Peromyscus boylii
brush mouse

Peromyscus boylii is found in mountainous regions from northern California and western Nevada, eastward to western Texas and Oklahoma, and southward to central Mexico (Hall 1981, Bradley and Schmidly 1999).

Peromyscus boylii generally occurs at elevations over 2,000 m where it inhabits chaparral, pinyon-juniper woodlands, and pine-oak forests (Baker 1968, Findley et al. 1975, Hoffmeister 1986). On a more local scale within these vegetation communities, P. boylii is most commonly associated with rocky and dense shrub covered areas (Wilson 1968, Holbrook 1978, Boyett 2001). Shrub cover, rock cover, and logs are important habitat components that provide nesting sites and refuge from weather and predators (Bradley and Schmidly 1999). (Baker, 1968; Boyett, 2001; Bradley and Schmidly, 1999; Findley et al., 1975; Hoffmeister, 1986; Holbrook, 1978; Wilson, 1968)

Peromyscus boylii is a medium-sized Peromyscus. No significant sexual dimorphism is evident. The tail is usually longer than the head and body, bicolored (dark gray above and white below), well haired, and tufted at the end. The hind foot is small and similar in length to the ear but sometimes longer. Skull is medium-sized and auditory bullae are not greatly inflated. Upperparts of pelage are medium brown; sides lighter brown with a broad orange lateral line extending from cheek to hindquarters; underparts whitish; ankles dusky gray; and feet whitish below ankle (Hoffmeister 1986, Schmidly et al. 1988, Bradley and Schmidly 1999). The mean and ranges (mm) of the following four external characters are taken from Schmidly et al. (1988): total length, 194.3 (175-210); length of tail, 103.6 (89-115); length of hind foot, 20.1 (19-22); and length of ear, 19.0 (18-21).

Peromyscus boylii may be confused with P. attwateri, P. crinitus, P. difficilis, P. leucopus, P. maniculatus, P. pectoralis, and P. truei. These species are similar in general appearance and have distributions that overlap with P. boylii. However, a few external characters may help distinguish P. boylii from these species (Hoffmeister 1986, Bradley and Schmidly 1999). Peromyscus boylii differs from P. attwateri in having a larger hind foot. A longer tail in relation to head and body length and larger overall size differentiates P. boylii from P. crinitus, P. leucopus, and P. maniculatus. Peromyscus boylii differs from P. difficilis and P. truei in having shorter ears and smaller hind feet. Darker ankles rather than white differentiates P. boylii from P. pectoralis. Several closely related species found in Mexico (P. aztecus, P. beatae, P. levipes, P. simulus, and P. spicilegus) are exceedingly difficult to distinguish from one another (Bradley and Schmidly 1999). They were all previously considered part of P. boylii, but are now considered separate species based on molecular differences. (Bradley and Schmidly, 1999; Hoffmeister, 1986; Schmidly, Bradley, and Cato, 1988)

Little information on the life span/longevity of P. boylii in the wild is available. Nevertheless, the longevity of Peromyscus is typically short with few living more than one year under natural conditions (Terman 1968). In a long-term mark-recapture study of P. boylii in Arizona, Abbott et al. (1999) found that of more than 300 individuals most survived one year or less. A few individuals survived up to 26 months. Although mark-recapture studies typically underestimate survival and longevity and do not distinguish between dispersal and mortality, they provide a rough estimate of these demographic parameters. (Abbott, Ksiazek, and Mills, 1999; Terman, 1968)

Peromyscus boylii is nocturnal and active year-round. These mice are skilled climbers (Holbrook 1979, Hoffmeister 1986). Nests, constructed with dry grasses and other vegetation, may be found in tree cavities and rock crevices, under logs and boulders, and in caves and mine shafts (Hoffmeister 1986, Bradley and Schmidly 1999). Hoffmeister (1986) notes that mice living in caves and mine shafts often build open nests similar to those of goldfinches. During periods of environmental stress (e.g., food and/or water scarcity), many species of Peromyscus, including P. boylii, may employ torpor to temporally escape such stressful conditions (see MacMillen 1983, MacMillen and Garland 1989 and references therein).

In New Mexico, Ribble and Stanley (1998), using radiotelemetry, found that home ranges of male P. boylii (mean of 0.47 ha) were significantly larger than home ranges of female P. boylii (mean of 0.26 ha). Additionally, home-range size also varied inversely with population density, and home ranges of females overlapped little with those of other females while home ranges of males overlapped those of several other males and females. These patterns of home-range size and overlap were consistent with a promiscuous mating system for P. boylii.

