Animal Diversity Web

Geographic Range

NW California, from San Francisco Bay north to Siskiyou Co. (USA) Hall (1981), Wilson and Reeder (1993)

• Biogeographic Regions
• nearctic
  • native

Habitat

Tamias sonomae requires habitat with trees, shrubs, logs, snags, and litter. It occurs in chaparral and open areas in redwood forests and the lower and drier forests of Ponderosa pine. It also lives in areas characterized by Douglas fir, black oak, and laurel. Elevationally, it lives in areas from near sea level to 1800 meters in elevation. Best (1993), California Wildlife Habitat Relationships Systems (1997), Nowak (1991)

• Terrestrial Biomes
• savanna or grassland
• chaparral

Physical Description

Length: 25-125 g Tail: 220-227 mm

Hind foot: 33-37 mm

Ear (length from notch): 15-23 mm

Cranium (greatest length): 36.6-39.7 mm

Cranium (breadth): 15.0-16.5 mm

Baculum: 3.03-3.30 mm

Males and females are externally similar; there is no apparent sexual dimorphism. The Sonoma chipmunk has five black longitudinal back stripes separated by four dull gray or brownish stripes. In contrast to other western American chipmunks, however, the back stripes are not clearly demarcated (this lack of clearly demarcated stripes is believed to help camouflage the sonoma chipmunk in its chaparral habitat). The remainder of the upper parts are reddish brown in color, except for a small patch of black fur immediately behind each eye and whitish stripes on either side of each eye. The sides of the body are rusty in color and the ventral surface is creamy white. The tail is quite bushy and follows the color of the body both ventrally and dorsally. The fur is soft and dense, and it becomes slightly woolly in the winter. There are two molts each year, giving rise to a summer pelage (July-September) and a winter pelage (November-June), the summer pelage being brighter than the winter pelage.

There are two recognized subspecies of T. sonomae, T. s. alleni and T. s. sonomae (the first occupying the southern most part of range of this species). T. s. alleni is slightly smaller than T. s. sonomae, and its pelage is darker.

The skull of the sonoma chipmunk is long and narrow, the zygomatic breadth averaging 54% of the greatest length of the skull. The rostrum is deep, the nasals are separated at the tips by a small median notch, and the braincase is long and inflated. The incisive foramina are short. The dental formula is 1/1, 0/0, 2/1, 3/3, for a total of 22 teeth. The upper incisors are recurved, with a sharp-angled notch in the occlusal surfaces, and the cheek teeth are relatively small. The baculum has a thin shaft and a low keel that extends 10% of the length of the tip. The tip is 27-31% of the length of the shaft. The angle formed by the tip and shaft is 130 degrees, and the distal end of the shaft is slightly compressed laterally.

Best (1993), Ingles (1965), Nowak (1991)

• Other Physical Features
• endothermic
• bilateral symmetry

Reproduction

Females breed once per year in the spring. However, females from low elevations may enter estrus five or more months before females from high elevations. The gestation period is 31 days, and lactation lasts 39-45 days. Litters consist of three to five (usually four) young, and females alone raise the litter. They stay with the young and suckle them, at least at night, for at least three weeks after the young emerge. Weaned young remain together for some weeks after the mother no longer associates with them. The testes of adult males enlarge during the breeding season, during which the males are said to have scrotal testes (December-June). In late March, the testes of adult males reach 13-16.5 mm in length. The age at sexual maturity for males or females is unknown.

Best (1993), Nowak (1991)

• Key Reproductive Features
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual

Behavior

Sonoma chipmunks are not a social species; they have a shy retiring nature. They reside in burrows or nests and live mostly on the ground. They are active during the day and may seek protection in old logs and in crevices among rocks, as well as in their burrows or nests. Most foraging is done by climbing through the smaller branches of bushes, but individuals often seek elevated positions such as stumps, lower limbs of pine and oak (Quercus) trees, and rock outcroppings, where they rest, watch intruders, and eat food gathered elsewhere (carried in their cheek pouches). These chipmunks also gather food for storage underground. Although detailed information on the Sonoma chipmunk's breeding behavior is not available, it is known that during the breeding season, adult males with scrotal testes enter the home range of a female in estrus to breed and then move on. They compete with other similarly attracted males for estrus females over a wide area. This male nomadism subjects adult males to greater sources of mortality than adult females, which maintain a limited home range.

