Marsh Rice Rat
Encyclopedia
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 have been a commensal
in corn-cultivating communities. It usually occurs in wet habitats such as swamps and saltmarshes. Weighing about 40 to 80 g (1.4 to 2.8 oz), the marsh rice rat is a medium-sized rodent that resembles the common black
and brown rat
. The upperparts are generally gray-brown, but reddish in many Florida populations. The feet show several specializations for life in the water. The skull is large and flattened and is short at the front.
John Bachman
discovered the marsh rice rat in 1816, and it was formally described in 1837. Several subspecies
have been described since the 1890s, mainly from Florida, but there is disagreement over their validity. The Florida Keys
population is sometimes classified as a different species, the silver rice rat (Oryzomys argentatus). Data from the mitochondrial
cytochrome b
gene indicate a deep divergence between populations east of Mississippi and those further west, which suggests that the western populations may be recognized as a separate species, Oryzomys texensis. The species is part of the genus Oryzomys
, which also includes several others occurring further south in Mexico, Central America, and northwestern South America, some of which have previously been regarded as subspecies of the marsh rice rat. One, Oryzomys couesi, occurs with the marsh rice rat in Tamaulipas and southern Texas
.
The marsh rice rat is active during the night, makes nests of sedge and grass, and occasionally builds runways. It has a diverse diet that includes plants, fungi, and a variety of animals. Population densities are usually below 10 per ha (4 per acre) and home ranges vary from 0.23 to 0.37 ha (0.57 to 0.91 acres), depending on sex and geography. Litters of generally three to five young are born after a pregnancy of about 25 days, mainly during the summer. Newborns are helpless at birth, but are weaned after a few weeks. Several animals prey on the marsh rice rat, including the barn owl
, and it usually lives for less than a year in the wild. It is infected by many different parasites and harbors a hantavirus
that also infects humans. The species is not of conservation concern, but some populations are threatened.
, which is distributed from the eastern United States (marsh rice rat) into northwestern South America (O. gorgasi
). 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, most of which occur in South and Central America. In the United States, the marsh rice rat is the only oryzomyine rodent except for Oryzomys couesi in a small area of southern Texas
; the only other sigmodontines present are several species of cotton rat
s (Sigmodon) in the southern half of the country.
by John Bachman
. Bachman intended to describe the species as Arvicola oryzivora, but sent a specimen to Richard Harlan
and Charles Pickering
at the Academy of Natural Sciences
in Philadelphia to confirm its identity. Another specimen, from New Jersey
, was found in the Academy's collection, and Harlan took it upon himself, against Pickering's wishes, to describe the new species as Mus palustris, proclaiming it one of the few true rats of the United States. The specific name palustris is Latin for "marshy" and refers to the usual habitat of the species.
In 1854, in The quadrupeds of North America, Bachman redescribed it as Arvicola oryzivora, considering it more closely related to the vole
s then placed in the genus Arvicola
, and also recorded it from Georgia
and Florida
. Three years later, Spencer Fullerton Baird
argued that the referral of the species to Arvicola was erroneous and introduced a new generic name for the marsh rice rat, Oryzomys. The name combines the Greek oryza "rice" and mys "mouse" and refers to the rat's habit of eating rice. At the time, Oryzomys was recognized either as a full genus or as a subgenus
of the now-defunct genus Hesperomys
, but since the 1890s, it has been universally recognized as a genus distinct from Hesperomys, with the marsh rice rat (Oryzomys palustris) as its type species
.
of the marsh rice rat were described from the United States: natator from Florida in 1893, texensis from Texas in 1894, and coloratus from elsewhere in Florida in 1898. Clinton Hart Merriam
recognized natator as a separate species in 1901 and described a subspecies of it, floridanus, but considered texensis to be nearly identical to nominate palustris. In his 1918 revision of North American Oryzomys, Edward Alphonso Goldman
again recognized all these as a single species, Oryzomys palustris. He distinguished four subspecies, which he said formed a "closely intergrading series"—O. p. palustris from New Jersey to southeastern Mississippi and eastern Missouri; O. p. natator in central Florida; O. p. coloratus (including O. natator floridanus Merriam) in southern Florida; and O. p. texensis from western Mississippi and southeastern Kansas to eastern Texas. Two additional subspecies were described by William J. Hamilton in 1955 from southern Florida: planirostris from Pine Island
and two miles (3 km) north of Fort Myers
and sanibeli from Sanibel Island
. Also in 1955, Claude W. Hibbard
described a new species of Oryzomys, O. fossilis, from Pleistocene
deposits in Kansas
, on the basis of small differences in characters of the tooth with living marsh rice rats. In 1965, Walter Dalquest demoted this species, later also found in Texas, to a subspecies, because it does not differ more from living marsh rice rats than the latter differ from each other.
Merriam and Goldman had recognized that a number of Central American species, including Oryzomys couesi and numerous forms with more limited distributions, are related to the marsh rice rat. O. couesi ranges north to southernmost Texas, where its distribution meets that of the marsh rice rat. In 1960, Raymond Hall argued that specimens from the contact zone were intermediate between the local forms of O. couesi and the marsh rice rat, and accordingly included the former in the marsh rice rat. While reporting on the ecology of Texan O. couesi in 1979, Benson and Gehlbach noted that populations of O. couesi and the marsh rice rat there were in fact distinct, with the latter being smaller and less brown and more gray in color; their karyotype
s were also distinct. Since then, the two have generally been retained as distinct species, as supported by further research; a 1994 study even found the two to occur at some of the same places (in sympatry
) in southern Texas and nearby Tamaulipas
, Mexico.
In 1973, rice rats were discovered at Cudjoe Key in the Florida Keys
, and in 1978 Spitzer and Lazell described this population as a new species, Oryzomys argentatus. The status of this form—either a distinct species or not even subspecifically distinct from O. palustris natator—has remained controversial since; the 2005 third edition of Mammal Species of the World
does not recognize O. argentatus as a separate species, but acknowledges a need for further research. A 2005 study using microsatellite DNA found that Florida Keys rice rats exhibit low genetic variation and are significantly different from Everglades
rice rats; the study concluded in favor of classifying the Keys rice rat as a "distinct vertebrate population". This population probably diverged from mainland rice rats about 2000 years ago.
Among the described subspecies, a 1989 morphometrical study by Humphrey and Setzer separated only two—natator from much of Florida (including coloratus, planirostris, sanibeli, and floridanus, as well as argentatus) and palustris from the rest of the range (including texensis). On the other hand, Whitaker and Hamilton in their 1998 book on the Mammals of the Eastern United States recognized planirostris and sanibeli as separate subspecies, but merged all others into O. palustris palustris, and placed O. argentatus as a separate species; their classification was based on their emphasis of overwater gaps as agents of biological diversification and a critique of shortcomings in Humphrey and Setzer's study, not on a reanalysis of the data.
In 2010, Delton Hanson and colleagues published a study of the relationships among populations of Oryzomys on the basis of data from three genes—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). The Cytb data placed all marsh rice rats studied sister to a clade
containing various populations of O. couesi; the mean genetic distance between the two groups was 11.30%. The marsh rice rats fell into two main groups, differing on average by 6.05%, one containing animals from Mississippi, southwestern Tennessee, and further west, and the other including specimens from Alabama and further east. Within the eastern group, variation was only about 0.65%, even though examples of the putative subspecies palustris, coloratus, sanibeli, and planirostris were all included. 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 the marsh rice rat as separate clades. In addition, Adh1-I2 placed a Costa Rican population within the marsh rice rat clade and some other southern Oryzomys specimens closer to the marsh rice rat than to the O. couesi group. The combined dataset supported the western and eastern clades within the marsh rice rat 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 western populations of the marsh rice rat be recognized as a separate species, Oryzomys texensis. They recommended further research in the Mississippi–Alabama–Tennessee region, where the ranges of the two meet.
s have been proposed for the marsh rice rat. Early describers used "Rice Meadow-Mouse" and "Rice-field Mouse" and in the early 1900s, name such as "rice rat", "marsh mouse", and "swamp rice rat" came into use. Some of the subspecies received their own common names, such as "Florida Marsh Mouse", "Swimming Rice Rat", and "Central Florida Rice Rat" for O. p. natator; "Bangs' Marsh Mouse", "Cape Sable Rice Rat", and "Everglades Rice Rat" for O. p. coloratus; and "Texas Rice Rat" for O. p. texensis. The species is now usually known as the "marsh rice rat", although "marsh oryzomys" has also been in recent use. The Florida Keys form (argentatus) is known as the "silver rice rat".
The marsh rice rat is a medium-sized rodent that looks much like the common black
and brown rat
s, but has greater differences in color between the upper- and underparts. The fur is thick and short. The upperparts are generally gray to grayish brown, with the head a bit lighter, and are sharply delimited from the underparts, which are off-white, as are the feet. There are small cheek pouches. The ears are about the same color as the upperparts, but there is a patch of light hairs in front of them. The tail is dark brown above and may be paler below. The guard hair
s are long and have unpigmented, silvery tips. When rice rats swim, air is trapped in the fur, which increases buoyancy and reduces heat loss. As in most other oryzomyines, females have eight mammae.
