Tamias senex

Geographic Range

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.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

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 ).

• Habitat Regions
• temperate temperate
• terrestrial terrestrial

• Terrestrial Biomes
• chaparral chaparral
• forest forest
• mountains mountains

Physical Description

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 .

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.

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.

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.

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.

• Other Physical Features
• endothermic endothermic
• heterothermic heterothermic
• homoiothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger

Reproduction

The mating system of this species has not been described. However, many other species in the genus Tamias are polygynous. Males may travel extensively during the breeding season, seeking females with whom to mate. It is likely that T. senex is similar.

Young animals start to reproduce in the breeding season following their birth. Mating season begins in mid-March, about one month after these chipmunks emerge from hibernation, and continues for about four weeks. Populations found in the Great Basin begin their mating season earlier than populations in the Sierra Nevadas and the Volcanic Plateau.

Gestation is about 28 days, and average litter size is 4.5. The young are born in late May or early June and are nursed by the mother until early August.

As the breeding season approaches, male reproductive organs increase dramatically in size. Testis length doubles, and the prostate and cauda epididymis triple in width. In addition, both the diameter of the bulbo-urethral glands and the length of the seminal vesicles increase five times. Although the swelling and post-breeding reduction of the testes are gradual, changes in the bulbo-urethral glands and semial vesicles occur abruptly.

In females, the vulva perforate at the onset of estrus. The genital region becomes swollen, and the uterine horns are inflated and twisted. Nipples become enlarged during estrus and pregnancy, and reach maximum size during lactation.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
• viviparous viviparous

Details on the parental investment of these animals are not available. However, in other species of Tamias , males provide no parental care. Tamias senex is altricial at birth. Females provide all protection and care to the young. Young usually disperse in the late summer or autumn of their birth year.

• Parental Investment
• altricial altricial
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
• pre-weaning/fledging
  • provisioning
  • protecting
    • female
• pre-independence
  • protecting
    • female

Lifespan/Longevity

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.

Behavior

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.

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.

• Key Behaviors
• arboreal arboreal
• scansorial
• diurnal diurnal
• motile motile
• sedentary sedentary
• hibernation hibernation
• social social

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.

Communication and Perception

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.

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.

• Communication Channels
• visual visual
• tactile tactile
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

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.

• Primary Diet
• herbivore herbivore
  • granivore granivore

• Animal Foods
• insects

• Plant Foods
• leaves
• seeds, grains, and nuts
• fruit
• flowers

• Other Foods
• fungus

• Foraging Behavior
• stores or caches food stores or caches food

Predation

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.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

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.

• Ecosystem Impact
• disperses seeds

Economic Importance for Humans: Positive

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.

Economic Importance for Humans: Negative

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.

Conservation Status

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

Other Comments

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.

Additional Links

Encyclopedia of Life

Contributors

Marie Winans (author), California State Polytechnic University, Pomona, Nancy Avalos (author), California State Polytechnic University, Pomona, John Demboski (editor, instructor), California State Polytechnic University, Pomona.

Nancy Shefferly (editor), Animal Diversity Web.

References

Gannon, W., R. Forbes. 1995. Tamias senex. Mammalian Species , 502: 1-6.

Hartson, T. 1999. Squirrels of the West . New York: Lone Pine Field Guide.

Harvey, T., C. Polite. 2005. "Department of Fish and Game" (On-line). Accessed November 01, 2005 at http://www.dfg.ca.gov .

Nowak, R. 1999. Walker's Mammals of the World, Sixth Edition . Baltimore and London: The Johns Hopkins University Press.

Owca, D. 2005. "www.chipmunks-chipmunks.com" (On-line). Accessed December 04, 2005 at http://www.chipmunks-chipmunks.com/chipmunks-management.htm .

Stewart, A., R. Elis, C. Krueger, A. Pertschuck. 2005. "National Wildlife Federation" (On-line). Accessed November 04, 2005 at http://enature.com/home/ .

Sutton, D., B. Patterson. 2000. Geographic variation of the western chipmunks Tamias senex and T. siskiyou with two new subspecies from California. Journal of Mammalogy , 81/2: 299-316.

Verts, B., L. Carraway. 1998. Land Mammals of Oregon . Canada: The Regents of the University of California.

Wilson, D., S. Ruff. 1999. The Smithsonian Book of North American Mammals . Canada: UBC Press.