Sonoma chipmunks have two types of vocalizations, alarm calls (a chipping sound) and courtship sounds (harsher than alarm calls). They make alarm calls from trees, bushes, the ground, and under bushes. In response to alarm calls, T. sonomae moves rapidly either over a direct path to the center of the tree or bush in which it is foraging or directly to the nearest log, tree, or bush from open ground, where it becomes still and attentive. This behavior appears to allow chipmunks responding to alarm calls to be less accessible to predators (one known predator is the red-tailed hawk) with the briefest amount of obvious movement. Alarm calls are emitted more often by females than males, and kin selection is probably responsible for this observation. Since juvenile females tend to stay near the natal area while males disperse, mother-daughter groups are formed and females are more likely to have close relatives nearby. Thus, it is advantageous for them to emit alarms. However, males tend to be removed from their mothers and siblings, and alarms calls are not as beneficial.

Best (1993), Nowak (1991)

• Key Behaviors
• motile

Communication and Perception

• Perception Channels
• tactile
• chemical

Food Habits

The sonoma chipunk has not been extensively studied and therefore little is known about its food habits. Nonetheless, considering its habitat, T. sonomae probably eats seeds and leaves of chaparral plants. Common plants within the sonoma chipmunk's range include whitethorn, chokecherry, serviceberry, and silk tassel. It is reasonable to assume that T. sonomae follows the food habits of other species within the genus Tamias. Other species within this genus are known to eat the fruits and seeds of various trees and herbs. The foliage and flowers of some herbs are also eaten, as are the tender buds of woody plants. Mushrooms, insects, bulbs, and birds' eggs are also consumed at times.

Best (1993), Nowak (1991)

Economic Importance for Humans: Positive

The skins of another species in the Tamias genus, T. sibiricus, are used to some extent in the fur industry. Also, the sonoma chipmunk may be an important disperser of seeds and the spores of mycorrhizal fungi in the habitats in which it lives. Nowak (1991)

Economic Importance for Humans: Negative

Other species in the genus Tamias that also prefer rugged or brush-covered land have been known to damage agricultural crops by eating planted seeds and young plantings. Fruit trees have also been damaged by Tamias species in some areas. Nowak (1991)

Conservation Status

The sonoma chipmunk's small range and the very small number of studies on its key habitats and forage resourses makes it difficult to determine its status. More research is needed to determine whether human activities or land use are a threat to this species. Nonetheless, it should be noted that several other Tamias species in the western United States may be in jeopardy becuase of human environmental disturbance. In fact, Tamias minimus atristriatus, of central New Mexico, evidently became extinct around 1980 when its limited habitat was lost to residential development. Nowak (1991)

• IUCN Red List

  Least Concern
  More information

Other Comments

Tamias evolved by the early Miocene, but no fossils of T. sonomae are known. Over a large area of northwestern California, the range of T. sonomae overlaps that of T. ochrogeys, T. siskiyou, or T. senex.

Some sources still place the Sonoma chipmunk in the genus Eutamias.

Tamias is from the Greek tamias meaning storer or distributor. The specific epithet sonomae apparently refers to Sonoma Co., where the type specimen was obtained.

Best (1993)

Contributors

Samir Hasmukh Shah (author), University of Michigan-Ann Arbor.

References

1997. California Wildlife Habitat Relationships System. http://danr.ucop.edu/ihrmp/bluemam.html

Best, T.L. 1993. Tamias sonomae. Mammalian Species. No. 444, pp. 1-5.

Hall, E.R. 1981. The Mammals of North America. Volume I. Second Edition. John Wiley & Sons, New York.

Ingles, L.G. 1965. Mammals of the Pacific States: California, Oregon, and Washington. Stanford University Press, Stanford, California.

Nowak, R. M. 1991. Walker's Mammals of the World. Fifth Edition. Johns Hopkins University Press, Baltimore.