The forefeet have four and the hindfeet five digits. On the forefeet, the ungual tuft
s (tufts of hair on the digits) are absent. The hindfeet are broad and have a short fifth digit. Many of the pads are reduced, as are the ungual tufts, but there are small interdigital webs
. The Florida Keys form, argentatus, has even more reduced ungual tufts. Many of these traits are common adaptations to life in the water in oryzomyines.
There is some geographic variation in fur color: western populations (texensis) are lighter than those from the east (nominate palustris), and Florida populations are generally more tawny or reddish than either, with those from southern Florida (coloratus) being brighter than those from the center of the state (natator). The Florida Keys form (argentatus) is silvery, and the two other Florida forms—planirostris and sanibeli—lack the reddish tones of mainland Florida populations and are instead grayish, resembling nominate palustris (planirostris), or brownish (sanibeli). In 1989, Humphrey and Setzer reviewed variation in color among Florida populations. They found argentatus to be substantially lighter and planirostris and sanibeli to be somewhat darker than mainland populations, and argentatus to have a less yellow fur, but found no significant differences in redness. There was also substantial variation within populations.
Total length is 226 to 305 mm (8.9 to 12.0 in), tail length 108 to 156 mm (4.3 to 6.1 in), hindfoot length 28 to 37 mm (1.1 to 1.5 in), and body mass 40 to 80 g (1.4 to 2.8 oz), with males slightly larger than females. The largest individuals occur in Florida and along the Gulf Coast east of the Mississippi River delta.
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 chromosomes and a fundamental number of 60 chromosomal arms (2n = 56, FN = 60). The form of the sex chromosomes has been used to distinguish the marsh rice rat from Oryzomys couesi, but may be too variable among Oryzomys to be useful in differentiating species. X chromosome inactivation
occurs in the marsh rice rat, even though the animal lacks LINE-1 retrotransposons, which have been suggested as components of the inactivation process. Mutants with fused or additional molars and with light fur have been recorded in laboratory colonies; the abnormal molars are apparently the result of a single autosomal recessive mutation. At about 50%, hematocrit
(the proportion of red blood cells in the blood) is high in the marsh rice rat compared to other rodents; this may be an adaptation that enables the rice rat to increase oxygen capacity while swimming underwater.
is long and robust, averaging 7.3 mm (0.29 in) long and 4.6 mm (0.18 in) broad, and the baculum
(penis bone) is 6.6 mm (0.26 in) long. As is characteristic of Sigmodontinae, the marsh rice rat has a complex penis, with the distal (far) end of the baculum ending in three digits. The central digit is notably larger than those 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 the marsh rice rat shares only with Oligoryzomys
and Oryzomys couesi 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 O. couesi and Holochilus brasiliensis. The baculum is deeper than it is wide.
Some features of the accessory glands in the male genital region vary among oryzomyines. In the marsh rice rat, a single pair of preputial gland
s is present at the penis. As is usual for sigmodontines, there are two pairs of ventral prostate
glands and a single pair of anterior and dorsal prostate glands. Part of the end of the vesicular gland is irregularly folded, not smooth as in most oryzomyines.
and premaxillary bones extend back beyond the point where the lacrimal
, frontal
, and maxillary bones meet. In planirostris, the rostrum is flatter than in mainland Florida forms, in which it is more convex, and the nasals are said to be relatively longer in argentatus. The zygomatic plate
, the flattened front part of the zygomatic arch
(cheekbone), is broad and develops a notch at its front end. The arches themselves are robust and contain small but distinct jugal bones. The sphenopalatine foramen
, a foramen
(opening) in the side of the skull above the molars, is large; it is much smaller in O. couesi. The narrowest part of the interorbital region
(located between the eyes) is towards the front and the edges are lined by prominent shelves. The marsh rice rat has a narrow braincase lined by prominent ridges and a narrow interparietal bone. According to Goldman, Florida animals (coloratus and natator) generally have the largest and broadest skulls, and western specimens (texensis) have somewhat smaller and narrower skulls than those from the east outside Florida (nominate palustris). In argentatus, the skull is also relatively narrow.
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 back part, near the third molars, is usually perforated by prominent posterolateral palatal pits
, which are recessed into fossae (depressions). The mesopterygoid fossa, the gap behind the end of the palate, is perforated by sphenopalatine vacuities
, which are set far to the front. 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 present, but not as large as in O. couesi.
), as usual in muroid rodents
. The upper incisors are well-developed and strongly 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 the marsh rice rat 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).
The upper molars have two longitudinal rows of cusps, not three as in the black and brown rats. The first and second upper molar are oval in form and 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), but does display a hollow in the middle, the anteromedian fossette, which divides it into separate cuspules at the labial and lingual (inner) sides of the molar. A crest, the anteroloph, is present behind the labial cuspule, but in older animals, the cusps and the crest are united into a single structure by wear. In the third upper molar, the cusps at the back are reduced and scarcely distinguishable. 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.
The first lower molar is rounded at the front end and the labial and lingual conules of the anteroconid, the frontmost cusp, are barely distinct. The second lower molar is elongate and has a crest, the anterolophid, before the two cusps that form the front edge of the molar in some other oryzomyines, the protoconid and metaconid. There is a distinct ridge (anterolabial cingulum) at the outer front (anterolabial) edge of the molar, before the protoconid. The lower third molar is about as long as the second and also has an anterolophid, albeit a less well-defined one. The first lower molar has large roots at the front and back of the tooth and usually one or two smaller ones in between, at the labial and lingual side. The second and third lowers molars have either two roots, one labial and one lingual, or only one at the front, and another large root 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).
may increase up to 200%, that rice rats are unable to conserve water well when dehydrated
, and that in water contaminated with oil they swim less and their mortality increases. The median amount of radiation needed to kill a marsh rice rat is 5.25 Gy
and the lethal dose of potassium cyanide
is 7.20 mg/kg; both values are relatively low for cricetid rodents. In one study, wild rice rats in radioactively contaminated areas did not show signs of disease. Experiments have found that exposure to more daylight and higher food availability cause increased development of the gonad
s in both adult and juvenile rice rats. When the pineal gland
is removed or melatonin
is administered in male rice rats, the testes are reduced and tend to regress into the body.
The marsh rice rat currently occurs in much of the eastern and southern United States, northeast to southern New Jersey, and south to southeastern Texas and far northeastern Tamaulipas
, Mexico. The northernmost records in the interior United States are in eastern Oklahoma
, southeastern Kansas
, southern Missouri
and Illinois
, and the southern half of Kentucky
, but the species is absent in much of the Appalachians. Fossils of the marsh rice rat are known from Rancholabrean
(late Pleistocene, less than 300,000 years ago) deposits in Florida and Georgia and remains referred to the extinct subspecies O. p. fossilis are from the Wisconsinan
and Sangamonian of Texas and Illinoian and Sangamonian of Kansas. In the Florida Keys, rice rats occur on most of the Lower Keys, but are absent from the Upper Keys, which are of a different geological origin and were probably never connected to the mainland. The western and eastern Cytb clades within the marsh rice rat may represent expansions from different glacial refugia which the species was restricted to during a glacial period
.
Cave and archeological remains indicate that the range of the marsh rice rat has extended substantially further north and west earlier in the Holocene
, into central Texas, eastern Nebraska
, southwestern Iowa
, central Illinois, southern Indiana
, southern Ohio
, West Virginia
, and southwestern Pennsylvania
. Most northern archeological sites date from about 1000 CE and are associated with corn
cultivation, but in some older cave sites the rice rat is found with the extinct giant armadillo Dasypus bellus, suggesting warm climatic conditions. Perhaps a warm period during the Quaternary
enabled the rice rat to disperse northward and when the climate cooled, relict
populations were able to survive in the north as commensals in corn-cultivating Native American communities. Some subfossil animals are slightly larger than living marsh rice rats, possibly because environmental constraints were relaxed in commensal populations.
In Tamaulipas and southern Texas, the ranges of the marsh rice rat and the related Oryzomys couesi meet; in parts of Kenedy, Willacy and Cameron
counties, Texas, and in far northeastern Tamaulipas, the two are sympatric (occur in the same places). In experimental conditions, they fail to interbreed and genetic analysis yields no evidence of gene flow
or hybridization in the wild. Compared to O. couesi, the marsh rice rat shows less genetic variability within but more between populations in the contact zone, probably because the species is restricted to isolated populations near the coast.
The marsh rice rat occurs in several habitats, ranging from coastal salt marshes to mountain streams and clearings. It is semiaquatic, spending much time in the water, and usually occurs in wetland habitats. It prefers areas where the ground is covered with grasses and sedges, which protect it from predators. In southern Illinois, marsh rice rats are more likely to occur in wetlands with more herbaceous cover, visual obstruction, and nearby grasslands. The species also occurs in drier uplands, which serve as sinks for young, dispersing animals and as refuges during high tide. Rice rats are adept overwater dispersers; studies on islands off Virginia
's Delmarva Peninsula
show that they readily cross 300-m (1000 ft) channels between islands.
