Marsh rice rat facts for kids

Quick facts for kids Marsh rice rat Temporal range: Rancholabrean (300,000 years before present) – present
Conservation status
Least Concern (IUCN 3.1)
Scientific classification
Genus:	Oryzomys
Species:	palustris
Current (blue) and approximate former (light blue) distribution of the marsh rice rat in the eastern United States. A small part of the distribution of Oryzomys couesi is also shown (red).
Synonyms
Mus palustris Harlan, 1837 Arvicola oryzivora Bachman, 1854 Oryzomys palustris: Baird, 1857 Oryzomys palustris natator Chapman, 1893 ?Oryzomys palustris texensis J.A. Allen, 1894 Oryzomys palustris coloratus Bangs, 1898 Oryzomys natator floridanus Merriam, 1901 Oryzomys fossilis Hibbard, 1955 Oryzomys palustris planirostris Hamilton, 1955 Oryzomys palustris sanibeli Hamilton, 1955 ?Oryzomys argentatus Spitzer and Lazell, 1978

The marsh rice rat (Oryzomys palustris) is a semiaquatic North American rodent in the family Cricetidae. It usually occurs in wetland habitats, such as swamps and salt marshes. 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. 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 are 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 disagreement exists 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. Its diverse diet includes plants, fungi, and a variety of animals. Population densities are usually below 10 per ha (four 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 around 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.

Taxonomy

The marsh rice rat is classified as one of eight species in the genus Oryzomys, 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 40 species from the genus. All are placed in the tribe Oryzomyini ("rice rats"), a diverse assemblage of over 100 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 rats (Sigmodon) in the southern half of the country.

Early history

The marsh rice rat was discovered in 1816 in South Carolina 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 voles 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.

Common names

Many common names 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 (O. p. argentatus) is known as the "silver rice rat".

Description

The marsh rice rat is a medium-sized rodent that looks much like the common black and brown rats, 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. It has small cheek pouches. The ears are about the same color as the upperparts, but a patch of light hairs is in front of them. The tail is dark brown above and may be paler below. The guard hairs 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 fore feet have four and the hind feet five digits. On the fore feet, the ungual tufts (tufts of hair on the digits) are absent. The hind feet are broad and have a short fifth digit. Many of the pads are reduced, as are the ungual tufts, but small interdigital webs are present. The Florida Keys form, P. o. argentatus, has even more reduced ungual tufts. Many of these traits are common adaptations to life in the water in oryzomyines.

Marsh rice rats in much of Florida are more reddish than those elsewhere.

Some geographic variation in fur color occurs; western populations (P. o. texensis) are lighter than those from the east (nominate P. o. palustris), and Florida populations are generally more tawny or reddish than either, with those from southern Florida (P. o. coloratus) being brighter than those from the center of the state (P. o. natator). The Florida Keys form (P. o. argentatus) is silvery, and the two other Florida forms—P. o. planirostris and P. o. sanibeli—lack the reddish tones of mainland Florida populations and are instead grayish, resembling P. o. planirostris, or brownish (P. o. sanibeli). In 1989, Humphrey and Setzer reviewed variation in color among Florida populations. They found P. o. argentatus to be substantially lighter and P. o. planirostris and P. o. sanibeli to be somewhat darker than mainland populations, and P. o. argentatus to have a less yellow fur, but found no significant differences in redness. Substantial variation within populations also was found.

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), hind foot 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.

Mandible of a marsh rice rat from New Jersey, seen labially (from the outer side)

Distribution and habitat

A marsh rice rat walking on mesh in Paynes Prairie, Florida

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.

Behavior and ecology

The barn owl is an important predator of the marsh rice rat.

Marsh rice rats are active during the night, so are rarely seen, although they may be among the most common small mammals in part of their range. They 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 wrens (Cistothorus palustris), red-winged blackbirds (Agelaius phoeniceus), muskrats (Ondatra zibethicus) or round-tailed muskrats (Neofiber alleni). Marsh rice rats sometimes make large runways or dig burrows. They are accomplished and willing swimmers, easily swimming more than 10 m (33 ft) under water, and often seek safety in the water when alarmed. Rice rats in the Florida Keys occasionally climb in vegetation, but never higher than 90 cm (3.0 ft). Marsh rice rats are very clean 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, their perceptual range (the distance from which an animal can detect a patch of suitable habitat) 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 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 owls, Strix varia); snakes (cottonmouths, Agkistrodon piscivorus, and others), alligators (Alligator mississippiensis), and carnivorans (raccoons, Procyon lotor; red foxes, Vulpes vulpes; minks, Neovison vison; weasels of the genus Mustela; and striped skunks, Mephitis mephitis). Many parasites have been recorded on the marsh rice rat, including various ticks and mites, lice, and fleas among external parasites and many nematodes and digeneans, a pentastomid, and several coccidians 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.

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 Everglades, 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 hectares (0.91 acres) and in females about 0.23 hectares (0.57 acres). A study in Florida found male home ranges to average 0.25 hectares (0.62 acres) and female 0.33 hectares (0.82 acres).

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, storms probably do not cause the population to decline substantially, and in Texas, inundation of its habitat did not significantly influence population density. However, in Mississippi, 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 no evidence shows 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.

Diet

Spartina alterniflora is eaten by the marsh 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. It prefers Spartina alterniflora that has 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 crabs, 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.

Reproduction and lifecycle

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, primarily because photoperiod influences their circadian rhythm which determines breeding. In both Texas and Virginia, variation in reproductive activity in females is less 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 cycle ranges from 6 to 9 days, with an average of 7.72 days. Estrus occurs again after a litter is born. Mating 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.

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 0.10 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 11th days, the eyes open, the fur develops, and the young begin to take solid food. Weaning occurs on the 11th to 20th day, according to different studies. Considerable variation is reported in body masses at different ages, perhaps because of geographic variation. 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 to which other rodents used at the time are not susceptible. 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 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 2016 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 areas. 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 polychlorinated biphenyls, but less heavy metal than white-footed mice (Peromyscus leucopus).

See also

In Spanish: Oryzomys palustris para niños