation. After a gestation period
Gestation period
For mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth. The duration of this period varies between species.-Duration:...
of 21 to 28 days, about four young are born. Species of Oryzomys are infected by numerous parasites and carry at least three 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, one of which (Bayou virus) also infects humans. Two, perhaps three, species have gone extinct over the last two centuries and at least one other is endangered, but the widespread marsh rice rat and O. couesi are not threatened.
Taxonomy
Oryzomys is one of about thirty genera within the tribe OryzomyiniOryzomyini
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...
, a diverse group of well over a hundred species, many of which were formerly also included in Oryzomys. Oryzomyini is one of several tribes within 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...
, which includes hundreds of other species of mainly small rodents, distributed mainly in the Americas and Eurasia.
Within Oryzomyini, a 2006 phylogenetic
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
analysis by Marcelo Weksler which used both morphological and 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 some evidence that Oryzomys is most closely related to a group including Holochilus
Holochilus
Holochilus is a genus of semiaquatic rodents in the tribe Oryzomyini of family Cricetidae, sometimes called marsh rats. It contains three living species, Holochilus brasiliensis, Holochilus chacarius, and Holochilus sciureus, which are widely distributed in South America east of the Andes, and a...
, Lundomys, and Pseudoryzomys. Although analyses based on morphological and combined data supported this relationship, sequences of the Rbp3
RBP3
Retinol-binding protein 3, interstitial , also known as IRBP is a protein that in humans is encoded by the RBP3 gene. RBP3 orthologs have been identified in most eutherians except tenrecs and armadillos.- Function :...
gene alone instead placed Oryzomys among a group that included 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...
, Sigmodontomys
Sigmodontomys
Sigmodontomys is a genus of rodent in the tribe Oryzomyini of family Cricetidae. It is related to Nectomys and Melanomys and was previously included in Nectomys. It includes two species, Sigmodontomys alfari and the much rarer Sigmodontomys aphrastus, but whether these are indeed each other's...
, and a few other genera. In all analyses, Oryzomys appeared within clade D of Oryzomyini. The relationship between Oryzomys and the Holochilus group was supported by five synapomorphies
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 characters)—absence or reduction of both the hypothenar and interdigital pads; reduction of 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 of hairs surrounding the claws; having the back margin of 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...
of the skull at the same level as the front of the first upper molar
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"....
; and the anterocone (front cusp) of the first upper molar divided by an anteromedian fossette. The first three are adaptations to the semiaquatic lifestyle that Oryzomys and the members of the Holochilus group share, and may thus be examples of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
.
Circumscription
The name Oryzomys was introduced in 1857 by Spencer Fullerton BairdSpencer Fullerton Baird
Spencer Fullerton Baird was an American ornithologist, ichthyologist and herpetologist. Starting in 1850 he was assistant-secretary and later secretary of the Smithsonian Institution in Washington, D.C...
for 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...
(now Oryzomys palustris) of the eastern United States, which had been first described twenty years earlier by Richard Harlan
Richard Harlan
Richard Harlan was an American naturalist, zoologist, physicist and paleontologist....
. The name combines the Greek oryza "rice" and mys "mouse" and refers to the feeding habits of the marsh rice rat. Baird placed Oryzomys as a subgenus
Subgenus
In biology, a subgenus is a taxonomic rank directly below genus.In zoology, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the generic name and the specific epithet: e.g. the Tiger Cowry of the Indo-Pacific, Cypraea tigris Linnaeus, which...
of 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 included only the marsh rice rat in it, a classification which was followed by 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...
in 1874 and 1877. In 1890, Oryzomys was raised to generic rank, and in subsequent years numerous additional species were ascribed to it, many of which were soon moved to separate genera. In the 1898 Catalogus Mammalium, Édouard Louis Trouessart
Édouard Louis Trouessart
Édouard Louis Trouessart was a French zoologist. He discovered the dust mite....
listed 67 species of Oryzomys, including some that are now placed in Calomys, Necromys
Necromys
Necromys is a genus of South American sigmodontine rodents allied to Akodon. This genus has also been known as Cabreramys or more recently Bolomys. N. urichi has recently been transferred from Akodon....
