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 (O. palustris) of the United States and O. couesi of Mexico and Central America—are widespread; the six others have more restricted distributions. The species have had eventful taxonomic histories, and most species were at one time included in the marsh rice rat; additional species may be recognized in the future. The name Oryzomys was established in 1857 by Spencer Fullerton Baird for the marsh rice rat and was soon applied to over a hundred species of American rodents. Subsequently, the genus gradually became more narrowly defined until its current contents were established in 2006, when ten new genera were established for species previously placed in Oryzomys. Species of Oryzomys are medium-sized rats with long, coarse fur. The upperparts are gray to reddish and the underparts white to buff. The animals have broad feet with reduced or absent ungual tufts of hair around the claws and, in at least some species, with webbing between the toes. The rostrum (front part of the skull) is broad and the braincase is high. Both the marsh rice rat and O. couesi have 56 chromosomes, lack a gall bladder, and have a complex penis (as is characteristic of the Sigmodontinae) with some traits that are rare among oryzomyines; these characteristics are unknown in the other species of this genus. The habitat includes various kinds of wetlands, such as lakes, marshes, and rivers. Oryzomys species swim well, are active during the night, and eat both plant and animal food. They build woven nests of vegetation. After a gestation period of 21 to 28 days, about four young are born. Species of Oryzomys are infected by numerous parasites and carry at least three hantaviruses, one of which (Bayou virus) also infects humans. Two, maybe three, species have gone extinct over the last two centuries and at least one other is endangered, but the widespread marsh rice rat and O. couesi are not threatened. Oryzomys is one of about thirty genera within the tribe Oryzomyini, a diverse group of well over a hundred species, many of which were formerly also included in Oryzomys. Oryzomyini is one of several tribes within the subfamily Sigmodontinae of the family Cricetidae, which includes hundreds of other species of mainly small rodents, distributed mainly in the Americas and Eurasia. Within Oryzomyini, a 2006 phylogenetic analysis by Marcelo Weksler which used both morphological and DNA sequence data found some evidence that Oryzomys is most closely related to a group including Holochilus, Lundomys, and Pseudoryzomys. Although analyses based on morphological and combined data supported this relationship, sequences of the Rbp3 gene alone instead placed Oryzomys among a group that included Nectomys, Sigmodontomys, and a few other genera. In all analyses, Oryzomys appeared within clade D of Oryzomyini. The relationship between Oryzomys and the Holochilus group was supported by five synapomorphies (shared derived characters)—absence or reduction of both the hypothenar and interdigital pads; reduction of ungual tufts of hairs surrounding the claws; having the back margin of the zygomatic plate of the skull at the same level as the front of the first upper molar; and the anterocone (front cusp) of the first upper molar divided by an anteromedian fossette. The first three are adaptations to the semiaquatic lifestyle that Oryzomys and the members of the Holochilus group share, and may thus be examples of convergent evolution. The name Oryzomys was introduced in 1857 by Spencer Fullerton Baird for the marsh rice rat (now Oryzomys palustris) of the eastern United States, which had been first described twenty years earlier by Richard Harlan. The name combines the Greek oryza 'rice' and mys 'mouse' and refers to the feeding habits of the marsh rice rat. Baird placed Oryzomys as a subgenus of the now-defunct genus Hesperomys and included only the marsh rice rat in it, a classification which was followed by Elliott Coues in 1874 and 1877. In 1890, Oryzomys was raised to generic rank, and in subsequent years numerous additional species were ascribed to it, many of which were soon moved to separate genera. In the 1898 Catalogus Mammalium, Édouard Louis Trouessart listed 67 species of Oryzomys, including some that are now placed in Calomys, Necromys, Thomasomys, and other genera unrelated to Oryzomys. Some of the new genera proposed were soon subsumed in Oryzomys again, and in The Families and Genera of Living Rodents (1941), John Ellerman listed Microryzomys, Oligoryzomys, Melanomys, Nesoryzomys, and Oecomys as synonyms of Oryzomys and included about 127 species in it. In 1948, Philip Hershkovitz suggested that other oryzomyines like Nectomys and Megalomys could as well be included in Oryzomys, and Clayton Ray followed this suggestion in 1962. Hershkovitz and Ray's classification was never widely followed, and from 1976 on authors started to reinstate some of the other groups lumped in Oryzomys as separate genera. The genus was reduced to 43 species (out of 110 in Oryzomyini) in the third edition (2005) of Mammal Species of the World, but it was still not a natural, monophyletic group; rather, it mostly united those oryzomyines that lacked the conspicuous specializations of other genera. In 2006, Marcelo Weksler's comprehensive phylogenetic analysis produced further evidence that the genus was polyphyletic, as species of Oryzomys were dispersed all over the oryzomyine tree. He proposed that eleven new genera should be created to accommodate those species that were not closely related to the type species of Oryzomys, the marsh rice rat; he considered other options that would require fewer new genera, but argued that that would result in less meaningful genus-level groups in Oryzomyini. Later in the same year, Weksler, Percequillo, and Voss created ten new genera—Aegialomys, Cerradomys, Eremoryzomys, Euryoryzomys, Hylaeamys, Mindomys, Nephelomys, Oreoryzomys, Sooretamys, and Transandinomys—for species formerly placed in Oryzomys and placed six more species related to 'Oryzomys' alfaroi in Handleyomys pending the description of more new genera for them. They left only five species in Oryzomys, which was now finally a natural, monophyletic group. Because of subsequent taxonomic work, the number of species has since increased to at least eight. Some problems remain: ?Oryzomys pliocaenicus, a Miocene fossil from Kansas, is of uncertain identity but may belong in Bensonomys, and fossils from the Miocene of Oregon and Pliocene of New Mexico have also been ascribed to Oryzomys, but probably incorrectly. A possible Oryzomys has been recorded from the Irvingtonian (Pleistocene) of Saskatchewan.