Marsh rice rats are active during the night and for this reason are rarely seen, although they may be among the most common small mammals in part of their range. They will build nests of sedge and grass, about 13 cm (5 in) large, which are placed under debris, near shrubs, in short burrows, or high in aquatic vegetation. They may also use old nests of marsh wren
s (Cistothorus palustris), red-winged blackbird
s (Agelaius phoeniceus), muskrat
s (Ondatra zibethicus) or round-tailed muskrat
s (Neofiber alleni). Marsh rice rats sometimes make large runways or dig burrows. They are accomplished and willing swimmers, easily swimming for more than 10 m (30 ft) under water, and will often seek safety in the water when alarmed. Rice rats in the Florida Keys occasionally climb in vegetation, but never higher than 90 cm (3 ft). Marsh rice rats are very cleanly and extensively groom
themselves, perhaps to keep their fur water-repellent. They are aggressive towards conspecifics and emit high-pitched squeaks while fighting. In dense vegetation, the perceptual range (the distance from which an animal can detect a patch of suitable habitat) of marsh rice rats is less than 10 m (33 ft). When released outside of their natural wetland habitat, marsh rice rats generally move either upwind or downwind (anemotaxis), perhaps in order to move in a straight line, which is an efficient strategy to find suitable habitat.
Many animals prey on marsh rice rats. The barn owl
(Tyto alba) is among the most important; one study found that 97.5% of vertebrate remains in barn owl pellets were marsh rice rats. Other predators include birds (marsh hawks, Circus cyaneus; and barred owl
s, Strix varia), snakes (cottonmouth mocassins, Agkistrodon piscivorus; and others), alligators
, and carnivora
ns (raccoon
s, Procyon lotor; red fox
es, Vulpes vulpes; mink
s, Neovison vison; weasel
s of the genus Mustela; and striped skunk
s, Mephitis mephitis). Many parasites have been recorded on the marsh rice rat, including various ticks and mites, lice
, and flea
s among external parasites and many nematode
s and digenea
ns, a pentastomid, and several coccidia
ns among internal parasites (see parasites of the marsh rice rat).
Periodontitis, a bacterial disease affecting the jaws, is particularly virulent in marsh rice rats; the animal has been proposed as a model for research on the disease in humans. The identity of the bacterial agent remains unknown. Vitamin E
, fluoride
, and iodide
protect against bone loss associated with this disease in the rice rat and a high-sucrose
diet increases the severity of periodontitis. A case of kyphosis
has been observed in a North Carolina marsh rice rat.
, densities may exceed 200 per ha (80 per acre) when flooding concentrates populations on small islands, In the Florida Keys, population density is less than 1 per ha (0.4 per acre). On Breton Island
, Louisiana, perhaps an atypical habitat, home ranges in males average about 0.37 ha (0.81 acre) and in females about 0.23 ha (0.51 acre). A study in Florida found male home ranges to average 0.25 ha (0.56 acre) and female 0.33 ha (0.72 acre).
Population size is usually largest during the summer and declines during winter, although populations in Texas and Louisiana may be more seasonally stable. Animals also often lose weight during winter. Population size varies dramatically from year to year in southern Texas. In coastal Mississippi, a study found that storms probably do not cause the population to decline substantially and in Texas inundation of its habitat did not significantly influence population density. However, another study in Mississippi found that flooding did cause a marked decline in rice rat abundance.
In the northern part of its range, the species often occurs with the meadow vole
(Microtus pennsylvanicus), but there is no evidence that they compete with each other. In the south, the hispid cotton rat
(Sigmodon hispidus) and the rice rat regularly occur together; water levels are known to influence relative abundance of these two species in Florida. The cotton rat is mainly active during the day, which may help differentiate its niche
from that of the rice rat.
The marsh rice rat takes both vegetable and animal food, and is more carnivorous than most small rodents are; dominant food items vary seasonally. Plants eaten include species of Spartina, Salicornia, Tripsacum
, and Elymus
, among others; it mainly eats seeds and succulent parts. A 2004 study found that it prefers Spartina alterniflora
that have been fertilized with nitrogen and mainly eats the inner tissue of the stem, perhaps because nitrogen-fertilized plants contain much less dimethylsulfoniopropionate
in their inner tissues. The marsh rice rat was a major pest on rice
plantations, feeding on the rice when it was newly planted. It also eats the fungus Endogone at times.
Animals that are important to the marsh rice rat's diet include insects, fiddler crab
s, and snails, but the species is known to eat many other animals, including fish, clams, and juvenile Graptemys
and Chrysemys turtles. They scavenge on carcasses of muskrats, deermice, and sparrows, and may be the most important predator on eggs and young of the marsh wren. Rice rats also eat eggs and young of the seaside sparrow
(Ammodramus maritimus) and are aggressive towards the sparrow, apparently leading it to avoid nesting in Juncus
in a seaside salt marsh in Florida. On islands in North Carolina, rice rats consume eggs of Forster's tern
(Sterna forsteri). They have been observed preying on alligator eggs in Georgia.
Laboratory studies have found that rice rats assimilate
88% to 95% of the energy in their food. They lose weight when fed on Spartina, fiddler crabs, or sunflower seeds alone, but a diet consisting of several of those items or of mealworms is adequate to maintain weight. In an experiment, marsh rice rats did not show hoarding
behavior, but wild rice rats have been observed carrying food to a nest. Even when they live in uplands, they mostly eat water plants and animals, although they consume some upland plants.
ranges from 6 to 9 days, with an average of 7.72 days. Estrus occurs again after a litter is born. A 1970 study reported that copulatory behavior in the marsh rice rat is similar to that in laboratory brown rats. Before mating starts, "the male pursues the running female from behind." The male then repeatedly mounts and dismounts the female; not all mounts result in an ejaculation. Penetrations only last for about 250 ms, but during mating the penetrations and the intervals between them become longer. Even when a male is satiated after mating, it is able to copulate again when a new female is introduced (the Coolidge effect
). Partly because of resistance by the female, the frequency of ejaculation during mating is rather low in marsh rice rats as compared to laboratory rats, hamster
s, and deermice.
After a gestation of about 25 days, three to five young are usually born, although litter sizes vary from one to seven. Females may have up to six litters a year. Newborns weigh 3 to 4 g (about 0.1 to 0.15 oz) and are blind and almost naked. About as many males as females are born. The external ears (pinnae) soon unfold and on the first day claws are visible and the young emit high-pitched squeaks. On the second day, they are able to crawl and during the third to fifth days the whiskers and eyelids develop. On the two subsequent days, the mammae and incisors become visible and the animals become more active. Between the eighth and eleventh day, the eyes open, the fur develops, and the young begin to take solid food. Weaning occurs on the eleventh to twentieth day according to different studies. There is considerable variation in reported body masses at different ages, perhaps because of geographic variation. Sexual activity commences when the animals are about 50 to 60 days old. In the wild, rice rats usually live for less than a year; one study suggested that the average lifespan is only seven months.
The marsh rice rat is the primary host of the Bayou virus (BAYV), the second most common agent of hantavirus
infections in the United States. About 16% of animals are infected and the virus is most prevalent in old, heavy males. The virus may be transmitted among rice rats through bites inflicted during fights. It is also present in rice rat saliva and urine and human infections may occur because of contact with these excreta. Two related hantaviruses, Catacama virus and Playa de Oro virus
, are known from Oryzomys couesi in Honduras and western Mexico, respectively. An arenavirus
normally associated with woodrats (Neotoma) has also been found in Florida marsh rice rats. Antibodies against Borrelia burgdorferi
, the bacterium that causes Lyme disease
in the United States, have been found in marsh rice rats in Virginia, Maryland, North Carolina, and Tennessee. Another pathogenic bacterium, Bartonella
, is known from Georgia marsh rice rats.
The 2009 IUCN Red List
assesses the conservation status of the marsh rice rat as "Least Concern
", because it is a common, widespread, and stable species without major threats that occurs in several protected area
s. The Florida Keys form is rare and in decline and is threatened by competition with the black rat, predation by domestic cats, habitat loss, and loss of genetic variation; it is considered endangered. At the northern edge of its distribution, the marsh rice rat is listed as threatened in Illinois and whether it persists in Pennsylvania is unclear; it probably formerly occurred in tidal marshes on the Delaware River
. In Illinois, its population may have regenerated because wetlands have been developed to protect waterfowl and shorebirds and because suitable wetlands often develop in abandoned coal-mining operations. A 2001 study projected that climate change
would reduce the range of the marsh rice rat in Texas, where it is now common but may become threatened by habitat loss in the future. A study at the Paducah Gaseous Diffusion Plant
found that rice rats accumulate more PCBs
but less heavy metal than white-footed mice
(Peromyscus leucopus).
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...
. It is found mostly in the eastern and southern 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 northeasternmost 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; its range previously extended further west and north, where it may have been a commensal
Commensalism
In ecology, commensalism is a class of relationship between two organisms where one organism benefits but the other is neutral...
in corn-cultivating communities. It usually occurs in wet habitats such as swamps and saltmarshes. Weighing about 40 to 80 g (1.4 to 2.8 oz), the marsh rice rat is a medium-sized rodent that resembles the common black
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....