, Thomasomys
Thomasomys
Thomasomys is a genus of rodent in the family Cricetidae, named after British zoologist Oldfield Thomas. It contains the following species:* Anderson's Oldfield Mouse * Apeco Oldfield Mouse...
, and other genera unrelated to Oryzomys. Some of the new genera proposed were soon subsumed in Oryzomys again, and in The Families and Genera of Living Rodents (1941), John Ellerman
Sir John Ellerman, 2nd Baronet
Sir John Reeves Ellerman, 2nd Baronet was an English shipowner, natural historian and philanthropist. The only son and heir of the English shipowner and investor John Ellerman, he was often said to be Britain's richest man...
listed Microryzomys
Microryzomys
Microryzomys is a genus of rodent in the tribe Oryzomyini of family Cricetidae. It is closely related to Oreoryzomys, Oligoryzomys, and Neacomys. It contains two species, both restricted to the Andes: Microryzomys altissimus and Microryzomys minutus.-Literature cited:*Musser, G.G. and Carleton,...
, 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...
, 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...
, Nesoryzomys
Nesoryzomys
Nesoryzomys is a genus of rodent in the tribe Oryzomyini of family Cricetidae, endemic to the Galápagos Islands. Other rodents restricted to the Galápagos include Megaoryzomys curioi and Aegialomys galapagoensis.-References:...
, and Oecomys
Oecomys
Oecomys is a genus of rodent within the tribe Oryzomyini of family Cricetidae. It contains about 17 species, which live in trees and are distributed across forested parts of South America, extending into Panama and Trinidad.-Literature cited:...
as synonym
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...
s of Oryzomys and included about 127 species in it. In 1948, 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...
suggested that other oryzomyines like 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...
and Megalomys
Megalomys
Megalomys is a genus of rodent in the family Cricetidae, part of the tribe Oryzomyini. The genus contains four large rodents from various Caribbean islands, all of which are now extinct. The last species to survive was M...
could as well be included in Oryzomys, and Clayton Ray followed this suggestion in 1962.
Hershkovitz and Ray's classification was never widely followed, and from 1976 on authors started to reinstate some of the other groups lumped in Oryzomys as separate genera. The genus was reduced to 43 species (out of 110 in Oryzomyini) in the third edition (2005) of Mammal Species of the World, but it was still not a natural, monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
group; rather, it mostly united those oryzomyines that lacked the conspicuous specializations of other genera. In 2006, Marcelo Weksler's comprehensive phylogenetic analysis produced further evidence that the genus was polyphyletic
Polyphyly
A polyphyletic group is one whose members' last common ancestor is not a member of the group.For example, the group consisting of warm-blooded animals is polyphyletic, because it contains both mammals and birds, but the most recent common ancestor of mammals and birds was cold-blooded...
, as species of Oryzomys were dispersed all over the oryzomyine tree. He proposed that eleven new genera should be created to accommodate those species that were not closely related to the type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
of Oryzomys, the marsh rice rat; he considered other options that would require fewer new genera, but argued that that would result in less meaningful genus-level groups in Oryzomyini. Later in the same year, Weksler, Percequillo, and Voss created ten new genera—Aegialomys
Aegialomys
Aegialomys is a genus of oryzomyine rodents from the lowlands and mountains of western Peru and Ecuador, including the Galápagos Islands. The species in this genus have historically been placed in Oryzomys, but according to cladistic research, the genus is more closely related to a group...
, Cerradomys
Cerradomys
Cerradomys is a genus of oryzomyine rodents from eastern Bolivia, Paraguay, and central Brazil found in cerrado, Caatinga and Gran Chaco habitats. The species in this genus have historically been placed in Oryzomys, but according to cladistic research, they are not more closely related to the type...
