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)
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. (Wilson and Ruff, 1999)
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. (Gannon and Forbes, 1995; Wilson and Ruff, 1999)
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. (Gannon and Forbes, 1995)
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. (Gannon and Forbes, 1995)
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. (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)
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)
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)
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.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
uses sound to communicate
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
Referring to an animal that lives in trees; tree-climbing.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.
uses smells or other chemicals to communicate
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
union of egg and spermatozoan
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
an animal that mainly eats seeds
An animal that eats mainly plants or parts of plants.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
having the capacity to move from one place to another.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
the area in which the animal is naturally found, the region in which it is endemic.
breeding is confined to a particular season
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
places a food item in a special place to be eaten later. Also called "hoarding"
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
uses sight to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
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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.