. The upperparts are generally gray-brown, but reddish in many Florida populations. The feet show several specializations for life in the water. The skull is large and flattened and is short at the front.
John Bachman
John Bachman
The Rev. John Bachman was an American Lutheran minister, social activist and naturalist who collaborated with J.J. Audubon to produce Viviparous Quadrapeds of North America and whose writings, particularly Unity of the Human Race, were influential in the development of the theory of evolution. He...
discovered the marsh rice rat in 1816, and it was formally described in 1837. Several 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...
have been described since the 1890s, mainly from Florida, but there is disagreement over their validity. The 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...
population is sometimes classified as a different species, the silver rice rat (Oryzomys argentatus). 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...
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 :...
gene indicate a deep divergence between populations east of Mississippi and those further west, which suggests that the western populations may be recognized as a separate species, Oryzomys texensis. The species is part of 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...
, which also includes several others occurring further south in Mexico, Central America, and northwestern South America, some of which have previously been regarded as subspecies of the marsh rice rat. One, Oryzomys couesi, occurs with the marsh rice rat in Tamaulipas and 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...
.
The marsh rice rat is active during the night, makes nests of sedge and grass, and occasionally builds runways. It has a diverse diet that includes plants, fungi, and a variety of animals. Population densities are usually below 10 per ha (4 per acre) and home ranges vary from 0.23 to 0.37 ha (0.57 to 0.91 acres), depending on sex and geography. Litters of generally three to five young are born after a pregnancy of about 25 days, mainly during the summer. Newborns are helpless at birth, but are weaned after a few weeks. Several animals prey on the marsh rice rat, including 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...
, and it usually lives for less than a year in the wild. It is infected by many different parasites and harbors a 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...
that also infects humans. The species is not of conservation concern, but some populations are threatened.
Taxonomy
The marsh rice rat is classified as one of eight species in 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...
, which is distributed from the eastern United States (marsh rice rat) into northwestern South America (O. 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...
). 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, most of which occur in South and Central America. In the United States, the marsh rice rat is the only oryzomyine rodent except for Oryzomys couesi in a small area of 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...
; the only other sigmodontines present are several species of cotton rat
Cotton rat
A cotton rat is any member of the rodent genus Sigmodon. They are called cotton rats because they build their nests out of cotton, and can damage cotton crops. Cotton rats have small ears and dark coats, and are found in North and South America....
s (Sigmodon) in the southern half of the country.
Early history
The marsh rice rat was discovered in 1816 in South CarolinaSouth Carolina
South Carolina is a state in the Deep South of the United States that borders Georgia to the south, North Carolina to the north, and the Atlantic Ocean to the east. Originally part of the Province of Carolina, the Province of South Carolina was one of the 13 colonies that declared independence...
by John Bachman
John Bachman
The Rev. John Bachman was an American Lutheran minister, social activist and naturalist who collaborated with J.J. Audubon to produce Viviparous Quadrapeds of North America and whose writings, particularly Unity of the Human Race, were influential in the development of the theory of evolution. He...
. Bachman intended to describe the species as Arvicola oryzivora, but sent a specimen to Richard Harlan
Richard Harlan
Richard Harlan was an American naturalist, zoologist, physicist and paleontologist....
and Charles Pickering
Charles Pickering (naturalist)
Charles Pickering was an American naturalist.Born in Susquehanna County, Pennsylvania, the grandson of Colonel Timothy Pickering, he grew up in Wenham, Massachusetts and received a medical degree from Harvard University in 1823...
at the Academy of Natural Sciences
Academy of Natural Sciences
The Academy of Natural Sciences of Drexel University, formerly Academy of Natural Sciences of Philadelphia, is the oldest natural science research institution and museum in the New World...
in Philadelphia to confirm its identity. Another specimen, 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...
, was found in the Academy's collection, and Harlan took it upon himself, against Pickering's wishes, to describe the new species as Mus palustris, proclaiming it one of the few true rats of the United States. The specific name palustris is Latin for "marshy" and refers to the usual habitat of the species.
In 1854, in The quadrupeds of North America, Bachman redescribed it as Arvicola oryzivora, considering it more closely related to the vole
Vole
A vole is a small rodent resembling a mouse but with a stouter body, a shorter hairy tail, a slightly rounder head, smaller ears and eyes, and differently formed molars . There are approximately 155 species of voles. They are sometimes known as meadow mice or field mice in North America...
s then placed in the genus Arvicola
Arvicola
The water voles are large voles in the genus Arvicola. They are found in both aquatic and dry habitat through Europe and much of northern Asia...
, and also recorded it from Georgia
Georgia (U.S. state)
Georgia is a state located in the southeastern United States. It was established in 1732, the last of the original Thirteen Colonies. The state is named after King George II of Great Britain. Georgia was the fourth state to ratify the United States Constitution, on January 2, 1788...
and 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...
. Three years later, 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...
argued that the referral of the species to Arvicola was erroneous and introduced a new generic name for the marsh rice rat, Oryzomys. The name combines the Greek oryza "rice" and mys "mouse" and refers to the rat's habit of eating rice. At the time, Oryzomys was recognized either as a full genus or 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...
, but since the 1890s, it has been universally recognized as a genus distinct from Hesperomys, with the marsh rice rat (Oryzomys palustris) as its 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...
.
Species boundaries and subspecies
In the 1890s, several subspeciesSubspecies
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...
of the marsh rice rat were described from the United States: natator from Florida in 1893, texensis from Texas in 1894, and coloratus from elsewhere in Florida in 1898. 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...
recognized natator as a separate species in 1901 and described a subspecies of it, floridanus, but considered texensis to be nearly identical to nominate palustris. In his 1918 revision of North American Oryzomys, 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....
again recognized all these as a single species, Oryzomys palustris. He distinguished four subspecies, which he said formed a "closely intergrading series"—O. p. palustris from New Jersey to southeastern Mississippi and eastern Missouri; O. p. natator in central Florida; O. p. coloratus (including O. natator floridanus Merriam) in southern Florida; and O. p. texensis from western Mississippi and southeastern Kansas to eastern Texas. Two additional subspecies were described by William J. Hamilton in 1955 from southern Florida: planirostris from Pine Island
Pine Island (Lee County, Florida)
Pine Island is an island located in Lee County, Florida, on the Gulf coast of southwest Florida. It is the largest island in the state of Florida, and the 118th largest island in the United States. The Intracoastal Waterway passes through Pine Island Sound, to the west of the island. Matlacha Pass...
and two miles (3 km) north of Fort Myers
Fort Myers, Florida
Fort Myers is the county seat and commercial center of Lee County, Florida, United States. Its population was 62,298 in the 2010 census, a 29.23 percent increase over the 2000 figure....
and sanibeli from Sanibel Island
Sanibel Island
Sanibel Island is an island located on the Gulf coast of Florida, just offshore of Fort Myers. In 2000, it had an estimated population of 6,064 people...
. Also in 1955, Claude W. Hibbard
Claude W. Hibbard
Claude W. Hibbard, popularly known as Hibbie , was an American paleontologist....
described a new species of Oryzomys, O. fossilis, from 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 ....
deposits in 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...
, on the basis of small differences in characters of the tooth with living marsh rice rats. In 1965, Walter Dalquest demoted this species, later also found in Texas, to a subspecies, because it does not differ more from living marsh rice rats than the latter differ from each other.
Merriam and Goldman had recognized that a number of Central American species, including Oryzomys couesi and numerous forms with more limited distributions, are related to the marsh rice rat. O. couesi ranges north to southernmost Texas, where its distribution meets that of the marsh rice rat. In 1960, Raymond Hall argued that specimens from the contact zone were intermediate between the local forms of O. couesi and the marsh rice rat, and accordingly included the former in the marsh rice rat. While reporting on the ecology of Texan O. couesi in 1979, Benson and Gehlbach noted that populations of O. couesi and the marsh rice rat there were in fact distinct, with the latter being smaller and less brown and more gray in color; their 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...
s were also distinct. Since then, the two have generally been retained as distinct species, as supported by further research; a 1994 study even found the two to occur at some of the same places (in sympatry
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...
) in southern 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.
In 1973, rice rats were discovered at Cudjoe Key in the 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...
, and in 1978 Spitzer and Lazell described this population as a new species, Oryzomys argentatus. The status of this form—either a distinct species or not even subspecifically distinct from O. palustris natator—has remained controversial since; the 2005 third edition of Mammal Species of the World
Mammal Species of the World
Mammal Species of the World, now in its 3rd edition, is a standard reference work in zoology giving descriptions and bibliographic data for the known species of mammals.An updated Third Edition of Mammal Species of the World was published late in 2005:...
does not recognize O. argentatus as a separate species, but acknowledges a need for further research. A 2005 study using microsatellite DNA found that Florida Keys rice rats exhibit low genetic variation and are significantly different from Everglades
Everglades
The Everglades are subtropical wetlands in the southern portion of the U.S. state of Florida, comprising the southern half of a large watershed. The system begins near Orlando with the Kissimmee River, which discharges into the vast but shallow Lake Okeechobee...
rice rats; the study concluded in favor of classifying the Keys rice rat as a "distinct vertebrate population". This population probably diverged from mainland rice rats about 2000 years ago.