, Eremoryzomys, Euryoryzomys
Euryoryzomys
Euryoryzomys is a genus of rodents in the tribe Oryzomyini of family Cricetidae. It includes six species, which are distributed in South America. Until 2006, its members were included in the genus Oryzomys, but they are not closely related to the type species of that genus, and therefore they were...
, Hylaeamys
Hylaeamys
Hylaeamys is a genus of South American oryzomyine rodents found principally in humid forested areas east of the Andes. The species in this genus have historically been placed in Oryzomys. They are most closely related to Euryoryzomys, Transandinomys, Nephelomys, Oecomys, and Handleyomys, and most...
, Mindomys, Nephelomys
Nephelomys
Nephelomys is a genus of South American oryzomyine rodents found in the Andes from Bolivia to Venezuela, with a westward extension into the mountains of Costa Rica...
, Oreoryzomys, Sooretamys, and Transandinomys
Transandinomys
Transandinomys is a genus of rodents in the tribe Oryzomyini of family Cricetidae. It includes two species—T. bolivaris and T. talamancae—found in forests from Honduras in Central America south and east to southwestern Ecuador and northwestern Venezuela in northern South America...
—for species formerly placed in Oryzomys and placed six more species related to "Oryzomys" alfaroi in Handleyomys
Handleyomys
Handleyomys is a genus of Central and South American rodents in the tribe Oryzomyini of family Cricetidae. It was first described in 2002 to include two species from the Colombian Andes which were previously included in distinct and unrelated genera, Aepeomys and Oryzomys, but which turned out to...
pending the description of more new genera for them. They left only five species in Oryzomys, which was now finally a natural, monophyletic group. Because of subsequent taxonomic work, the number of species has since increased to at least eight.
Some problems remain: ?Oryzomys pliocaenicus
?Oryzomys pliocaenicus
?Oryzomys pliocaenicus is a fossil rodent from the Hemphillian of Kansas, central United States. It is known from a single mandible with the back part missing. All three molars are present, but very worn. Together, the molars are 3.6 mm long...
, a Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
fossil from Kansas, is of uncertain identity but may belong in Bensonomys, and the identity of Oryzomys anoblepas
Oryzomys anoblepas
Juliomys anoblepas is a rodent in the genus Juliomys of the subfamily Sigmodontinae known from a single broken skull. The specimen was collected by Peter Wilhelm Lund in the caves of Lagoa Santa in Minas Gerais, Brazil, in the first half of the 19th century and described by Herluf Winge in 1887 as...
, a Brazilian subfossil, is yet to be determined. Fossils from the Miocene of Oregon and Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
of New Mexico have also been ascribed to Oryzomys, but probably incorrectly. A possible Oryzomys has been recorded from the Irvingtonian
Irvingtonian
The Irvingtonian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology , typically set from 1,800,000 to 300,000 years BP, a period of . It is usually considered to overlap the Lower Pleistocene and Middle...
(Pleistocene) of Saskatchewan
Saskatchewan
Saskatchewan is a prairie province in Canada, which has an area of . Saskatchewan is bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, and on the south by the U.S. states of Montana and North Dakota....
.
Species
The current concept of Oryzomys derives from the palustris-mexicanus group recognized within a much larger genus Oryzomys by Merriam (1901) and the palustris group proposed by Goldman (1918). Merriam recognized 21 species within his group, but Goldman consolidated them into eight—the marsh rice rat in the United States, O. couesi in far southern Texas, Mexico, and Central America, and six others with small distributions. In 1960, Raymond Hall united O. couesi and the marsh rice rat into a single species, Oryzomys palustris, and thereafter, other localized forms were also included in O. palustris. Hershkovitz described another species in the group, O. gorgasiOryzomys gorgasi
Oryzomys gorgasi, also known as Gorgas's Oryzomys or Gorgas's Rice Rat, is a rodent in the genus Oryzomys of family Cricetidae. First collected as a living animal in 1967, it is known from only a few localities, including a freshwater swamp in the lowlands of northwestern Colombia and a mangrove...
