Peromyscus californicus
California mouse

Peromyscus californicus is found from San Francisco Bay south and east along the coast ranges and in the eastern Sierra Nevada from Mariposa Co. south to Kern Co. in California south to Bahia San Quintin on the Pacific coast of northwestern Baja California (Bryiski and Harris, 1984; Alvarez-Casteñada and Cortés-Calva, 1999). (Brylski and Harris, 1990)

Peromyscus californicus is generally restricted to dense chaparrel and broad-sclerophyll woodland (Meritt 1974). The limiting factor for its small geographic range may be the need for naturally occurring burrow holes of the proper size for these larger animals (Grinnell and Orr 1934), as they are poor natural burrowers. The co-occurrence of woodrat houses and a distributional association with the California laurel complex have also been noted as potential limiting factors (Meritt 1974), although there is no direct evidence to implicate any of these. (Grinnell and Orr, 1934; Meritt, 1974; Merritt, 1999)

Peromyscus californicus is the largest species in its genus. Its total length is between 220-285 mm, with tail length ranging from approximately 117-156 mm (Whitaker 1997). It is distinctly bicolored. Adults have a yellowish brown or gray mixed with black dorsal coloring, and a white underside, and feet. Many individuals have a distinctly fulvous throat patch and a fulvous lateral line separating dorsal from ventral pelage in the shoulder region, sometimes extending to the thigh. Juveniles are gray on top with a white underside. The tail matches the dorsal pelage and is not sharply bicolored. The ears are large, ranging from 20-25 mm (Whitaker 1997).

Unlike most mice P. californicus is truly monogamous, and once mated will stay paired for life (McCade and Blanchard 1950; Ribble 1991; Ribble and Salvioni 1990; Guvernick and Nelson 1990). (McCabe and Blanchard, 1950; Ribble, 1992)

Mating may occur year round, but mainly from March to September. In the lab P. californicus can have up to 6 litters per year, but in the wild the average is 3-4. Gestation is 30 to 33 days and average litter size is 2 (from 1 to 3), with a slight increase in litter size with the age of the female. Compared to other species of Peromyscus, P. californicus young are rather precocious, although weaning is not completed for about 5 weeks. They also have a long period before reaching sexual maturity, approximately 11 weeks for females and even longer for males. (McCabe and Blanchard, 1950)

Males help extensively in caring for and protecting the young. (Merritt, 1999)

California mice generally live for 9 to 18 months. Populations tend to be fairly stable and at low densities, as compared to other Peromyscus species. (Merritt, 1999)

Peromyscus californicus is nocturnal, with a slight activity peak just before dawn (Hudson 1967). Usually a docile mouse, it will actively defend its nest from conspecifics of the same sex. A poor burrower, Peromyscus californicus readily uses woodrat (Neotoma fuscipes) houses for dens, and where found together, P. californicus actively excludes its conspecific P. truei from these woodrat houses (Meritt 1974). They build globular nests out of grasses or sticks, lined with fine grass. California mice are excellent climbers and are often found in trees and shrubs. Males and females form long-term pair bonds and these mice live in small family groups. (Meritt, 1974; Merritt, 1999)

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

Peromyscus californicus specializes on the fruits, seeds and flowers of shrubs (Meserve 1976). In woodland habitat the seeds of California bay laurel (Umbellularia californica) are the major food (Meritt 1974). Arthropods may make up a small percentage on the diet but these are not actively hunted (Meserve 1976). Water is most likely obtained from the food that it eats and supplimented with dew; P. californicus is not as good at water conservation as other species in the genus (Meritt 1974). (Meritt, 1974; Merritt, 1999; Meserve, 1977)

California mice, like other Peromyscus species, are an important prey base for many predators throughout their range. They are preyed on by hawks, owls, rattlesnakes, and small mammalian predators. Their nocturnal and secretive habits help to protect them from many predators.

California mice are important seed predators in the ecosystems in which they live and they form an important prey base for rattlesnakes, owls, and other predators.

Like other members of Peromyscus, California mice fecal matter may transmit hantavirus.

Despite its relatively narrow habitat, limited geographic range, and generally low population densities, Peromyscus californicus populations remain healthy. (Merritt, 1999)