Among the described subspecies, a 1989 morphometrical study by Humphrey and Setzer separated only two—natator from much of Florida (including coloratus, planirostris, sanibeli, and floridanus, as well as argentatus) and palustris from the rest of the range (including texensis). On the other hand, Whitaker and Hamilton in their 1998 book on the Mammals of the Eastern United States recognized planirostris and sanibeli as separate subspecies, but merged all others into O. palustris palustris, and placed O. argentatus as a separate species; their classification was based on their emphasis of overwater gaps as agents of biological diversification and a critique of shortcomings in Humphrey and Setzer's study, not on a reanalysis of the data.
In 2010, Delton Hanson and colleagues published a study of the relationships among populations of Oryzomys on the basis of data from three genes—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). The Cytb data placed all marsh rice rats studied sister to 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...
containing various populations of O. couesi; the mean genetic distance between the two groups was 11.30%. The marsh rice rats fell into two main groups, differing on average by 6.05%, one containing animals from Mississippi, southwestern Tennessee, and further west, and the other including specimens from Alabama and further east. Within the eastern group, variation was only about 0.65%, even though examples of the putative subspecies palustris, coloratus, sanibeli, and planirostris were all included. 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 the marsh rice rat as separate clades. In addition, Adh1-I2 placed a Costa Rican population within the marsh rice rat clade and some other southern Oryzomys specimens closer to the marsh rice rat than to the O. couesi group. The combined dataset supported the western and eastern clades within the marsh rice rat 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 western populations of the marsh rice rat be recognized as a separate species, Oryzomys texensis. They recommended further research in the Mississippi–Alabama–Tennessee region, where the ranges of the two meet.
Common names
Many 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 the marsh rice rat. Early describers used "Rice Meadow-Mouse" and "Rice-field Mouse" and in the early 1900s, name such as "rice rat", "marsh mouse", and "swamp rice rat" came into use. Some of the subspecies received their own common names, such as "Florida Marsh Mouse", "Swimming Rice Rat", and "Central Florida Rice Rat" for O. p. natator; "Bangs' Marsh Mouse", "Cape Sable Rice Rat", and "Everglades Rice Rat" for O. p. coloratus; and "Texas Rice Rat" for O. p. texensis. The species is now usually known as the "marsh rice rat", although "marsh oryzomys" has also been in recent use. The Florida Keys form (argentatus) is known as the "silver rice rat".
Description
| Population | n | Total length | Tail length | Hindfoot length |
|---|---|---|---|---|
| O. p. palustris (New Jersey) | 4 | 242 (237–245) | 112 (109–116) | 31 (30–31.5) |
| O. p. natator (Florida) | 10 | 281.2 (246–318) | 140.6 (122–173) | 33.1 (28–37) |
| O. p. coloratus (Florida) | 11 | 283.0 (250–326) | 143.5 (123–171) | 33.4 (31–38) |
| O. p. texensis (Texas) | 8 | 242 (226–279) | 120 (108–133) | 29 (28.5–30.5) |
| O. p. planirostris (Florida) | 14 | 247.5 (226–266) | 129.6 (108–128) | 31 (29–33) |
| O. p. sanibeli (Florida) | 11 | 257.5 (233–274) | 123.6 (111–138) | 31.0 (29–33) |
| O. argentatus (Florida Keys) | 2 | 251, 259 | 121, 132 | 32, 32 |
| Measurements are all in millimeters and are in the form "average (minimum–maximum)", except those of the Florida Keys population. n=Number of specimens measured. |
||||
The marsh rice rat is a medium-sized rodent that looks much like the common black
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....
s, but has greater differences in color between the upper- and underparts. The fur is thick and short. The upperparts are generally gray to grayish brown, with the head a bit lighter, and are sharply delimited from the underparts, which are off-white, as are the feet. There are small cheek pouches. The ears are about the same color as the upperparts, but there is a patch of light hairs in front of them. The tail is dark brown above and may be paler below. The guard hair
Guard hair
Guard hairs are the longest, coarsest hairs in a mammal's coat, forming the topcoat . They taper to a point and protect the undercoat from the elements. They are often water repellent and stick out above the rest of the coat...
s are long and have unpigmented, silvery tips. When rice rats swim, air is trapped in the fur, which increases buoyancy and reduces heat loss. As in most other oryzomyines, females have eight mammae.
The forefeet have four and the hindfeet five digits. 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 absent. The hindfeet are broad and have a short fifth digit. Many of the pads are reduced, as are the ungual tufts, but there are 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...
. The Florida Keys form, argentatus, has even more reduced ungual tufts. Many of these traits are common adaptations to life in the water in oryzomyines.
There is some geographic variation in fur color: western populations (texensis) are lighter than those from the east (nominate palustris), and Florida populations are generally more tawny or reddish than either, with those from southern Florida (coloratus) being brighter than those from the center of the state (natator). The Florida Keys form (argentatus) is silvery, and the two other Florida forms—planirostris and sanibeli—lack the reddish tones of mainland Florida populations and are instead grayish, resembling nominate palustris (planirostris), or brownish (sanibeli). In 1989, Humphrey and Setzer reviewed variation in color among Florida populations. They found argentatus to be substantially lighter and planirostris and sanibeli to be somewhat darker than mainland populations, and argentatus to have a less yellow fur, but found no significant differences in redness. There was also substantial variation within populations.
Total length is 226 to 305 mm (8.9 to 12.0 in), tail length 108 to 156 mm (4.3 to 6.1 in), hindfoot length 28 to 37 mm (1.1 to 1.5 in), and body mass 40 to 80 g (1.4 to 2.8 oz), with males slightly larger than females. The largest individuals occur in Florida and along the Gulf Coast east of the Mississippi River delta.
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 chromosomes and a fundamental number of 60 chromosomal arms (2n = 56, FN = 60). The form of the sex chromosomes has been used to distinguish the marsh rice rat from Oryzomys couesi, but may be too variable among Oryzomys to be useful in differentiating species. X chromosome inactivation
X-inactivation
X-inactivation is a process by which one of the two copies of the X chromosome present in female mammals is inactivated. The inactive X chromosome is silenced by packaging into transcriptionally inactive heterochromatin...
occurs in the marsh rice rat, even though the animal lacks LINE-1 retrotransposons, which have been suggested as components of the inactivation process. Mutants with fused or additional molars and with light fur have been recorded in laboratory colonies; the abnormal molars are apparently the result of a single autosomal recessive mutation. At about 50%, hematocrit
Hematocrit
The hematocrit or packed cell volume or erythrocyte volume fraction is the percentage of the concentration of red blood cells in blood. It is normally about 45% for men and 40% for women...
(the proportion of red blood cells in the blood) is high in the marsh rice rat compared to other rodents; this may be an adaptation that enables the rice rat to increase oxygen capacity while swimming underwater.
Male reproductive anatomy
The glans penisGlans penis
The glans penis is the sensitive bulbous structure at the distal end of the penis. The glans penis is anatomically homologous to the clitoral glans of the female...
is long and robust, averaging 7.3 mm (0.29 in) long and 4.6 mm (0.18 in) broad, and 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) is 6.6 mm (0.26 in) long. As is characteristic of Sigmodontinae, the marsh rice rat has a complex penis, with the distal (far) end of the baculum ending in three digits. The central digit is notably larger than those 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 the marsh rice rat 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 Oryzomys couesi 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 O. couesi and Holochilus brasiliensis. The baculum is deeper than it is wide.
Some features of the accessory glands in the male genital region vary among oryzomyines. In the marsh rice rat, a single pair of preputial gland
Preputial gland
Preputial glands are exocrine glands that are located in front of the genitals of some mammals and produce pheromones. The preputial glands of female animals are sometimes called clitoral glands....
s is present at the penis. As is usual for sigmodontines, there are two pairs of ventral prostate
Prostate
The prostate is a compound tubuloalveolar exocrine gland of the male reproductive system in most mammals....
glands and a single pair of anterior and dorsal prostate glands. Part of the end of the vesicular gland is irregularly folded, not smooth as in most oryzomyines.
Skull
The marsh rice rat has a large, flattened skull with a short and broad rostrum (front part). The nasalNasal 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....
and premaxillary bones extend back beyond the point where the lacrimal
Lacrimal 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. In planirostris, the rostrum is flatter than in mainland Florida forms, in which it is more convex, and the nasals are said to be relatively longer in argentatus. 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
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...
(cheekbone), is broad and develops a notch at its front end. The arches themselves are robust and contain small but distinct jugal bones. 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) in the side of the skull above the molars, is large; it is much smaller in O. couesi. The narrowest part 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....
(located between the eyes) is towards the front and the edges are lined by prominent shelves. The marsh rice rat has a narrow braincase lined by prominent ridges and a narrow interparietal bone. According to Goldman, Florida animals (coloratus and natator) generally have the largest and broadest skulls, and western specimens (texensis) have somewhat smaller and narrower skulls than those from the east outside Florida (nominate palustris). In argentatus, the skull is also relatively narrow.
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
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...
, reach backward between the molars. The palate is long, extending substantially beyond the third molars. 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). The mesopterygoid fossa, the gap behind the end of the palate, is perforated by 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...