from Colombia, in 1970 and the next year he noted that O. dimidiatus, previously classified as a Nectomys, was similar to O. palustris. After 1979, the marsh rice rat and O. couesi were again regarded as separate as a result of further work in Texas, where their ranges meet. While reviewing O. gorgasi in 2001, J. Sánchez H. and colleagues redefined and characterized the O. palustris group and listed O. couesi, O. dimidiatus, O. gorgasi, and the marsh rice rat as its members; Guy Musser
Guy Musser
Guy Graham Musser is an American zoologist. His main research field is the subfamily Murinae, in which he has described many new species.Musser was born in Salt Lake City, Utah...
and Michael Carleton in the 2005 third edition of Mammal Species of the World additionally listed O. nelsoni from María Madre Island in western Mexico.
In 2006, Weksler and colleagues followed the 2001 definition by Sánchez and others for the restricted genus Oryzomys, but added O. antillarum from Jamaica
Jamaica
Jamaica is an island nation of the Greater Antilles, in length, up to in width and 10,990 square kilometres in area. It is situated in the Caribbean Sea, about south of Cuba, and west of Hispaniola, the island harbouring the nation-states Haiti and the Dominican Republic...
as a species. Carleton and Joaquin Arroyo-Cabrales reviewed Oryzomys from western Mexico in 2009 and in this context provided an extended diagnosis of Oryzomys. They recognized eight species: the six previously mentioned plus 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...
and 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...
. Also in 2009, Robert Voss and Weksler identified the subfossil
Subfossil
Subfossil refers to remains whose fossilization process is not complete, either for lack of time or because the conditions in which they were buried were not optimal for fossilization....
Oryzomys curasoae from Curaçao
Curaçao
Curaçao is an island in the southern Caribbean Sea, off the Venezuelan coast. The Country of Curaçao , which includes the main island plus the small, uninhabited island of Klein Curaçao , is a constituent country of the Kingdom of the Netherlands...
as an island population of O. gorgasi. The next year, Delton Hanson and colleagues published a study using DNA sequence data from the 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 :...
, interphotoreceptor retinoid-binding protein, and 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...
1 genes to assess relationships within Oryzomys. They recommended that the marsh rice rat be split into two species and that O. couesi be split into four species on the basis of the observed sequence divergence and other data.
Merriam divided his palustris-mexicanus group in two "series" according to the color of the underparts (white or fulvous). Goldman divided his palustris group in two "sections"—a couesi section with O. couesi and six related species, and a palustris section with O. palustris only. He noted that the latter differed from the former in the generally darker, more brownish, longer fur, and larger 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 mesopterygoid fossa, the gap behind the end of the 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...
). As Weksler's 2006 analysis included only O. couesi and the marsh rice rat among species of Oryzomys in the strict sense, he could not test those groups. Carleton and Arroyo-Cabrales concurred with Goldman's division, listing additional characters, and noted that the palustris group may be more semiaquatically adapted than the members of the couesi group are. In the latter, the fur is usually reddish-brown, as opposed to grayish-brown in the palustris group. Members of the couesi group have smaller sphenopalatine vacuities and a smaller 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) in the side of the skull above the molars, and a more highly developed anterolabial cingulum on the third lower molar (a crest at the front of the tooth). The hypothenar pad of the hindfoot, located on the sole far from the fingers, is present in the couesi group, but absent in the palustris group. Interdigital webbing
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 more highly developed in the palustris group. Using morphological data, Voss and Weksler found a closer relationship between O. couesi and O. gorgasi to the exclusion of O. palustris, but with low confidence. The DNA sequence data of Hanson and colleagues supported a deep separation between the palustris and couesi groups, but a Costa Rican sample (assigned to O. couesi) was about as distant from the two groups as they were from each other.
