Tamias senex
Allen's chipmunk

Shadow chipmunks, Tamias senex, occur in California, Oregon, and Nevada. The species ranges northward from the central Sierra Nevadas in California, skirting Nevada near Lake Tahoe, and into the eastern Cascade Range of Oregon. In northern California, this species is found from the Pacific Coast to the Warner mountains between the Eel and Klamath rivers.

In the Sierras, shadow chipmunks are limited to elevations above 1,500 m. They do not move to lower elevations on the eastern or western slopes of the range, where other species of chipmunk occur. (Gannon and Forbes, 1995; Verts and Carraway, 1998)

Tamias senex is primarily arboreal and resides in fairly dense, moist, mature forests. It forages on low-strewn forest floors and in adjacent chaparral.

Shadow chipmunk nests are found in hollow trees and fallen logs. The most common nesting trees include Jeffrey pine (Pinus jeffreyi), Ponderosa pine (Pinus ponderosa), sugar pine (Pinus lambertiana), Douglas fir (Pseudotsuga menziesii), white fir (Abies concolor), red fir (Abies magnifica), California black oak (Quercus kellogii), incense cedar (Libocedrus decurrens), and mountain hemlock (Tsiga mertensiana).

The common shrubs occurring in areas inhabited by T. senex include manzanita (Arctostaphylos), California lilac and buckbrush (Ceanothus), chinquapin (Castanopsis sempervirens), and blackberry (Rubus). (Sutton and Patterson, 2000)

Tamias senex is a relatively large chipmunk. The total length of T. senex is between 229 and 261 mm, and the hind foot measures 34 to 38 mm. As is the case for most members of the genus Tamias, females are slightly larger than males, with an average dimorphism ratio of 1.033. Mass of females ranges between 73.0 g and 108.5 g, compared to males, which measure 66.8 g to 99.3 g. Mean head and body lengths also differ between sexes, with females measuring 144.12 mm, compared to 139.54 mm for males.

Pelage is gray. The tail, which is shorter and less bushy than seen in Tamias merriami and Tamias sonomae, has faintly buffy edging, rather than the "distintly buffy" edging seen in Tamias speciosus. (Gannon and Forbes, 1995)

The pelage of inland popluations of T. senex varies seasonally in color, length, and quality. In both the winter and summer, pelage is white ventrally, gray on the top of the head and the rump, and has a marked black dorsal stripe. Two molts occur annually. The winter pelage is replaced by a summer molt just after breeding activity in spring. The summer molt begins at the head and continues toward the rump, resulting in short, coarse, brightly-colored pelage. The winter molt occurs in September, October, and November (just before hibernation) beginning at the rump and continuing toward the head. The winter molt results in long, dense, silky, dull-colored pelage with gray upper parts. (Gannon and Forbes, 1995; Sutton and Patterson, 2000)

Tamias senex is polymorphic. Coastal populations are larger and darker in color, whereas inland populations are smaller and paler. The transition from the dark pelage of coastal specimens to the paler pelage of inland specimens is quite abrupt about 32 km inland from the Pacific coast, near the eastern margin of the coastal forest. This variation in pelage may be an adaptive response which makes these chipmunks more cryptically colored in different habitats. (Sutton and Patterson, 2000)

Genital bones of each sex are diagnostic of T. senex, and allow these animals to be distinguished from Tamias orchrogenys, a species with which it is often confused. Compared to T. orchrogenys, the baculum is longer (3.09 mm versus 2.47 mm) and more slender. (Gannon and Forbes, 1995)

Chipmunks are endothermic. Because they enter hibernation, their body temperature varies, making them heterothermic. However, they may also be considered homoiothermic. Individuals maintain their body temperature around set points; higher when they are active, and lower when they are torpid. (Nowak, 1999; Wilson and Ruff, 1999)

Members of the subgenus Neotamias are reported to live as long as 8 years in the wild, and may reach almost 11 years of age in captivity. Although there are no firm data for T. senex, we may assume that they are similar to other members of their subgenus. (Nowak, 1999)

Tamias senex is generally social. These chipmunks are diurnal, and hibernate from about November through March. Tamias senex is one of the few species in North America which puts on a layer of fat to survive the winter months. Seasonal fat accumulation increases the weight of individuals as much as 20% before November. They experience fewer bouts of wakefulness during hibernation than do other members of the genus. (Gannon and Forbes, 1995; Hartson, 1999; Harvey and Polite, 2005)

These chipmunks are largely arboreal, and like other members of their subgenus, build nests in trees. Such nests may be located as high as 23 m in the trees. Other western chipmunks may also occupy night or winter nests in rocky ground locations. Although data for this species are not available, they are likely to resemble other arboreal chipmunks of western North America in this regard. (Nowak, 1999)

Home Range

Home ranges for Tamias amoenus, a related species, are recorded at 1.6 hectares for males and 0.93 hectares for females. It is likely that T. senex occupies similar home ranges.

Tamias senex show differences in call notes compared to other chipmunks, like T. sonomae. Unlike other species, T. senex does not produce a whistle vocalization. Tamias senex produces an alarm call in 3 to 5 metallic barks with maximum frequency 12.98 kHz and minimum frequency 4.56 kHz. The average call duration is 0.81 seconds, followed by a single chirped note. (Gannon and Forbes, 1995; Hartson, 1999)

In addition to vocal communication, these diurnal mammals use visual cues, such as body posture, in their interactions. During mating season, there are visual changes in female genitalia, indicating that males may use such cues in finding mates. Tactile communication between a mother and her young undoubtedly occurs in the nest, and may play some role in mating behaviors. The role of chemical communication has not been studied in these animals. (Nowak, 1999)

Tamias senex is primarily herbivorous, spending the majority of its time collecting and storing food. Diet consists mainly of the seeds of various herbs and grasses, but also contains fungi, fruits of shrubs and forest trees, and occasional insects. Some examples of food choices are acorns, chinquapins, spruces, hazelnuts, cherries, hemlocks, and berries. (Sutton and Patterson, 2000; Verts and Carraway, 1998)

Natural predators including coyotes, badgers, weasels, snakes, foxes, and large birds of prey, have the most influence on the population of T. senex. Owl attacks are also a threat, although not very common because these chipmunks are usually diurnal, whereas owls are usually nocturnal. (Owca, 2005)

Most small rodents, including the T. senex, play important roles in their ecosystem. Besides being an abundant food source for their predators, they are also an important biotic vector in seed dispersal. They may provide habitat for certain parisites. (Owca, 2005)

When T. senex populations become too dense, this species may become a pest. Chipmunks can damage pine seeds, preventing reforestation. This may cause negative economic consequences to humans. (Owca, 2005)

Direct impact of this species on humans is limited. Tamias senex may have an indirect economic importance to humans as a result of their seed dispersal. (Owca, 2005)

Although the range of this species is not large, these chimpmunks are not thought to be at great risk.

The genus name Tamias is derived from the Greek word tamias, meaning steward. The species name senex means dim or shadowy, in Latin. Common names for T. senex include Allen's chipmunk, gray chipmunk, California chipmunk, Was-La of the Klamath, large mountain chipmunk, and shadow chipmunk. Shadow chipmunk was most recently used by Jameson and Peeters (1988), referring to the dark coloration of the pelage and the preference of this species for densely shaded areas of the forest. (Gannon and Forbes, 1995)