Marsh rice rat

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. 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. 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. In the 1890s, several subspecies of the marsh rice rat were described from the United States: O. p. natator from Florida in 1893, O. p. texensis from Texas in 1894, and O. p. coloratus from elsewhere in Florida in 1898. Clinton Hart Merriam recognized O. p. natator as a separate species in 1901 and described a subspecies of it, O. p. floridanus, but considered O. p. texensis to be nearly identical to nominate O. p. 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: O. p. planirostris from Pine Island and two miles (3 km) north of Fort Myers and O. p. 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 karyotypes 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.