The genus currently includes the following species:
| Name | Distribution | Characteristics | Taxonomic comments |
|---|---|---|---|
| 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... |
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... , Guanajuato Guanajuato Guanajuato officially Estado Libre y Soberano de Guanajuato is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 46 municipalities and its capital city is Guanajuato.... , 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... (inland western Mexico) |
Large, long tail, robust skull | Described in 1901 as a species; subspecies of O. couesi in 1918; reinstated as a species in 2009 |
| Oryzomys antillarum | Jamaica Jamaica Jamaica is an island nation of the Greater Antilles, in length, up to in width and 10,990 square kilometres in area. It is situated in the Caribbean Sea, about south of Cuba, and west of Hispaniola, the island harbouring the nation-states Haiti and the Dominican Republic... (extinct) |
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, short 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:... (perforations in the 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... ), robust 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) |
Described in 1898 as a species; subspecies of O. couesi in 1966; reinstated as a species in 1993/2006 |
| Oryzomys couesi | 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 coastal Sonora Sonora 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.... (northwestern Mexico) to northwestern Colombia Colombia Colombia, officially the Republic of Colombia , is a unitary constitutional republic comprising thirty-two departments. The country is located in northwestern South America, bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the... |
Upperparts buff to reddish, underparts white to buff, sphenopalatine vacuities small to absent | Described in 1877 as a species; subspecies of marsh rice rat in 1960; reinstated as a species in 1979; various other species split from and lumped into it from time to time. Genetic data suggest four species may be present: one along the Pacific coast from Sonora to El Salvador; one from Texas to Nicaragua; one in Costa Rica; and one in Panama. |
| Oryzomys dimidiatus | Southeastern Nicaragua Nicaragua Nicaragua is the largest country in the Central American American isthmus, bordered by Honduras to the north and Costa Rica to the south. The country is situated between 11 and 14 degrees north of the Equator in the Northern Hemisphere, which places it entirely within the tropics. The Pacific Ocean... |
Gray underparts, brown feet, tail nearly the same color above as below | Described in 1905 as a species of 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... ; reclassified in Oryzomys in 1948 |
| Oryzomys gorgasi Oryzomys gorgasi Oryzomys gorgasi, also known as Gorgas's Oryzomys or Gorgas's Rice Rat, is a rodent in the genus Oryzomys of family Cricetidae. First collected as a living animal in 1967, it is known from only a few localities, including a freshwater swamp in the lowlands of northwestern Colombia and a mangrove... |
Northwestern Colombia Colombia Colombia, officially the Republic of Colombia , is a unitary constitutional republic comprising thirty-two departments. The country is located in northwestern South America, bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the... , northwestern Venezuela Venezuela Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south... ; extinct on Curaçao Curaçao Curaçao is an island in the southern Caribbean Sea, off the Venezuelan coast. The Country of Curaçao , which includes the main island plus the small, uninhabited island of Klein Curaçao , is a constituent country of the Kingdom of the Netherlands... |
Robust rostrum (front part of skull); incisive foramina tapering at the back; sphenopalatine vacuities absent; 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... (opening at the back of the skull) small |
Described in 1970; Oryzomys curasoae described in 2001; both synonymized 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... in 2009 |
| Oryzomys nelsoni | María Madre Island, Nayarit, western Mexico (extinct) | Large, long tail, rostrum heavy and bent downward, incisor Incisor Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:... s large and wide |
Described in 1898 as a species; subspecies of O. palustris in 1971; otherwise regarded as a distinct species |
| 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... (Oryzomys palustris) |
In the eastern United States from New Jersey New Jersey New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware... and Kansas Kansas Kansas is a US state located in the Midwestern United States. It is named after the Kansas River which flows through it, which in turn was named after the Kansa Native American tribe, which inhabited the area. The tribe's name is often said to mean "people of the wind" or "people of the south... south to Florida Florida Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it... and 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 into 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; previously further north to Iowa Iowa Iowa is a state located in the Midwestern United States, an area often referred to as the "American Heartland". It derives its name from the Ioway people, one of the many American Indian tribes that occupied the state at the time of European exploration. Iowa was a part of the French colony of New... and southwestern Pennsylvania Pennsylvania The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to... |
