Mammal Species of the World
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- Original description: Grinnell, J., 1915. Eutamias sonomae, a new chipmunk from the inner northern coast belt of California, p. 321. University of California Publications in Zoology, 12:321-325.
endemic to a single state or province
Global Range: Northwestern California, from northern San Francisco Bay area north to Siskiyou County; Upper Sonoran and Transition life zones, from near sea level to 2100 m (Johnson 1943, Best 1993).
NW California, from San Francisco Bay north to Siskiyou Co. (USA) Hall (1981), Wilson and Reeder (1993)
Biogeographic Regions: nearctic (Native )
Regularity: Regularly occurring
Type of Residency: Year-round
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)
Length: 28 cm
Comments: Dense chaparral, brushy clearings in forests, and streamside thickets; often associated with black oak, ponderosa pine, digger pine, Douglas-fir, white fir, redwood, sticky laurel, incense cedar, madrone, manzanita, and serviceberry; associated with tan oak on high ridges in Trinity and Humboldt counties, with whitethorn, chokecherry, serviceberry, and silk tassel in large warm tracts of brush in Trinity County (see Best 1993). A tree nest was on a limb 15 m up in a large Douglas-fir at the base of a forested slope (see Best 1993). Often uses elevated perches such as stumps, rocks, or lower limbs of trees.
Habitat and Ecology
Breeds once a year in spring. Young are born during May-July; timing of reproduction varies with elevation. Litter size is 3-5 (usually four). Female raises litter alone; remains with young for at least three weeks after young emerge. Weaned young may stay together for some weeks after separation from mother.
Males disperse as juveniles; females tend to stay near natal site (Best 1993). Diet includes seeds, fruits, fungi, etc. Forages on ground and in bushes. Active throughout most of the year. Probably remains in its burrow only during severe winter storms.
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
Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: No. No populations of this species make annual migrations of over 200 km.
Comments: Probably feeds primarily on seeds, fruits, fungi, etc. Forages on ground and in bushes.
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)
Males disperse as juveniles; females tend to stay near natal site (see Best 1993).
Life History and Behavior
Comments: Active throughout most of the year. Probably remains in its burrow only during severe winter storms.
Breeds once a year in spring. Young are born during May-July; timing of reproduction varies with elevation. Litter size is 3-5 (usually 4). Female raises litter alone; remains with young for at least 3 weeks after young emerge. Weaned young may stay together for some weeks after separation from mother.
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)
National NatureServe Conservation Status
Rounded National Status Rank: N4 - Apparently Secure
NatureServe Conservation Status
Rounded Global Status Rank: G4 - Apparently Secure
IUCN Red List Assessment
Red List Category
Red List Criteria
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 of Threatened Species: least concern
Relevance to Humans and Ecosystems
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)
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)
Names and Taxonomy
Comments: Formerly included in genus Eutamias, which recently was included in the genus Tamias (Levenson et al. 1985; Jones et al. 1992, Hoffmann et al., in Wilson and Reeder 1993). Based on patterns of variation in ectoparasites (Jameson 1999) and molecular phylogenetics (Piaggio and Spicer 2001), the North American mammal checklist by Baker et al. (2003) placed all North American chipmunks (except Tamias striatus) in the genus Neotamias. Thorington and Hoffmann (in Wilson and Reeder 2005) noted that chipmunks could be legitimately allocated to one (Tamias), two (Neotamias, Tamias), or three (Tamias, Neotamias, Eutamias) genera; they chose to adopt the single-genus (Tamias) arrangement.
See Sutton (1992) for a key to the species of chipmunks.