, which are set far to the front. 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 present, but not as large as in O. couesi.
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"....
), 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 well-developed and strongly 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 the marsh rice rat 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).
The upper molars have two longitudinal rows of cusps, not three as in the black and brown rats. The first and second upper molar are oval in form and 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), but does display a hollow in the middle, the anteromedian fossette, which divides it into separate cuspules at the labial and lingual (inner) sides of the molar. A crest, the anteroloph, is present behind the labial cuspule, but in older animals, the cusps and the crest are united into a single structure by wear. In the third upper molar, the cusps at the back are reduced and scarcely distinguishable. 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.
The first lower molar is rounded at the front end and the labial and lingual conules of the anteroconid, the frontmost cusp, are barely distinct. The second lower molar is elongate and has a crest, the anterolophid, before the two cusps that form the front edge of the molar in some other oryzomyines, the protoconid and metaconid. There is a distinct ridge (anterolabial cingulum) at the outer front (anterolabial) edge of the molar, before the protoconid. The lower third molar is about as long as the second and also has an anterolophid, albeit a less well-defined one. The first lower molar has large roots at the front and back of the tooth and usually one or two smaller ones in between, at the labial and lingual side. The second and third lowers molars have either two roots, one labial and one lingual, or only one at the front, and another large root 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).
Physiology
Studies have shown that in poor conditions the weight of the adrenal glandAdrenal gland
In mammals, the adrenal glands are endocrine glands that sit atop the kidneys; in humans, the right suprarenal gland is triangular shaped, while the left suprarenal gland is semilunar shaped...
may increase up to 200%, that rice rats are unable to conserve water well when dehydrated
Dehydration
In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...
, and that in water contaminated with oil they swim less and their mortality increases. The median amount of radiation needed to kill a marsh rice rat is 5.25 Gy
Gray (unit)
The gray is the SI unit of absorbed radiation dose of ionizing radiation , and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter ....
and the lethal dose of potassium cyanide
Potassium cyanide
Potassium cyanide is an inorganic compound with the formula KCN. This colorless crystalline compound, similar in appearance to sugar, is highly soluble in water. Most KCN is used in gold mining, organic synthesis, and electroplating. Smaller applications include jewelry for chemical gilding and...
is 7.20 mg/kg; both values are relatively low for cricetid rodents. In one study, wild rice rats in radioactively contaminated areas did not show signs of disease. Experiments have found that exposure to more daylight and higher food availability cause increased development of the gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s in both adult and juvenile rice rats. When the pineal gland
Pineal gland
The pineal gland is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions...
is removed or melatonin
Melatonin
Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...
is administered in male rice rats, the testes are reduced and tend to regress into the body.
Distribution and habitat
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. The northernmost records in the interior United States are in eastern Oklahoma
Oklahoma
Oklahoma is a state located in the South Central region of the United States of America. With an estimated 3,751,351 residents as of the 2010 census and a land area of 68,667 square miles , Oklahoma is the 28th most populous and 20th-largest state...
, southeastern 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...
, southern Missouri
Missouri
Missouri is a US state located in the Midwestern United States, bordered by Iowa, Illinois, Kentucky, Tennessee, Arkansas, Oklahoma, Kansas and Nebraska. With a 2010 population of 5,988,927, Missouri is the 18th most populous state in the nation and the fifth most populous in the Midwest. It...
and Illinois
Illinois
Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...
, and the southern half of Kentucky
Kentucky
The Commonwealth of Kentucky is a state located in the East Central United States of America. As classified by the United States Census Bureau, Kentucky is a Southern state, more specifically in the East South Central region. Kentucky is one of four U.S. states constituted as a commonwealth...
, but the species is absent in much of the Appalachians. Fossils of the marsh rice rat are known from 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...
(late Pleistocene, less than 300,000 years ago) deposits in Florida and Georgia and remains referred to the extinct subspecies O. p. fossilis are from the Wisconsinan
Wisconsinan glaciation
The Wisconsin Glacial Episode was the most recent major advance of the North American Laurentide ice sheet. Globally, this advance is known as the last glacial period. The Wisconsin glaciation extended from approximately 110,000 to 10,000 years ago, between the Eemian interglacial and the current...
and Sangamonian of Texas and Illinoian and Sangamonian of Kansas. In the Florida Keys, rice rats occur on most of the Lower Keys, but are absent from the Upper Keys, which are of a different geological origin and were probably never connected to the mainland. The western and eastern Cytb clades within the marsh rice rat may represent expansions from different glacial refugia which the species was restricted to during a glacial period
Glacial period
A glacial period is an interval of time within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate within an ice age...
.
Cave and archeological remains indicate that the range of the marsh rice rat has extended substantially further north and west earlier in the Holocene
Holocene
The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...
, into central Texas, eastern Nebraska
Nebraska
Nebraska is a state on the Great Plains of the Midwestern United States. The state's capital is Lincoln and its largest city is Omaha, on the Missouri River....
, southwestern 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...
, central Illinois, southern Indiana
Indiana
Indiana is a US state, admitted to the United States as the 19th on December 11, 1816. It is located in the Midwestern United States and Great Lakes Region. With 6,483,802 residents, the state is ranked 15th in population and 16th in population density. Indiana is ranked 38th in land area and is...
, southern Ohio
Ohio
Ohio is a Midwestern state in the United States. The 34th largest state by area in the U.S.,it is the 7th‑most populous with over 11.5 million residents, containing several major American cities and seven metropolitan areas with populations of 500,000 or more.The state's capital is Columbus...
, West Virginia
West Virginia
West Virginia is a state in the Appalachian and Southeastern regions of the United States, bordered by Virginia to the southeast, Kentucky to the southwest, Ohio to the northwest, Pennsylvania to the northeast and Maryland to the east...
, 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...
. Most northern archeological sites date from about 1000 CE and are associated with corn
Maize
Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...
cultivation, but in some older cave sites the rice rat is found with the extinct giant armadillo Dasypus bellus, suggesting warm climatic conditions. Perhaps a warm period during the Quaternary
Quaternary
The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...
enabled the rice rat to disperse northward and when the climate cooled, relict
Relict
A relict is a surviving remnant of a natural phenomenon.* In biology a relict is an organism that at an earlier time was abundant in a large area but now occurs at only one or a few small areas....
populations were able to survive in the north as commensals in corn-cultivating Native American communities. Some subfossil animals are slightly larger than living marsh rice rats, possibly because environmental constraints were relaxed in commensal populations.
In Tamaulipas and southern Texas, the ranges of the marsh rice rat and the related Oryzomys couesi meet; 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 experimental conditions, they 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 O. couesi, the marsh rice rat shows less genetic variability within but more between populations in the contact zone, probably because the species is restricted to isolated populations near the coast.
The marsh rice rat occurs in several habitats, ranging from coastal salt marshes to mountain streams and clearings. It is semiaquatic, spending much time in the water, and usually occurs in wetland habitats. It prefers areas where the ground is covered with grasses and sedges, which protect it from predators. In southern Illinois, marsh rice rats are more likely to occur in wetlands with more herbaceous cover, visual obstruction, and nearby grasslands. The species also occurs in drier uplands, which serve as sinks for young, dispersing animals and as refuges during high tide. Rice rats are adept overwater dispersers; studies on islands off Virginia
Virginia
The Commonwealth of Virginia , is a U.S. state on the Atlantic Coast of the Southern United States. Virginia is nicknamed the "Old Dominion" and sometimes the "Mother of Presidents" after the eight U.S. presidents born there...
's Delmarva Peninsula
Delmarva Peninsula
The Delmarva Peninsula is a large peninsula on the East Coast of the United States, occupied by most of Delaware and portions of Maryland and Virginia...
show that they readily cross 300-m (1000 ft) channels between islands.
Behavior and ecology
Marsh Wren
The Marsh Wren is a small North American songbird of the wren family. It is sometimes called Long-billed Marsh Wren to distinguish it from the Sedge Wren, also known as Short-billed Marsh Wren....
s (Cistothorus palustris), red-winged blackbird
Red-winged Blackbird
The Red-winged Blackbird is a passerine bird of the family Icteridae found in most of North and much of Central America. It breeds from Alaska and Newfoundland south to Florida, the Gulf of Mexico, Mexico, and Guatemala, with isolated populations in western El Salvador, northwestern Honduras, and...
s (Agelaius phoeniceus), muskrat
Muskrat
The muskrat , the only species in genus Ondatra, is a medium-sized semi-aquatic rodent native to North America, and introduced in parts of Europe, Asia, and South America. The muskrat is found in wetlands and is a very successful animal over a wide range of climates and habitats...
s (Ondatra zibethicus) or round-tailed muskrat
Round-Tailed Muskrat
The Round-tailed Muskrat is a species of rodent in the family Cricetidae, sometimes called the Florida water rat. It is the only species in the genus Neofiber....
s (Neofiber alleni). Marsh rice rats sometimes make large runways or dig burrows. They are accomplished and willing swimmers, easily swimming for more than 10 m (30 ft) under water, and will often seek safety in the water when alarmed. Rice rats in the Florida Keys occasionally climb in vegetation, but never higher than 90 cm (3 ft). Marsh rice rats are very cleanly and extensively groom
Personal grooming
Personal grooming is the art of cleaning, grooming, and maintaining parts of the body. It is a species-typical behavior that is controlled by neural circuits in the brain.- In humans :...
themselves, perhaps to keep their fur water-repellent. They are aggressive towards conspecifics and emit high-pitched squeaks while fighting. In dense vegetation, the perceptual range (the distance from which an animal can detect a patch of suitable habitat) of marsh rice rats is less than 10 m (33 ft). When released outside of their natural wetland habitat, marsh rice rats generally move either upwind or downwind (anemotaxis), perhaps in order to move in a straight line, which is an efficient strategy to find suitable habitat.
