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.
Mating systems in cactus mice are not well understood. Like other Peromyscus species they are likely to have a promiscuous mating system.
The gestation period for Peromyscus eremicus is 20 to 25 days. Litter size is 1 to 4 offspring and average weight at birth is 2.1 to 2.9 grams (Parker 1990). Females may have up to 4 litters per year. The female has two pairs of teats rather than three, which has been suggested to correlate with the number of offspring (Parker 1990). The weaning period is unknown. Females reach sexual maturity after about two months, and the age of male sexual maturity is unknown. The average age for the female cactus mouse's first estrus is 39.2 days (Nowak 1991). Females have been observed to reproduce continuously and year round, and specifically during January, February, June, and September in central Arizona (Spotorno 1992). (Caire, 1999)
Male Peromyscus eremicus have a simple penis, distinct from the complex penis of some other murids. The os baculum is simple, lacking the three distal prongs typical of complex penes. Young cactus mouse penes were found to have three bluish cartilaginous condensations near the tip of a well-ossified baculum. One condensation was located at the apex and two were located laterally, where the normal distal prongs in a complex penis would occur (Spotorno 1992). This characteristic is not only distinguishing, but it suggests that possession of a 3-pronged baculum may be primitive. The penis is nevertheless broad, and its size has been suggested to increase "locking" during 3.5 percent of copulations (Species Information Library Peromyscus eremicus 1994). Additionally, bone-ossification and sexual maturity are both controlled by testosterone.
Young cactus mice are born with their ears and eyes closed. The ears open in the first day after birth and the eyes open in 11 to 15 days. Females nurse and care for their young in their nest until they are weaned, probably within a few weeks of birth. Juveniles go through their first molt at 5 weeks old. (Caire, 1999)
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.
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.
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).
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.
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).
Tanya Dewey (editor), Animal Diversity Web.
Amanda Myers (author), University of Michigan-Ann Arbor.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
uses sound to communicate
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.
uses smells or other chemicals to communicate
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
an animal that mainly eats all kinds of things, including plants and animals
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
uses sight to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
breeding takes place throughout the year
Anderson, S. and J.K. Jones, Jr. 1984. Orders and Families of Recent Mammals of the World. John Wiley and Sons: New York.
Djawdan, M. and T. Garland, Jr. 1988. Maximal Running Speeds of Bipedal and Quadrupedal Rodents. Journal of Mammology 69(4): 765-772.
Glazier, D.S. 1985. Relationship Between Metabolic Rate and Energy Expenditure for Lactation in Peromyscus. Comparative Biochemistry and Physiology, Vol. 80A: 587-590.
Hanney, P.W. 1975. Rodents: Their Lives and Habits. Taplinger Publishing Company, New York.
Hulbert, A.J., Hinds, D.S., MacMillen, R.E. 1985. Minimal Metabolism, Summit Metabolism and Plasma Thyroxine in Rodents from Different Environments. Comparative Biochemistry and Physiology, Vol 81A, No.3: 687-693. Nowak, R.M. 1991. Walker's Mammals of the World. Fifth Edition, Volume II. The Johns Hopkins University Press: Baltimore.
Parker, S.P. 1990. Grzimek's Encyclopedia of Mammals. Volume 3. McGraw-Hill Publishing Company: New York.
Price, M.V. and Waser, N.M. 1984. On the Relative Abudance of Species: Postfire Changes in a Coastal Sage Scrub Rodent Community. Ecology 65(4): 1161-1169.
Species Information Library. 1994. Mouse, Cactus, Peromyscus eremicus. CD-ROM, Version 1.0. National Information Services Corporation: Baltimore.
Species Information Library. 1994. Mouse, Cactus, Peromyscus eremicus eremicus. CD-ROM, Version 1.0. National Information Services Corporation: Baltimore.
Spotorno, A. E. 1992. Parallel Evolution and Ontogeny of Simple Penis Among New World Cricetid Rodents. Journal of Mammology 73(3): 504-514.
Caire, W. 1999. Cactus mouse. Pp. 567-568 in D Wilson, S Ruff, eds. The Smithsonian Book of North American Mammals. Washington, D.C.: Smithsonian Institution Press.