Tawny-Bellied Cotton Rat
Order Rodentia : Family Muridae : Sigmodon fulviventer J.A. Allen

Description. A small to medium sized cotton rat with brownish, buff brown, or fulvous underparts from throat to anus. Dorsal coloration is light brown heavily speckled with dark brown to black, giving a "salt-and-pepper" or "hispid" appearance. The tail is uniformly blackish and the tops of the feet are buff brown.

The tawny-bellied cotton rat is similar in appearance to two other cotton rats that occur in Texas. From the yellow-nosed cotton rat (S. ochrognathus), S. fulviventer differs in having a rich buffy ventral coloration rather than whitish, a more heavily speckled dorsal coloration, buff-colored tops to the feet rather than grayish, and a uniformly dark colored tail. Also, S. fulviventer lacks the tawny colored nose of S. ochrognathus. The hispid cotton rat (S. hispidus) differs from S. fulviventer in having gray or whitish underparts, a bicolored tail that is lighter below than above, grayish tops of the feet, and slightly larger ears and hind feet.

External measurements reported for the holotype of S. f. dalquesti from Fort Davis, Texas were: total length, 242 mm; length of tail, 90 mm; hind foot, 28 mm; ear, 18 mm.

Distribution in Texas. In the United States known from grassland habitats in southeastern Arizona and southwestern New Mexico to middle Rio Grande valley. In Texas known only from an isolated population near Fort Davis in Jeff Davis County.

Habits. The tawny-bellied cotton rat inhabits grassy areas interspersed with shrubby growth that affords cover and allows dense growth of grasses. In Mexico these rats are associated with bunch grasses in the mesquite-grassland vegetation type. Over their limited range in Arizona and New Mexico, tawny-bellied cotton rats are found in weedy and grassy places in pinyon-juniper-live oak woodland, Mexican oak-pine woodland, and mesquite-yucca-grassland vegetation types, where their runways are hidden in the thick, grassy cover.

In Texas, tawny-bellied cotton rats have been taken in similar habitat at one site near Fort Davis. Within a general area described as a "heavily grazed, level valley plain" with "small, scattered mesquite, catclaw, and a fence line" that protected against livestock grazing, tawny-bellied cotton rats were caught in dense grasses along fencerows and in adjacent grassy areas protected by clumps of mesquite and catclaw. Hispid cotton rats were also caught in these areas.

Of 20 Texas specimens captured in late March, eight were juveniles. Both adult males were in reproductive condition and of the 10 mature females, one was lactating and four were pregnant. Embryo counts revealed litter sizes of three, four, four, and four.

Remarks. Previously unknown in the state, the tawny-bellied cotton rat was first recorded in Texas in the spring of 1991 near Fort Davis by Fred Stangl of Midwestern State University. This isolated population represents not only a new species of mammal for Texas, but appears to be a new subspecies as well — S. f. dalquesti. The extent of this rat’s range and population numbers in Texas remain unknown. The brief life history notes available on the Texas specimens are taken from Stangl’s paper published in the Southwestern Naturalist.