Oryzomys albiventer

Oryzomys albiventer is a rodent in the genus "Oryzomys" of family Cricetidae from interior western Mexico, in the states of Jalisco, Guanajuato, and Michoacán. First described in 1901 as a separate species, it was later lumped under "O. couesi" and the marsh rice rat ("O. palustris") until it was reinstated as a species in 2009. It differs from neighboring "Oryzomys" populations in size and measurements and is a large, brightly colored species with a long tail and robust skull and molars. Its range has been much impacted by agricultural development, but isolated populations are thought to persist.
Taxonomy.
"Oryzomys albiventer" was first described by C.H. Merriam in 1901 on the basis of ten specimens from Ameca, Jalisco. He named the animal "albiventer" after the white color of its underparts and considered it most closely related to "Oryzomys aquaticus" (currently included in "Oryzomys couesi"). Two years later, D.G. Eliot described "Oryzomys molestus" on the basis of a single individual from Ocotlán, Jalisco; the name "molestus" means "troublesome, irksome". Eliot considered "Oryzomys fulgens", another current synonym of "O. couesi", as the closest relative of his new species. In his 1918 review of North American "Oryzomys", E.A. Goldman assessed the holotype of "O. molestus", an old male, as merely a large example of "albiventer", and reduced "albiventer" to one of many subspecies of "O. couesi". He considered it closely related to three other highland Mexican forms. In 1960, E.R. Hall argued that "O. couesi" was the same species as the marsh rice rat ("O. palustris") of the United States, and listed "albiventer" as a subspecies of the latter. Later, "O. couesi" was again accepted as separate from the marsh rice rat, but "O. albiventer" was still classified under "O. couesi".
In a 2009 revision of the "Oryzomys" of western Mexico, M.D. Carleton and J. Arroyo-Cabrales noted substantial differences in coloration and measurements between highland populations ("albiventer") and lowland populations ("mexicanus") in Jalisco. On the basis of these differences, they recognized "O. albiventer" as a species separate from the lowland populations, which they classified under "O. couesi". They confirmed that Eliot's "molestus" was based on a large example of "O. abiventer", but left the status of the three forms Goldman had associated with "albiventer"—"crinitus", "aztecus", and "regillus"—open, noting that there was no convincing evidence that these represented the same species as "O. albiventer". The identity and exact provenance of "fulgens" (supposed to be from the Valley of Mexico), and consequently its relationship to "O. albiventer", remain unknown.
"O. albiventer" is part of the genus "Oryzomys", which currently includes about eight species distributed from the eastern United States ("O. palustris") into northwestern South America ("O. gorgasi"). "O. albiventer" is further part of the "O. couesi" section, which is centered on the widespread Central American "O. couesi" and also includes various other species with more limited and peripheral distributions. Many aspects of the systematics of the "O. couesi" section remain unclear and it is likely that the current classification underestimates the true diversity of the group. "Oryzomys" previously included many other species, which were progressively removed in various studies culminating in a contribution by Marcelo Weksler and coworkers in 2006 that removed more than forty species from the genus. All are classified in the tribe Oryzomyini ("rice rats"), a diverse assemblage of American rodents of over a hundred species, and on higher taxonomic levels in the subfamily Sigmodontinae of family Cricetidae, along with hundreds of other species of mainly small rodents.
In 1904, Eliot used the common name "white-bellied rice rat" for "O. albiventer" and "Ocotlan rice rat" for "O. molestus". In 1918, Goldman also used "white-bellied rice rat" for "O. albiventer".
Description.
"Oryzomys albiventer" is a large and long-tailed "Oryzomys". The upperparts are brightly ochraceous, becoming grayer toward the front. The hairs on the underparts are pale gray near the bases and white in the outer half, so that the underparts appear pale grayish according to Carleton and Arroyo-Cabrales (not white as claimed by Merriman). The tail is dark above and light below. The skull and molars are relatively robust. "O. albiventer" has broad zygomatic arches (cheekbones), long incisive foramina (perforations of the palate between the incisors and the molars), and long nasal bones that extend behind the premaxillary bones. Compared to its lowland relative "O. couesi mexicanus", "O. albiventer" is larger and more brightly colored and has larger molars but narrower incisive foramina.
In twelve specimens, total length is , averaging ; head and body length is , averaging ; tail length is , averaging ; hindfoot length is , averaging ; and skull length (occipitonasal length) is , averaging .
Distribution and conservation.
"O. albiventer" occurs at about elevation in northern Michoacán, southern Guanajuato, and central and eastern Jalisco, mostly in the area around Lake Chapala. Its range has seen massive agricultural development and although populations may survive, the current distribution of the species is certainly highly fragmented. More survey work is needed to assess the distribution and status of "O. albiventer".