Many animals prey on marsh rice rats. 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 among the most important; one study found that 97.5% of vertebrate remains in barn owl pellets were marsh rice rats. Other predators include birds (marsh hawks, Circus cyaneus; and barred owl
Barred Owl
The Barred Owl is a large typical owl. It goes by many other names, including eight hooter, rain owl, wood owl, and striped owl, but is probably best known as the hoot owl.-Description:...
s, Strix varia), snakes (cottonmouth mocassins, Agkistrodon piscivorus; and others), alligators
American Alligator
The American alligator , sometimes referred to colloquially as a gator, is a reptile endemic only to the Southeastern United States. It is one of the two living species of alligator, in the genus Alligator, within the family Alligatoridae...
, and carnivora
Carnivora
The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...
ns (raccoon
Raccoon
Procyon is a genus of nocturnal mammals, comprising three species commonly known as raccoons, in the family Procyonidae. The most familiar species, the common raccoon , is often known simply as "the" raccoon, as the two other raccoon species in the genus are native only to the tropics and are...
s, Procyon lotor; red fox
Red Fox
The red fox is the largest of the true foxes, as well as being the most geographically spread member of the Carnivora, being distributed across the entire northern hemisphere from the Arctic Circle to North Africa, Central America, and the steppes of Asia...
es, Vulpes vulpes; mink
Mink
There are two living species referred to as "mink": the European Mink and the American Mink. The extinct Sea Mink is related to the American Mink, but was much larger. All three species are dark-colored, semi-aquatic, carnivorous mammals of the family Mustelidae, which also includes the weasels and...
s, Neovison vison; weasel
Weasel
Weasels are mammals forming the genus Mustela of the Mustelidae family. They are small, active predators, long and slender with short legs....
s of the genus Mustela; and striped skunk
Striped Skunk
The striped skunk, Mephitis mephitis, is an omnivorous mammal of the skunk family Mephitidae. Found over most of the North American continent north of Mexico, it is one of the best-known mammals in Canada and the United States.-Description:...
s, Mephitis mephitis). Many parasites have been recorded on the marsh rice rat, including various ticks and mites, lice
Louse
Lice is the common name for over 3,000 species of wingless insects of the order Phthiraptera; three of which are classified as human disease agents...
, and flea
Flea
Flea is the common name for insects of the order Siphonaptera which are wingless insects with mouthparts adapted for piercing skin and sucking blood...
s among external parasites and many 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 and digenea
Digenea
Digenea is a subclass within the Platyhelminthes consisting of parasitic flatworms with a syncytial tegument and, usually, two suckers, one ventral and one oral. Adults are particularly common in the digestive tract, but occur throughout the organ systems of all classes of vertebrates...
ns, a pentastomid, and several coccidia
Coccidia
Coccidia is a subclass of microscopic, spore-forming, single-celled obligate parasites belonging to the apicomplexan class Conoidasida. Coccidian parasites infect the intestinal tracts of animals, and are the largest group of apicomplexan protozoa....
ns among internal parasites (see parasites of the marsh rice rat).
Periodontitis, a bacterial disease affecting the jaws, is particularly virulent in marsh rice rats; the animal has been proposed as a model for research on the disease in humans. The identity of the bacterial agent remains unknown. Vitamin E
Vitamin E
Vitamin E is used to refer to a group of fat-soluble compounds that include both tocopherols and tocotrienols. There are many different forms of vitamin E, of which γ-tocopherol is the most common in the North American diet. γ-Tocopherol can be found in corn oil, soybean oil, margarine and dressings...
, fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
, and iodide
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
protect against bone loss associated with this disease in the rice rat and a high-sucrose
Sucrose
Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...
diet increases the severity of periodontitis. A case of kyphosis
Kyphosis
Kyphosis , also called roundback or Kelso's hunchback, is a condition of over-curvature of the thoracic vertebrae...
has been observed in a North Carolina marsh rice rat.
Population dynamics
The population density of the marsh rice rat usually does not reach 10 per ha (4 per acre). The weather may influence population dynamics; in the EvergladesEverglades
The Everglades are subtropical wetlands in the southern portion of the U.S. state of Florida, comprising the southern half of a large watershed. The system begins near Orlando with the Kissimmee River, which discharges into the vast but shallow Lake Okeechobee...
, densities may exceed 200 per ha (80 per acre) when flooding concentrates populations on small islands, In the Florida Keys, population density is less than 1 per ha (0.4 per acre). On Breton Island
Breton Island
Breton Island is a small rocky island lying southwest of Empereur Island. It was charted in 1950 by the French Antarctic Expedition and named by them for their largely Breton crew....
, Louisiana, perhaps an atypical habitat, home ranges in males average about 0.37 ha (0.81 acre) and in females about 0.23 ha (0.51 acre). A study in Florida found male home ranges to average 0.25 ha (0.56 acre) and female 0.33 ha (0.72 acre).
Population size is usually largest during the summer and declines during winter, although populations in Texas and Louisiana may be more seasonally stable. Animals also often lose weight during winter. Population size varies dramatically from year to year in southern Texas. In coastal Mississippi, a study found that storms probably do not cause the population to decline substantially and in Texas inundation of its habitat did not significantly influence population density. However, another study in Mississippi found that flooding did cause a marked decline in rice rat abundance.
In the northern part of its range, the species often occurs with the meadow vole
Meadow Vole
The Meadow Vole , sometimes called the Field Mouse or Meadow Mouse, is a North American vole found across Canada, Alaska and the northern United States. Its range extends further south along the Atlantic coast. One subspecies, the Florida Salt Marsh Vole , is found in Florida, and is classified as...
(Microtus pennsylvanicus), but there is no evidence that they compete with each other. In the south, the hispid cotton rat
Hispid Cotton Rat
The Hispid Cotton Rat is a rodent species long thought to occur in parts of South America, Central America, and southern North America. However, recent taxonomic revisions, based on mitochondrial DNA sequence data, have split this widely distributed species into three separate species. Carroll et...
(Sigmodon hispidus) and the rice rat regularly occur together; water levels are known to influence relative abundance of these two species in Florida. The cotton rat is mainly active during the day, 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 rice rat.
Diet
Tripsacum
Tripsacum is a genus of grass in the Poaceae family.Species include:* T. andersonii* T. australe* T. cundinamarce* T. dactyloides* T. fasciculatum * T. floridanum...
, and Elymus
Elymus (genus)
Elymus is a genus of perennial grasses in the tribe which contains many of the main cereal grains. Plants of this genus are known generally as wildrye or wheatgrass...
, among others; it mainly eats seeds and succulent parts. A 2004 study found that it prefers Spartina alterniflora
Spartina alterniflora
Spartina alterniflora is a perennial deciduous grass which is found in intertidal wetlands, especially estuarine salt marshes. It grows 1-1.5 m tall, and has smooth, hollow stems which bear leaves up to 20-60 cm long and 1.5 cm wide at their base, which are sharply tapered and bend down...
that have been fertilized with nitrogen and mainly eats the inner tissue of the stem, perhaps because nitrogen-fertilized plants contain much less dimethylsulfoniopropionate
Dimethylsulfoniopropionate
Dimethylsulfoniopropionate , is an organosulfur compound with the formula 2S+CH2CH2COO−. This zwitterionic metabolite found in marine phytoplankton, seaweeds, and some species of terrestrial and aquatic vascular plants...
in their inner tissues. The marsh rice rat was a major pest on rice
Rice
Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...
plantations, feeding on the rice when it was newly planted. It also eats the fungus Endogone at times.
Animals that are important to the marsh rice rat's diet include insects, fiddler crab
Fiddler crab
A fiddler crab, sometimes known as a calling crab, may be any of approximately 100 species of semi-terrestrial marine crabs which make up the genus Uca. As members of the family Ocypodidae, fiddler crabs are most closely related to the ghost crabs of the genus Ocypode. This entire group is composed...
s, and snails, but the species is known to eat many other animals, including fish, clams, and juvenile Graptemys
Graptemys
Graptemys is a genus of turtles known commonly as map turtles or sometimes sawback turtles. They are aquatic, freshwater turtles that are found throughout the eastern half of the United States and northward into southern Canada. They superficially resemble many other species of aquatic turtle,...
and Chrysemys turtles. They scavenge on carcasses of muskrats, deermice, and sparrows, and may be the most important predator on eggs and young of the marsh wren. Rice rats also eat eggs and young of the seaside sparrow
Seaside Sparrow
The Seaside Sparrow, Ammodramus maritimus, is a small American sparrow. The 11 Ammodramus species inhabit marshes and grasslands.Adults have brownish upperparts with grey on the crown and nape, and a grayish buff colored breast with dark streaks; they have a dark face with grey cheeks, a white...
