Northern Rock Mouse
Order Rodentia : Family Muridae : Peromyscus nasutus (J.A. Allen)

Description. A rather large, long-tailed, grayish buff mouse; tail sharply bicolor, brownish to blackish above and white below, slightly tufted, more than 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. External measurements average: total length, 193 mm; tail, 104 mm; hind feet, 22.5 mm. Resembles both P. boylii and P. pectoralis, but differs from the former in having white ankles like pectoralis. Differs from pectoralis in having a noticeably longer tail and heavier molars (length of maxillary toothrow 4.5 mm as opposed to less than 4.0 mm). Differs from P. truei in grayish buff rather than ochraceous buff upperparts and with smaller ears; differs from P. attwateri in shorter hind feet (less than 24 mm) and in white rather than dusky ankles.

Distribution in Texas. 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.

Habits. This species is found among boulders on rocky mountain slopes and in rockpiles 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 rock mouse 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 captivity the rock mouse 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, July, and 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.