The Nicaraguan Rice Rat (Oryzomys dimidiatus) is a rare species of oryzomyine rodent in the genus Oryzomys. Only two specimens are known, which were obtained in two nearby localities in southeastern Nicaragua. It was originally described as a species of genus Nectomys, but is now considered to be a member of the restricted genus Oryzomys. It is one of two mammal species endemic to Nicaragua (the other is Richmond's Squirrel); the low degree of endemism in Nicaragua is explained by the nation's continuous habitat shared with its neighbors.

Description

The Nicaraguan Rice Rat has a thick, glossy fur with a velvety underfur. The hairs on the back are about 6 mm in length. The back is brown, with the sides being slightly different in colour. A faint buff line extends from the sides to the inner sides of the hindlegs. The belly is buffy. The hands and feet are white above. The tail contains about 15 rings per centimeter and is greyish above and whitish below. It resembles the sympatric Coues's Rice Rat, but is smaller and darker and has a relatively shorter tail. Measurements for the two known species are as follows (in each case, the first measurement given is from the holotype, taken in 1904, the second, from the other specimen, taken in 1966): total length 240 and 228 mm, tail length 115 and 110 mm, hindfoot length 27 and 28 mm, ear length 13 and 15 mm, skull length 29.8 and 29.0 mm. The 1966 specimen weighed 46.0 g; the weight of the 1904 specimen is unknown.

Taxonomy

The first known specimen was obtained by W. G. Palmer on 5 November 1904 on the Río Escondido near El Rama in what is now the Región Autónoma del Atlántico Sur. Oldfield Thomas of the British Museum of Natural History described it as a new species of Nectomys in 1905. This specimen, the holotype remains in the Natural History Museum, London, as specimen BM 5.3.4.2. No further primary information was published in the next four decades, although N. dimidiatus was listed as a species of Nectomys in several taxonomic lists. In 1948, however, Philip Hershkovitz of the Field Museum of Natural History transferred it to the genus Oryzomys as the only member of subgenus Micronectomys. In 1957, another species, Oryzomys (Micronectomys) borreroi, was allocated to the subgenus, but it is now considered to represent a synonym of the Brown Cane Mouse (Zygodontomys brunneus); no evidence for a relationship between the Nicaraguan Rice Rat and the Colombian form has been published since. In 1970, Hershkovitz wrote that his 1948 description of subgenus Micronectomys had not been in agreement with the International Code of Zoological Nomenclature, so that it had been a nomen nudum. By that time, Hershkovitz acknowledged a morphological resemblance with the Marsh Rice Rat (Oryzomys palustris), which at the time included Coues's Rice Rat (Oryzomys couesi) and other members of the current genus Oryzomys.

On 26 July 1966, a second specimen of the Nicaraguan Rice Rat was obtained at El Recreo in the Región Autónoma del Atlántico Sur (then still the Zelaya Department), 15 km from the location of the first specimen. The find was published in a 1971 article in the Journal of Mammalogy. The specimen, a male, is in the collections of the University of Kansas Museum of Natural History as number 106607. The Nicaraguan Rice Rat's relationship to the palustris group was confirmed in the early 2000s and when most species of Oryzomys were split off into new genera in 2006, O. dimidiatus was retained in the genus. Its exact relationship to nearby populations of Coues's Rice Rat, itself a composite of at least three species, remains unclear.

Ecology

The first specimen was captured on very wet clay in a banana plantation. The second specimen was taken in a stand of cane at the south bank of the Río Mico. At the same location, the Dusky Rice Rat (Melanomys caliginosus; reported as Oryzomys caliginosus), Fulvous Colilargo (Oligoryzomys fulvescens; reported as Oryzomys fulvescens), Coues's Rice Rat (Oryzomys couesi; reported as Oryzomys palustris), Southern Cotton Rat (Sigmodon hirsutus; reported as Sigmodon hispidus) and Tapeti (Sylvilagus brasiliensis) were also taken.

Conservation status

The conservation status of the Nicaraguan Rice Rat has not been considered in the primary scientific literature. It has however been assessed as Near Threatened by the International Union for Conservation of Nature, despite its small distribution and apparent rarity.

Notes

References

• Baillie, J. 1996. Oryzomys dimidiatus 2006 IUCN Red List of Threatened Species. Downloaded on 19 July 2007.
• Genoways, H. H., and J. K. Jones, Jr. 1971. Second specimen of Oryzomys dimidiatus. Journal of Mammalogy 52: 833-834.
• Hershkovitz, P. 1944. A systematic review of the Neotropical water rats of the genus Nectomys (Cricetinae). Miscellaneous Publications Museum of Zoology, University of Michigan 58: 1–88.
• Hershkovitz, P. 1970. Supplementary notes on Neotropical Oryzomys dimidiatus and Oryzomys hammondi (Cricetinae). Journal of Mammalogy 51(4): 789-794.
• Jones, J. K., Jr., and M. D. Engstrom 1986. Synopsis of the rice rats (genus Oryzomys) of Nicaragua. Occasional Papers, The Museum, Texas Tech University 103: 1-23.
• Musser, G. G. and M. D. Carleton. 2005. Superfamily Muroidea. Pp. 894-1531 in Mammal Species of the World: a Taxonomic and Geographic Reference. D. E. Wilson and D. M. Reeder eds. Johns Hopkins University Press, Baltimore.
• Weksler, M., A. R. Percequillo, and R.S. Voss. 2006. Ten new genera of oryzomyine rodents (Cricetidae: Sigmodontinae). American Museum Novitates 3537:1-29.