Compared to sympatric Sympatry In biology, two species or populations are considered sympatric when they exist in the same geographic area and thus regularly encounter one another. An initially-interbreeding population that splits into two or more distinct species sharing a common range exemplifies sympatric speciation... O. couesi where the two meet: short tail, white underparts, sphenopalatine foramen large |
Described in 1827; specific status of Florida Keys Florida Keys The Florida Keys are a coral archipelago in southeast United States. They begin at the southeastern tip of the Florida peninsula, about south of Miami, and extend in a gentle arc south-southwest and then westward to Key West, the westernmost of the inhabited islands, and on to the uninhabited Dry... form (argentatus; first described in 1978) disputed; genetic data suggest populations west of Alabama Alabama Alabama is a state located in the southeastern region of the United States. It is bordered by Tennessee to the north, Georgia to the east, Florida and the Gulf of Mexico to the south, and Mississippi to the west. Alabama ranks 30th in total land area and ranks second in the size of its inland... may be a separate species |
| 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... |
Southern tip of Baja California Sur Baja California Sur Baja California Sur , is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. Before becoming a state on October 8, 1974, the area was known as the South Territory of Baja California. It has an area of , or 3.57% of the land mass of Mexico and comprises... (perhaps extinct) |
Moderately large, gray on head and forequarters, broad, squared zygomatic arches, long, broad incisive foramina, upper incisor nearly orthodont | Described in 1899 as a species; subspecies of O. palustris in 1971 and of O. couesi in 1994; reinstated as a species in 2009 |
Description
| Species | nNumber of specimens measured. | Total length | Tail | Hindfoot |
|---|---|---|---|---|
| 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... |
12 | 285.4 (245–314) | 155.4 (129–173) | 36.1 (33–40) |
| Oryzomys antillarum | 3 | 247 (228–260) | 119.7 (108–132) | 29.3 (28–30) |
| Oryzomys couesi from 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... |
62 | 244.8 (210–288) | 125.1 (105–150) | 30.5 (27–33) |
| Oryzomys dimidiatus | 3 | 249 (228–278) | 129 (110–150) | 28.3 (27–31) |
| Oryzomys gorgasi Oryzomys gorgasi Oryzomys gorgasi, also known as Gorgas's Oryzomys or Gorgas's Rice Rat, is a rodent in the genus Oryzomys of family Cricetidae. First collected as a living animal in 1967, it is known from only a few localities, including a freshwater swamp in the lowlands of northwestern Colombia and a mangrove... |
6–10Six for total length, eight for tail length, and ten for hindfoot length. | 259 (220–290) | 130 (116–138) | 31 (30–32) |
| Oryzomys nelsoni | 4 | 322 (288–344) | 181.5 (160–191) | 37.3 (35–39) |
| 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... |
– | 226–305 | 108–156 | 28–37 |
| 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... |
14 | 265.6 (227–305) | 136.8 (114–156) | 32.0 (29–34) |
| Measurements are in millimeters and in the form "average (minimum–maximum)". | ||||
Oryzomys contains medium-sized, semi-aquatically specialized oryzomyine rodents. They have long, coarse fur that is grayish to reddish on the upperparts and white to buff on the underparts. The marsh rice rat superficially resembles the introduced species
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...
black rat
Black Rat
The black rat is a common long-tailed rodent of the genus Rattus in the subfamily Murinae . The species originated in tropical Asia and spread through the Near East in Roman times before reaching Europe by the 1st century and spreading with Europeans across the world.-Taxonomy:The black rat was...
and brown rat
Brown Rat
The brown rat, common rat, sewer rat, Hanover rat, Norway rat, Brown Norway rat, Norwegian rat, or wharf rat is one of the best known and most common rats....
, but has larger differences in color between the upper- and underparts. The vibrissae (whiskers) are short and the ears are small and well-haired. The tail is usually as long as or longer than the head and body and is sparsely haired, but the hairs on the lower side are longer than those above. Females have eight mammae, as in most oryzomyines. The hindfeet are broad and have the first and fifth digits notably shorter than the middle three. The upper surface is hairy, but the underside is naked and covered with small irregularities (squamae
Squamae
In some rodents, squamae are small tubercles resembling scales on the sole of the hindfeet. Among oryzomyine rodents, their development is variable; most have well-developed squamae, but in others they are indistinct or entirely absent. Delomys sublineatus and Peromyscus maniculatus also have...