(Ammodramus maritimus) and are aggressive towards the sparrow, apparently leading it to avoid nesting in Juncus
Juncus
Juncus is a genus in the plant family Juncaceae. It consists of some 200 to 300 or more species of grassy plants commonly called rushes...
in a seaside salt marsh in Florida. On islands in North Carolina, rice rats consume eggs of Forster's tern
Forster's Tern
The Forster's Tern, Sterna forsteri, is a member of the tern family Sternidae. It breeds inland in North America and winters south to the Caribbean and northern South America....
(Sterna forsteri). They have been observed preying on alligator eggs in Georgia.
Laboratory studies have found that rice rats assimilate
Assimilation (biology)
Biological assimilation, or bio assimilation, is the combination of two processes to supply animal cells with nutrients. The first is the process of absorbing vitamins, minerals, and other chemicals from food within the gastrointestinal tract...
88% to 95% of the energy in their food. They lose weight when fed on Spartina, fiddler crabs, or sunflower seeds alone, but a diet consisting of several of those items or of mealworms is adequate to maintain weight. In an experiment, marsh rice rats did not show hoarding
Hoarding
Hoarding or caching is a general term for a behavior that leads people or animals to accumulate food or other items in anticipation of future need or scarcity.-Animal behavior:...
behavior, but wild rice rats have been observed carrying food to a nest. Even when they live in uplands, they mostly eat water plants and animals, although they consume some upland plants.
Reproduction and life cycle
Breeding occurs mostly during the summer. Some studies report that breeding ceases entirely during the winter, but winter breeding occurs as far north as Virginia. This is primarily because photoperiod influences their circadian rhythm which determines breeding. In both Texas and Virginia, there is less variation in reproductive activity in females than in males. In the south of its range, animals may breed less when the summer is at its warmest. The duration of the estrous cycleEstrous cycle
The estrous cycle comprises the recurring physiologic changes that are induced by reproductive hormones in most mammalian placental females. Estrous cycles start after puberty in sexually mature females and are interrupted by anestrous phases or pregnancies...
ranges from 6 to 9 days, with an average of 7.72 days. Estrus occurs again after a litter is born. A 1970 study reported that copulatory behavior in the marsh rice rat is similar to that in laboratory brown rats. Before mating starts, "the male pursues the running female from behind." The male then repeatedly mounts and dismounts the female; not all mounts result in an ejaculation. Penetrations only last for about 250 ms, but during mating the penetrations and the intervals between them become longer. Even when a male is satiated after mating, it is able to copulate again when a new female is introduced (the Coolidge effect
Coolidge effect
In biology and psychology, the Coolidge effect is a phenomenon—seen in nearly every mammalian species in which it has been tested—whereby males exhibit continuous sexual activity if they are introduced to receptive sexual partners, but will eventually stop having sex if the partner has already...
). Partly because of resistance by the female, the frequency of ejaculation during mating is rather low in marsh rice rats as compared to laboratory rats, hamster
Hamster
Hamsters are rodents belonging to the subfamily Cricetinae. The subfamily contains about 25 species, classified in six or seven genera....
s, and deermice.
| Age (days) | Body mass (g) | Body mass (oz) |
|---|---|---|
| 10 | 8–17 | 0.3–0.6 |
| 20 | 18–27 | 0.6–1.0 |
| 40 | 27–40 | 1.0–1.4 |
| 60 | 40–60 | 1.4–2.1 |
| 120 | 50–80 | 1.8–2.8 |
After a gestation of about 25 days, three to five young are usually born, although litter sizes vary from one to seven. Females may have up to six litters a year. Newborns weigh 3 to 4 g (about 0.1 to 0.15 oz) and are blind and almost naked. About as many males as females are born. The external ears (pinnae) soon unfold and on the first day claws are visible and the young emit high-pitched squeaks. On the second day, they are able to crawl and during the third to fifth days the whiskers and eyelids develop. On the two subsequent days, the mammae and incisors become visible and the animals become more active. Between the eighth and eleventh day, the eyes open, the fur develops, and the young begin to take solid food. Weaning occurs on the eleventh to twentieth day according to different studies. There is considerable variation in reported body masses at different ages, perhaps because of geographic variation. Sexual activity commences when the animals are about 50 to 60 days old. In the wild, rice rats usually live for less than a year; one study suggested that the average lifespan is only seven months.
Human interactions
The marsh rice rat is generally of little importance to humans, which is perhaps why it is not as well-studied as some other North American rodents. In 1931, Arthur Svihla noted that virtually no information had been published on the habits and life history of the marsh rice rat since the 1854 publication of Audubon and Bachman's description. Writing on Everglades mammals, Thomas E. Lodge notes that although the name "rat" may associate it unpleasantly with the introduced black and brown rats, its appearance is more endearing, even cute. J.S. Steward proposed the marsh rice rat as a model organism in 1951 to study certain infections that other rodents used at the time are not susceptible to. The marsh rice rat is quite susceptible to periodontitis and has been used as a model system for the study of that disease.The marsh rice rat is the primary host of the Bayou virus (BAYV), the second most common agent 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...
infections in the United States. About 16% of animals are infected and the virus is most prevalent in old, heavy males. The virus may be transmitted among rice rats through bites inflicted during fights. It is also present in rice rat saliva and urine and human infections may occur because of contact with these excreta. Two related hantaviruses, Catacama 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...
, are known from Oryzomys couesi in Honduras and western Mexico, respectively. An arenavirus
Arenavirus
Arenavirus is a genus of virus that infects rodents and occasionally humans. At least eight Arenaviruses are known to cause human disease. The diseases derived from Arenaviruses range in severity. Aseptic meningitis, a severe human disease that causes inflammation covering the brain and spinal...
normally associated with woodrats (Neotoma) has also been found in Florida marsh rice rats. Antibodies against Borrelia burgdorferi
Borrelia burgdorferi
Borrelia burgdorferi is a species of Gram negative bacteria of the spirochete class of the genus Borrelia. B. burgdorferi is predominant in North America, but also exists in Europe, and is the agent of Lyme disease....
, the bacterium that causes Lyme disease
Lyme disease
Lyme disease, or Lyme borreliosis, is an emerging infectious disease caused by at least three species of bacteria belonging to the genus Borrelia. Borrelia burgdorferi sensu stricto is the main cause of Lyme disease in the United States, whereas Borrelia afzelii and Borrelia garinii cause most...
in the United States, have been found in marsh rice rats in Virginia, Maryland, North Carolina, and Tennessee. Another pathogenic bacterium, Bartonella
Bartonella
Bartonella is a genus of Gram-negative bacteria. Facultative intracellular parasites, Bartonella species can infect healthy people but are considered especially important as opportunistic pathogens. Bartonella are transmitted by insect vectors such as ticks, fleas, sand flies and mosquitoes...
, is known from Georgia marsh rice rats.
The 2009 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 the conservation status of the marsh rice rat 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...
", because it is a common, widespread, and stable species without major threats that occurs in several 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. The Florida Keys form is rare and in decline and is threatened by competition with the black rat, predation by domestic cats, habitat loss, and loss of genetic variation; it is considered endangered. At the northern edge of its distribution, the marsh rice rat is listed as threatened in Illinois and whether it persists in Pennsylvania is unclear; it probably formerly occurred in tidal marshes on the Delaware River
Delaware River
The Delaware River is a major river on the Atlantic coast of the United States.A Dutch expedition led by Henry Hudson in 1609 first mapped the river. The river was christened the South River in the New Netherland colony that followed, in contrast to the North River, as the Hudson River was then...
. In Illinois, its population may have regenerated because wetlands have been developed to protect waterfowl and shorebirds and because suitable wetlands often develop in abandoned coal-mining operations. A 2001 study projected 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 reduce the range of the marsh rice rat in Texas, where it is now common but may become threatened by habitat loss in the future. A study at the Paducah Gaseous Diffusion Plant
Paducah Gaseous Diffusion Plant
The Paducah Gaseous Diffusion Plant is a facility located in McCracken County, Kentucky, near Paducah, Kentucky, that produces enriched uranium, for nuclear power plants. The plant is now operated by United States Enrichment Corporation, a subsidiary of USEC Incorporated, a publicly traded...
found that rice rats accumulate more PCBs
Polychlorinated biphenyl
Polychlorinated biphenyls are a class of organic compounds with 2 to 10 chlorine atoms attached to biphenyl, which is a molecule composed of two benzene rings. The chemical formula for PCBs is C12H10-xClx...
but less heavy metal than white-footed mice
White-footed mouse
White-footed Mouse is a rodent native to North America. It ranges from Ontario, Quebec, Labrador and the Maritime Provinces to the southwest USA and Mexico. It is also known as the Woodmouse, particularly in Texas.Adults are in length, not counting the tail, which can add another . A young adult...
(Peromyscus leucopus).
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