Stanislaus River Basin and Calaveras River Water Use Program
Threatened and Endangered Species Report - March 1995
Bay Delta and Special Water Projects Division, CA Dept of Fish and Game
San Joaquin Pocket Mouse
Perognathus inornatus
California Department of Fish and Game "Special Animal"
Life History
The San Joaquin pocket mouse is a small buff-orange colored mouse with a sprinkling of
darker guard hairs on its back; it does not contain the spiny hairs found in some of the other
species of pocket mice. An indistinct lateral line which separates the lighter belly hairs from
the darker back can be observed on most individuals. Some distinctive characteristics are
short ears that may have a patch of lighter hair at their base; a hind foot that has hair on the
sole; a long tail that is covered with hair, is unicolored, and has a small tuft of hair on the tip.
The external fur-lined cheek patches are used to store seeds of grasses and forbs which are
carried back to, or near, their dens for eating.
The diet of the pocket mouse consists of seeds and soft bodied insects. Seeds of grasses,
forbs, and shrubs such as Atriplex are the main food source and soft bodied insects such as
cutworms and even grasshoppers are also eaten. The pocket mouse lives in arid habitats,
therefore all water needs are metabolized through seed digestion. The foraging habits of the
pocket mouse tend to occur under the cover of shrubs and even above the ground within a
shrub. They generally do not travel far to forage and stay out of relatively open areas.
The breeding season for the San Joaquin pocket mouse is from March to July and the
females have at least two litters of four to six young per litter. It is believed that the young
will remain in the birthing den until mature, however, the length of time to maturity is
uncertain (Jameson and Peeters 1988).
Findings and Conclusions
There are no reported occurrences of the San Joaquin pocket mouse along or adjacent to
the Stanislaus River. With the majority of the river out of the pocket mouse's range and
because of its preference for arid land habits, it is not considered likely to occur within the
study area and no further surveys are required.
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