Description
A mouse with a white belly, white feet, large membranous ears lacking a white rim, and naked soles on the hind feet; dorsal pelage varies from dark to buffy to grayish, with ochraceous or fulvous wash; tail nearly always is longer than the head and body, is not distinctly bicolored, has a broad brownish dorsal stripe, and is nearly naked, with hairs about 2-4 mm long at the tip; total length 160-218 mm, tail length 84-128 mm, hind foot length 18-22 mm, ear length 13.4-20.0, mass 18-40 g (usually 18-27 g in subadult and adult males and nonpregnant females); ordinarily no accessory cusps on the first upper molar; premaxillary bones extend noticeably beyond the ends of the nasals posteriorly; zygomatic arches weak and not flared out (Ingles 1965, Veal and Caire 1979, Hoffmeister 1986).
Diagnostic Characteristics
Differs from P. MERRIAMI usually in having a hind foot length of usually less than 21.6 mm rather than larger, smaller total length (185-223 mm in MERRIAMI), lacking a pectoral spot, having whitish rather than frequently cream-colored underparts, mastoidal breadth less than 11.55 mm rather than larger, alveolar length of maxillary toothrow less than 4.18 mm rather than more, greatest length of skull less than 25.75 mm rather than more, baculum curving dorsally rather than ventrally, length of baculum less than 8.5 mm rather than more (these differences are usual, not absolute; identifications should be made using a combination of characters; Hoffmeister 1986). Differs from P. CRINITUS in lacking a noticeable tuft of hairs at the tip of the tail (which in CRINITUS is well haired), having shorter and less silky fur, and having the premaxillaries extending posterior to the ends of the nasals (Hoffmeister 1986, which see for further cranial distinctions). Differs from P. LEUUCOPUS in having a actually and relatively longer tail (tail is shorter than head and body in LEUCOPUS), lacking white border on ears, having a less prominent dorsal tail stripe, weaker and less bowed zygomatic arches, a smaller hind foot (usually 21-24 mm in LEUCOPUS), and 2 pairs of mammary glands instead of 3 pairs (Hoffmeister 1986). Differs from P. BOYLII in less heavily haired and less bicolored tail, smaller ears, upper molars 1-2 without mesolophs, auditory bullae less inflated, nasal branches of premaxillaries extending posteriorly beyond nasala rather than the reverse, glans penis relatively broader, 2 pairs of mammary glands instead of 3 pairs, and hind foot usually 18-21 mm rather than 21-24 mm (Hoffmeister 1986). Differs from P. CALIFORNICUS by being smaller (CALIFORNICUS: 220-266 mm total length, 117-148 mm tail, 25-29 mm hind foot, 20-25 mm ear) and lacking a well-haired tail (see Veal and Caire 1979). PEROMYSCUS MANICULATUS has extra mesolophs on upper molars 1-2, tail shorter than the head and body, and 3 pairs of mammary glands (Hoffmeister 1986). See Veal and Caire (1979) for distinctions between EREMICUS and P. EVA in southern Baja California. Most of the preceding comparisons are based on data from Arizona; details may differ in other locations.
Habitat
Rocky areas as well as localities with sandy substrates and loamy soils (Veal and Caire 1979). Deserts and pinyon-juniper zone. Nests have been found in rock heaps, stone walls, burrows, and brush fences (Veal and Caire 1979); also in woodrat houses.
Ecology
Average home range has been estimated at 0.3 hectares (Veal and Caire 1979).
Reproduction
Gestation lasts 3-5 weeks, longest in lactating females. Litter size is 1-6 (average 2-3). Young are weaned in about 18 days. Males and females in breeding condition have been trapped throughout the year (Veal and Caire 1979). Apparently monogamous mating system.