Cactus mouse
The cactus mouse ("Peromyscus eremicus") is a species of rodent in the family Cricetidae. It is a species of the genus "Peromyscus", a closely related group of New World mice often called "deermice" (although the term more commonly refers specifically to "P. maniculatus"). They are native to desert areas of western North America. Description. Cactus mice are small cricetid rodents, with large eyes and ears, a pointed snout, and a long monocolor tail. Total length, 160mm to 211mm; length of body, 72mm to 100mm; length of tail, 84mm to 120mm; length of  hind foot, 18mm to 22mm; length of ear, 13.4mm to 20.0mm; greatest length of skull, 22.7mm to 25.9mm and zygomatic breadth, 11.2mm to 13.5mm. Adults weigh between . Females weigh slightly more than males and are significantly larger in body length, ear length, length of mandible, and bullar width of skull. Cactus mice can be identified by having naked soles on their hind feet and almost naked flesh-colored tails (as opposed to the furry bicolored tail common in most "Peromyscus" species), which are usually the same length or longer than the animals ' body length. Their ears are nearly hairless, large, and membranous. Their fur is long and soft; coloration varies between subspecies and between different populations. Color of fur varies from ochre to cinnamon, with white ventral areas, and the sides and top of head slightly grayish. Females tend to be slightly paler in color than males, while juveniles appear more gray than their parents. Distribution. Cactus mice are found in dry desert habitats, steppe, and mountain foothills in arid regions of the southwestern United States and northern Mexico, as well as islands off the coast of the Baja California peninsula and  in the Gulf of Mexico. Low average temperatures and lack of the mesquite species "Prosopis juliflora" might limit northern expansion. The cactus mouse occurs sympatrically with five other mouse species, including the California mouse, canyon mouse, Eva's desert mouse, mesquite mouse, and the western deermouse. Diet. The cactus mouse feeds on seeds, mesquite beans, hackberry nutlets, insects, and green vegetation. In winter, they rely more on insects; on seeds and flowers in the spring; and seeds, leafy greens, and insects in the summer. In autumn, they transition to their winter diet. Behavior. Cactus mice, like most muroids, exhibit crepuscular  behavior and may even appear in midday, but are mainly nocturnal. They may be less active during the full moon. They have been described as "shy and excitable, and seldom bites when handled". They vocalize a shrill, high pitched squeak when frightened. In laboratory studies, cactus mice average a running speed of . Compared to other "Peromyscus" species, cactus mice have a lower metabolism. This is thought to be an adaptation to arid environments, and maintained by lower activity of the thyroid gland. They may enter torpor when deprived of food in the winter, and food and/or water in the summer . Below , they enter torpor more slowly, which may partially explain their southern distribution. In the laboratory, onset of torpor was observed to occur within half an hour of food deprivation. They are important as seed predators and as prey items for other animals. Reproduction. The mating habits of cactus mice are currently not well studied, but are inferred from other "Peromyscus" species to be promiscuous. They mate year around, but more so during warmer months. Unlike other muroids, male cactus mice have a simple penis, rather than a complex one, and females have two pairs of teats, rather than three . The number of teats in the female cactus mouse correlates with the number of offspring they can raise in their arid environment. Females mate after about two months, however, male sexual maturity age is unknown. Relationship with humans. Cactus mice are ideal laboratory animals since they keep clean, live well in captivity, and have a high rate of productivity. They have been proposed for physiological and genetic studies. There are no known negative effects on humans or human-related activities. Specimens from Southern California have tested positive for hantavirus, however, infections in this species are likely incidental and localized, rather  than a common reservoir.