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Arizona Cotton Rat

Sigmodon arizonae

French: Sigmodon d/Arizona / German: Arizona-Baumwollratte / Spanish: Rata de algodén de Arizona

Other common names: Colorado River Cotton Rat

Taxonomy. Sigmodon hispidus arizonae Mearns, 1890, “Fort Verde.” Corrected by D. F. Hoffmeister in 1986 to “Bell’s Ranch, 3 mi [= 7- 6 km] SE Camp Verde, Yavapai Co., Arizona,” USA .

Sigmodon arizonae was included in S. hispidus, butits specific status was confirmed by chromosomal and morphological differences. Two subspecies, nominotypical arizonae named by E. A. Mearns in 1890 and known only from Fort Verde, and jacksoni named by E. A. Goldman in 1912 from Fort Whipple, both from central Arizona, are extinct. Three extant subspecies are currently recognized.

Subspecies and Distribution.

S.a.cienegaeA.B.Howell,1919—CArnizona,USA,StoCSonora,Mexico.

S.a.majorBailey,1902—SSonora,Sinaloa,WDurango,andNayarit,Mexico.

S. a. plenus Goldman, 1928 — lower Colorado River in E California and W Arizona, USA; extinct in S Nevada .

Descriptive notes. Head-body 245-220 mm, tail 95-155 mm, ear 17-24 mm, hindfoot 34-43 mm; weight 140-300 g.

Sexual dimorphism is minimal, with males only slightly larger than females (mean total lengths 289 mm for males and 270 mm for females). The Arizona Cotton Ratis one of the larger species in Sigmodon, characterized by hispid, light brownish black dorsal pelage and lightsilver to whitish underparts, with gray-based hair; forefeet and hindfeet covered dorsally by grayish to dull brown hair; and indistinctly bicolored tail with dark dorsum grading into lighter brown orsilvery below. Tail is sparsely covered by long hairs and hasvisibly large scales. Key cranial features characterizing the Arizona Cotton Rat include broad presphenoid, wide distance between parietal and squamosal crests (equal to or greater than 3-2 mm), large squamosal crests (equalto or greater than 7 mm), curved dorsal border of supraoccipital; posteriorly tapered palatine slits, and angular anteroventral border of foramen magnum. Chromosomal complementis 2n = 22 or 24, FNa = 38.

Habitat. Semi-desert grasslands and riparian communities in northern distribution, upland desert and thorn scrub in northern Mexico, and savanna-woodland, deciduous tropical forest, palm forest, and mangrove swamp at southern distributional terminus, from sea level to elevations of 2000 m. Microhabitats at collection sites of Arizona Cotton Rats include thick grass or herbaceous vegetation, almost always near natural (streams and rivers) or man-made (irrigation canals, ponds, and agriculturalfields) water sources.

Food and Feeding. The Arizona Cotton Rat is presumably primarily herbivorous to perhaps seasonally omnivorous, feeding on stems,leaves, roots, seeds, and insects.

Breeding. Pregnant Arizona Cotton Rats have been collected in early spring and late summer—early autumn, but young have been collected in nearly every month suggesting that breeding is relatively continuous throughout the year. Embryo counts are 1-12 (average five). Gestation is ¢.27 days; young are precocial and are weaned within seven days.

Activity patterns. Arizona Cotton Rats are active day and night.

Movements, Home range and Social organization. Arizona Cotton Rats construct and travel along more or less defined runways in grass; burrows are located in dense vegetation or constructed underground. Local density in southern Arizona averaged 4-4-7-9 ind/9-8ha plot; manipulations reducing grass density and structure negatively impacted densities. Individuals moved average distances of 30-9-43-4 m in trapping grids.

Status and Conservation. Classified as Least Concern on The IUCN Red List. The Arizona Cotton Rat has a wide overall distribution and presumably large overall population,is locally common, readily uses human-modified habitats, lacks any known major conservation threats, and occurs in several protected areas. Nevertheless, nominate subspecies and jackson: are now extinct, and plenus, with its highly restricted distribution along the lower Colorado River, is extinct in Nevada and listed as a Species of Special Concern in California .

Bibliography. Alvarez-Castafieda, Castro-Arellano & Lacher (2016a), Bailey (1902), Carleton et al. (1999), Ceballos (2014a), Elder (1980), Frey et al. (2002), Goldman (1918b, 1928), Gwin et al. (2011), Hoffmeister (1986), Howell (1919), Mearns (1890), Ramirez & Chavez (2014b), Severinghaus & Hoffmeister (1978), Swier et al. (2009), Webb & Baker (1962), Zimmerman (1970), Zimmerman & Sihvonen (1973).