). The pads are generally poorly developed, as are the ungual tufts. Interdigital webbing
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, but its development is variable within the genus.
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...
has been recorded in various populations of the marsh rice rat and O. couesi and is apparently stable within the genus at 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, with the fundamental number of chromosomal arms ranging from 56 to 60 (2n = 56, FN = 56–60). In both species, 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. Furthermore, the gall bladder is absent, a synapomorphy of Oryzomyini.
Oryzomys species have a large skull with a short rostrum and high braincase. 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....
, located between the eyes, is narrowest to the front and is flanked by well-developed beads at its margins. 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...
is broad and has a well-developed zygomatic notch at its front. The zygomatic arch is robust and contains a small but distinct jugal bone. The interparietal bone, part of the roof of the braincase, is narrow and short; its narrowness is a synapomorphy for O. couesi plus the marsh rice rat according to Weksler's analysis. The incisive foramina are long, with their back margin at the front of the first molars or further back. The palate is also long, extending beyond the back margin of the maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
ry bone, and is perforated near the third molars by well-developed 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...
. 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 other oryzomyines separates two foramina in the skull. The auditory bulla
Auditory bulla
The auditory bulla is a hollow bony structure on the ventral, posterior portion of the skull of placental mammals that encloses parts of the middle and inner ear. In most species, it is formed by the tympanic part of the temporal bone.In extant primates, the structure is found in tarsiers,...
e are large. 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...
. 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 coronoid process
Coronoid process of the mandible
The mandible's coronoid process is a thin, triangular eminence, which is flattened from side to side and varies in shape and size....
, a 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:...
at the back, is well developed and 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 mandibular bone housing the root of the lower incisor, is conspicuous.
As usual in oryzomyines, the molars are pentalophodont (have the mesolophs and mesolophids, accessory crests, well developed) and bunodont, with the cusps higher than the connecting crests. The cusps on the upper molars are arranged in two longitudinal series, not three as in the black and brown rats. The front cusps of the first upper and lower molar (anterocone and anteroconid, respectively) are broad and not divided completely by an anteromedian flexus or flexid. Behind the anterocone, the anteroloph (a smaller crest) is complete and separated from the anterocone. On both the second and third lower molars, the anterolophid (a crest on the inner front corner) is present, a putative synapomorphy of the genus. The first molars have additional small roots in addition to the main ones, so that the upper first molar has four and the lower has three or four roots.
As is characteristic of Sigmodontinae, the marsh rice rat and O. couesi have 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) displaying large protuberances at the sides. 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 these two species share 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...
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 Holochilus brasiliensis. Both traits are recovered as synapomorphies of O. couesi plus the marsh rice rat in Weksler's analysis.
Distribution, ecology, and behavior
The range of Oryzomys extends from New JerseyNew Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...
in the eastern United States through Mexico and Central America south to northwestern Colombia and east to northwestern Venezuela and Curaçao
Curaçao
Curaçao is an island in the southern Caribbean Sea, off the Venezuelan coast. The Country of Curaçao , which includes the main island plus the small, uninhabited island of Klein Curaçao , is a constituent country of the Kingdom of the Netherlands...
. Species of Oryzomys usually live in wet habitats such as marshes, streams, and mangrove
Mangrove
Mangroves are various kinds of trees up to medium height and shrubs that grow in saline coastal sediment habitats in the tropics and subtropics – mainly between latitudes N and S...
s, but both the marsh rice rat and O. couesi are also occasionally encountered in drier habitats. They occur or occurred on many continental-shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
islands and one oceanic island, Jamaica
Jamaica
Jamaica is an island nation of the Greater Antilles, in length, up to in width and 10,990 square kilometres in area. It is situated in the Caribbean Sea, about south of Cuba, and west of Hispaniola, the island harbouring the nation-states Haiti and the Dominican Republic...