Like other Peromyscus species, brush mice have keen eyesight and vision and extensively use chemical cues in communication.

Like other species of Peromyscus, P. boylii is omnivorous. Its flexible diet varies both temporally and spatially reflecting variation in the availability of food resources across seasons, years, and habitats. In California, Jameson (1952) found that P. boylii fed extensively on acorns and conifer seeds in winter; insects in spring; manzanita (Arctostaphylos patula) berries and insects in summer; and acorns, conifer seeds, and fungi in fall. In New Mexico, Smartt (1978) found that over a one-year period important food items included, 31% arthropods, 26% juniper fruit and cones, 11% mistletoe, and 5% parts of prickly pear cactus (Opuntia). (Jameson, 1952; Smartt, 1978)

P. boylii escapes predation through their nocturnal and secretive habits. Their fecundity allows populations to withstand high predation pressures. They are important prey items for many predatory mammals, birds of prey, and snakes, including foxes, owls, hawks, and rattlesnakes.

Peromyscus boylii are often one of the most common and abundant small mammals within parts of their range (Hoffmeister 1986, Boyett 2001) and undoubtedly serve as important prey for a variety of terrestrial and avian predators. They are reported to be an important food source for the federally threatened Mexican spotted owl (Strix occidentalis lucida — Ward and Block 1995). (Boyett, 2001; Hoffmeister, 1986)

In the southwestern United States, P. boylii has been identified as a reservoir host for hantavirus (Mills et al. 1997, Abbott et al. 1999) and therefore may pose a health risk to humans. Persons living or working in areas where they may come into contact with potentially infected rodents should therefore follow specific safety precautions and guidelines (e.g., Mills et al. 1995, Centers for Disease Control and Prevention 2000). Sanchez et al. (2001) recently identified a previously unknown strain of hantavirus from P. boylii, which they named Limestone Canyon virus. However, they found no evidence that Limestone Canyon virus causes hantavirus pulmonary syndrome. (Abbott, Ksiazek, and Mills, 1999; Centers for Disease Control and Prevention, 2000; Mills et al., 1997; Mills et al., 1995; Sanchez, Abbott, and Nichol, 2001)

Most information indicates that P. boylii is probably of little positive economic importance. Jameson (1952), however, suggested that P. boylii may be beneficial to reforestation projects by consuming harmful insect pests in such areas, but he also noted that these mice probably consume the planted seeds. On a large scale, P. boylii likely has little or no positive economic benefit for humans. (Jameson, 1952)

Peromyscus boylii has no special conservation status.

Considerable taxonomic revision has taken place within P. boylii. Many taxa from Mexico and Central America previously assigned as subspecies of P. boylii (aztecus, beatae, levipes, madrensis, simulus, and spicilegus) have been elevated to species status (see Alvarez 1961, Carleton 1977, Schmidly et al. 1988) while others (ambiguus, cordillerae, and sacarensis) have been realigned under other species of Peromyscus (see Carleton 1979, Castro-Campillo et al. 1999, Bradley et al. 2000). Bradley and Schmidly (1999) recognize four subspecies: P. b. boylii (northern California and western Nevada); P. b. glasselli (San Pedro Nolasco Island, Sea of Cortéz, Mexico); P. b. rowleyi (southern California to Colorado and western Texas and Oklahoma, south to central Mexico); and P. b. utahensis (central Utah). Hafner et al. (2001) recommended that the insular P. stephani (San Esteban Island, Sea of Cortéz, Mexico) be included as a subspecies of P. boylii based on mitochondrial DNA sequence data.

Other small mammal species frequently found in association with P. boylii include Neotoma albigula (white-throated wood rats), N. stephensi (Stephen's wood rats), Spermophilus variegatus (rock squirrels), Tamias dorsalis (cliff chipmunks), and T. quadrivittatus (Colorado chipmunks) (Wilson 1968, Hoffmeister 1986, Boyett 2001). (Alvarez, 1961; Boyett, 2001; Bradley and Schmidly, 1999; Bradley et al., 2000; Carleton, 1977; Carleton, 1979; Castro-Campillo et al., 1999; Hafner, Riddle, and Alvarez-Castañeda, 2001; Hoffmeister, 1986; Schmidly, Bradley, and Cato, 1988; Wilson, 1968)