NORTHERN ROCK DEERMOUSE
Peromyscus nasutus (J. A. Allen 1891)
Order Rodentia : Family Cricetidae
DESCRIPTION. A rather large, long-tailed, grayish buff mouse; tail sharply bicolor, brownish to blackish above and white below, slightly tufted, >100 mm in length and longer than the combined length of head and body; tops of front and hind feet (including ankles) white; ears about as long as hind feet. Dental formula: I 1/1, C 0/0, Pm 0/0, M 3/3 × 2 = 16. Averages for external measurements: total length, 193 mm; tail, 104 mm; hind feet, 22.5 mm.
Resembles both P. boylii and P. laceianus; differs from boylii in having white ankles like laceianus, and differs from laceianus in having a noticeably longer tail and heavier molars (length of maxillary tooth row 4.5 mm as opposed to <4 mm). Differs from P. truei in grayish buff rather than ochraceous buff upperparts and in smaller ears; differs from P. attwateri in shorter hind feet (<24 mm) and in white rather than dusky ankles.
DISTRIBUTION. Known in Texas only from the Trans-Pecos, where it has been recorded from mountainous regions of Brewster, Culberson, El Paso, Jeff Davis, and Presidio counties. It has been recorded along the northwestern escarpment of the Llano Estacado in Quay and Curry counties in New Mexico, but it has yet to be recorded on the otherwise similar escarpment in Deaf Smith County, Texas, just a few miles to the east.
SUBSPECIES. Peromyscus n. nasutus from the Guadalupe Mountains in Culberson County, the Davis Mountains in Jeff Davis County, the Chisos Mountains in Brewster County, and the Chinati Mountains in Presidio County; P. n. penicillatus in El Paso County.
HABITS. This species is found among boulders on rocky mountain slopes and in rock piles in Texas madrone and oak associations where the crevices and cracks are covered with a thick layer of leaves. In the Franklin Mountains (El Paso County), the northern rock deermouse is common in rocky areas and talus slopes. P. boylii is absent from rocky areas where nasutus is abundant, but boylii is abundant in adjacent areas with fewer rocks and more vegetation. It appears that nasutus prefers rugged, rocky habitat with sparse vegetation. In the Davis Mountains, where we trapped this species, it showed a decided preference for the cooler and more mesic situations in shaded rocky canyons at the highest elevations.
In captivity, the northern rock deermouse is docile and easily handled. It is highly gregarious, and a dozen or more individuals of young and adults of both sexes often crowd into one nest without apparent conflict.
Little information is available on its breeding habits. Pregnant females have been taken in June–August. Vernon Bailey reported capture in New Mexico of a female containing four large fetuses in late July and another in late August with six fetuses. Individuals captured alive in the Franklin Mountains readily bred in captivity and produced several litters of two to six young. The gestation period is about 30 days.
POPULATION STATUS. Uncommon. The northern rock deermouse has a spotty distribution in the isolated mountains of far western Texas, where it occupies the mesic canyons associated with the forested woodlands of the largest ranges. Nowhere has it been found to be abundant.
CONSERVATION STATUS. The IUCN lists the northern rock deermouse as a species of least concern, and it does not appear on the federal or state lists of concerned species. In portions of the Davis Mountains this species was commonly collected in the late 1990s; however, trapping efforts 10 years later at the same localities produced no captures. Clearly, it could be threatened by climate change favoring increased aridity, under which conditions it likely would face serious competition from the brush deermouse, P. boylii. This is a species that warrants careful monitoring in the future.
From The Mammals of Texas, Seventh Edition by David J. Schmidly and Robert D. Bradley, copyright © 1994, 2004, 2016. Courtesy of the University of Texas Press.