; their adeptness at colonizing islands may be caused by their close association with water and frequent occurrence in coastal wetlands. The oldest fossils date to the Rancholabrean
Rancholabrean
The Rancholabrean North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology , typically set from less than 240,000 years to 11,000 years BP, a period of . It is usually considered to overlap the Middle Pleistocene...
of the United States, about 300,000 years ago; although there have been some earlier North American records, those are not in fact referable to Oryzomys or even Oryzomyini. Oryzomyines likely evolved in South America east of the Andes; the presence of Oryzomys in Central America and other trans-Andean regions is thought to be the result of one of several independent invasions of this region by oryzomyines. Alternatively, Oryzomys may have evolved from the Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
North American Jacobsomys. O. antillarum may have reached Jamaica during the last glacial period while sea levels were low.
Behavior is known mainly from the marsh rice rat and O. couesi, with some scattered data from the other species. Oryzomys are semiaquatic, spending much time in the water, and otherwise mainly live on the ground; both the marsh rice rat and O. couesi are known to be excellent swimmers and will flee into the water when disturbed. Both are also active during the night and build nests of interwoven vegetation, which may be suspended above the water. Breeding may occur throughout the year in both species, but is known to be seasonally variable in the marsh rice rat. In both, gestation takes about 21 to 28 days and litter size is usually one to seven, averaging three to five. Young marsh rice rats and O. couesi become reproductively active when about 50 days old.
The marsh rice rat, O. couesi, and O. gorgasi are known to be omnivore
Omnivore
Omnivores are species that eat both plants and animals as their primary food source...
s, eating both plant and animal material. They eat both seeds and green plant parts and consume a variety of animals, including insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s, crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s, and many others. The barn owl
Barn Owl
The Barn Owl is the most widely distributed species of owl, and one of the most widespread of all birds. It is also referred to as Common Barn Owl, to distinguish it from other species in the barn-owl family Tytonidae. These form one of two main lineages of living owls, the other being the typical...
(Tyto alba) is a major predator on the marsh rice rat and remains of O. antillarum, O. couesi, and O. gorgasi have been found in owl pellet deposits. Several other animals are known to prey on Oryzomys. A variety of parasites are known from O. couesi and the marsh rice rat and two parasitic 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...
s have been found in O. gorgasi.
Human interactions
Two species of Oryzomys, O. antillarum and O. nelsoni, have gone extinct since the 19th century, and a third, O. peninsulae, is unlikely to be still extant. Their extinction may have been caused by habitat destruction and by introduced speciesIntroduced 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...
such as the small Asian mongoose and the brown
Brown Rat
The brown rat, common rat, sewer rat, Hanover rat, Norway rat, Brown Norway rat, Norwegian rat, or wharf rat is one of the best known and most common rats....
and black rat
Black Rat
The black rat is a common long-tailed rodent of the genus Rattus in the subfamily Murinae . The species originated in tropical Asia and spread through the Near East in Roman times before reaching Europe by the 1st century and spreading with Europeans across the world.-Taxonomy:The black rat was...
. These same causes may threaten O. gorgasi, which the IUCN Red List
IUCN Red List
The IUCN Red List of Threatened Species , founded in 1963, is the world's most comprehensive inventory of the global conservation status of biological species. The International Union for Conservation of Nature is the world's main authority on the conservation status of species...
assesses as "Endangered". O. albiventer has been affected by human alteration of its habitat, but likely still survives. In contrast, the widespread species, the marsh rice rat and O. couesi, are common and of no conservation concern—indeed, both have been considered a pest—but some populations are threatened. Like these two species, O. dimidiatus is assessed as "Least Concern
Least 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...
" by the Red List.
The marsh rice rat is the natural 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...
of the Bayou virus, the second most common cause of hantavirus pulmonary syndrome in the United States. Two other hantaviruses, Catacamas virus 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...
, occur in O. couesi in Honduras and western Mexico, respectively, but are not known to infect humans.
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