326.

Yellow-nosed Cotton Rat

Sigmodon ochrognathus

French: Sigmodon a nez jaune / German: Gelbnasen-Baumwollratte / Spanish: Rata de algod 6n de hocico dorado

Taxonomy. Sigmodon ochrognathus Bailey, 1902, Chisos Mountains, 8000 ft [= 2438 m], Brewster County, Texas, USA.

This species is monotypic.

Distribution. SE Arizona, extreme SW New Mexico, and Trans-Pecos of W Texas, USA, S through NE Sonora, Chihuahua, and W Coahuila to C Durango, Mexico.

Descriptive notes. Head-body 132-144 mm, tail 85-110 mm, ear 16-22 mm, hindfoot 25-30 mm; weight 50-133 g.

Sexes are monomorphic in size. The Yellow-nosed Cotton Rat is moderately small to medium in size; dorsal hairs are dark gray at their bases and tipped with brown, but pelage is also dotted with yellow hairs that provide an overall grizzled grayish color. Venter is white, but gray bases of hairs are visible. Tail is bicolored, dark on top and gray below. Eyes are ringed in yellow. Distinct ocherous nose is conspicuous, underscoring its common name and distinguishing the Yellow-nosed Cotton Rat from all species Sigmodon . Distinguishing cranial characteristics from other members of the fulviventer species group include pronounced median keel on basioccipital, small and elongate auditory bullae, obviouslateral bulges of capsular projections of upper incisors, and median posterior notch with distinctive basal notches. The Yellow-nosed Cotton Rat differs from members of the hispidus species group by having small tail scales (0-5 mm wide); more heavily haired tail; short, broad, and more strongly arched (in lateral view) skull; short basioccipital; and deep palatal pits. Chromosomal complement is 2n = 52, FNa = 66.

Habitat. Desert mountain ranges in south-western North America at elevations of 990-2375 m. The Yellow-nosed Cotton Ratis the most xerophilous species of Sigmodon, living on dry, rocky slopes with scattered bunches of grasses in oak-pinyon pine-juniper woodland and grassy montane flats or alluvial fans where soils are deep and rocks are few; it is confined to drier slops when other species of Sigmodon —notably the Hispid Cotton Rat (S. hispidus) or the White-eared Cotton Rat (S. leucotis)—are present.

Food and Feeding. The Yellow-nosed Cotton Rat is probably primarily herbivorous and occasionally omnivorous.

Breeding. Length of breeding season of the Yellow-nosed Cotton Rat varies geographically, with young born essentially year-round in some areas but seasonally in others. At the northern end of the distribution in Arizona and Texas, breeding season is in March—October. In all areas, individual females likely produce multiple litters in a season. Gestation is ¢.35 days, and litters have 2-9 young. Young are precocial, gain 2 g/ day, are weaned in c.2 weeks, and reach sexual maturity by 45 days of age.

Activity patterns. The Yellow-nosed Cotton Rat is apparently active primarily during daylight hours, despite inhabiting more open, sparsely vegetated areas.

Movements, Home range and Social organization. Runways of Yellow-nosed Cotton Rats are prominent in thick grass covering deep soils but indistinct on sparsely vegetated rocky hillsides, often radiating from underground dens. Cuttings of grass blades and fecalpiles are usually conspicuous, and caches of grass clippings have been found in abandoned pocket gopher ( Thomomys umbrinus, Geomyidae) burrow systems. Nests of Yellow-nosed Cotton Rats are constructed in thickets of dense grass, under dead blades of Agave (Asparagaceae), and among roots ofjunipers or other small trees.

Status and Conservation. Classified as Least Concern on The IUCN Red List. The Yellownosed Cotton Rat has a wide distribution and can be locally common. Nevertheless, population trends are apparently variable, declining in some areas but stable or even expanding in others. Most species of Sigmodon exhibit irruptive population cycles, so care must be taken in interpretation of short-term studies of population trends.

Bibliography. Allen (1903c), Bailey (1902), Baker & Greer (1962), Baker & Shump (1978b), Benson (1940), Carroll et al. (2002), Ceballos (2014a), Elder & Lee (1985), Goldman & Gardner (1947), Hoffmeister (1963, 1986), Jiménez (1971, 1972), Lacher, Timm & Alvarez-Castaneda (2016), Ramirez & Chavez (2014e), Swier etal. (2009), Zimmerman (1970).