Peromyscus eremicus
cactus mouse

Peromyscus eremicus is found in the southwestern United States, northern Mexico, Baja California and several nearby islands (Nowak 1991). The subspecies Peromyscus eremicus eremicus can be found farther north in Utah (Species Information Library, Peromyscus eremicus eremicus 1994).

Peromyscus eremicus primarily lives in desert areas with cacti, hence the name cactus mouse. It lives in steppes and semi-arid deserts. Peromyscus eremicus may be found in the rocky foothills of desert mountain ranges. Subspecies found in Utah (Peromyscus eremicus eremicus) can be semi-arboreal and inhabit shrubs (Species Information Library 1994). Cactus mice build nests in self-dug tunnels, lodges of other rodents, brickwork buildings, and piles of rock or brushwood (Parker 1990). Price and Waser's study of the post-fire reinhabition of an area of California costal sage scrub revealed Peromyscus eremicus preference for rock and brush habitats over open areas and debris (1984).

The range of masses given includes both male and females, as little research has been conducted on sexual dimorphisms in size. Average body length (not including the tail) is 8.0 to 9.0 centimeters and the average tail length is 10.0 to 14.0 centimeters (Parker 1990). This species is noted for its unusually long tail. A possible function for such a long tail is body temperature regulation (Hanney 1975). The color of the thick pelage appears to vary. Parker remarks that Peromyscus eremicus has a pale gray back (1990). The Species Information Library, however, reports that species have been found in New Mexico with a spectrum of fur shades between pale yellowish and blackish (1994). A possible explanation is that two subspecies, one with darker fur (Peromyscus eremicus anthonyi) and lighter fur (Peromyscus eremicus eremicus) have interbred in New Mexico. Nowak describes the underparts of the cactus mouse as white or near-white (1991). The tail is usually less haired than that of other mice in its genus. Cactus mice have naked soles on their hind feet, which distinguish them from other southwestern Peromyscus species. Facial and skull characteristics are also important in distinguishing the cactus mouse from other Peromyscus mice. Peromyscus eremicus has small ears and one to two upper molars with usually one mesoloph. The zygomatic arches of the skull are weak and not flared out and the auditory bulla are not greatly inflated (Species Information Library Peromyscus eremicus 1994). The nasal branches of the premaxillae extend posteriorly behind the nasals.

Like other Peromyscus, most cactus mice live about 1 year.

Peromyscus eremicus is nocturnal. The cactus mouse has been described as shy and excitable, and seldom bites when handled (Species Information Library Peromyscus eremicus 1994). In a study of running speed, four males and three females averaged a speed of 13.1 kilometers per hour (Djawdan and Garland 1988). Speed is important in more open habitats as a method of predator evasion. Cactus mice may enter a state of torpor during the day or aestivate during dry, hot times of the year.

Home Range

The average territory size including males and females is 0.8 acre, or 0.3 hectare (Parker 1990). The male's range is typically larger, however, and may overlap with other males' ranges (Nowak 1991).

The vocalizations of Peromyscus species have been described as thin squeaks and shrill buzzings. Most species, when excited, thump their forefeet rapidly upon the ground to produce a drumming noise (Nowak 1991). Like other Peromyscus species, cactus mice have keen vision and hearing and use chemical cues extensively in communication.

Cactus mice are opportunistic omnivores. They eat mainly fruits and blossoms of shrubs and annual seeds such as hackberry (Ulmaceae) and mesquite (Leguminoseae) (Species Information Library, Peromyscus eremicus eremicus 1994). Pinus and Juniperus seeds are eaten during the winter. In addition, cactus mice eat insects, leaves, and green vegetation (Parker 1990). (Caire, 1999)

Cactus mice, like other Peromyscus species, are abundant small mammals in the areas in which they live. They form an important prey base for predators such as owls, rattlesnakes, foxes, and other predators. They can run quickly and are generally secretive and nocturnal, which helps to protect them from some predation. Their ability to reproduce rapidly also means that populations respond robustly to heavy predation pressure. (Caire, 1999)

Cactus mice are important seed predators and form an important prey base for a variety of predators in their desert habitats.

No negative effects on humans are known. However, the closely related species Peromyscus maniculatus is harmful to forest regeneration because it eats seeds, particularly those of conifers (Nowak 1991). Cactus mice are known to eat Pinus and Juniperus seeds as well, so it makes sense that they could be an additional threat to forest regeneration. They also prey on insects pests of forests, so the impact of their seed predation may be balanced by their impact on insects.

Peromyscus eremicus is an excellent model for physiological and genetic studies. It is clean, lives well in cages, and has a high rate of productivity (Nowak 1991).

Cactus mice are generally abundant within their range.

Peromyscus eremicus is in the subgenus Haplomylomys. Cactus mice have two main adaptations for their desert habitat, a lowered metabolism and an ability to enter torpor when deprived of food and water. To investigate the claim that the low metabolism of desert rodents is maintained by low thyroid gland activity, Hulbert et al. (1985) measured the level of plasma thyroxine in heteromyids versus cricetids (including Peromyscus eremicus). Their results suggested that the environment affects thyroxine levels, since all species displayed lower metabolisms in a desert versus a coastal habitat. In extreme conditions such as deprivation of both water and food, cactus mice may enter torpor within twelve hours. Below thirty degrees Celsius, there is a depression in body temperature and oxygen consumption. At temperatures lower than fifteen degrees Celsius, cactus mice enter torpor more slowly (Species Information Library Peromyscus eremicus 1994). This effect of temperature on torpor has been suggested as a possible explanation for their southern distribution. The desert adaptation of lowered metabolism comes with a cost. A study of five species of Peromyscus established that for female Peromyscus eremicus, the basal rate of metabolism is correlated to the rate it processes energy for reproduction during lactation (Glazier 1985). In practical terms, this means that the higher the rate of metabolism for the mother and the young, the faster food can be transformed into milk and eventually offspring growth. Thus, Peromyscus eremicus's drier habitat limits it to a litter size of about two to three offspring (Parker 1990).