Northeastern Marsh Rice Rat

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.

Some geographic variation in fur color occurs; western populations (P. o. texensis) are lighter than those from the east (nominate P. o. …

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. …

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 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.