Tamias striatus - Eastern Chipmunk
Due to the distinctive striped pattern of its pelt, the eastern chipmunk (Tamias striatus), is an easily identified squirrel (Sciuridae). There are five dark and two white or buff stripes that run the length (5.5 to 6.5inches) of their reddish-gray and brown bodies to the rump where an obvious red patch is located. There are two dark stripes on their face, one through the eye and the other underneath it. Their bellies are either white or light tan with eight mammae. This coloration helps them to blend into the forest floor by breaking up the shape that the predators see. Predators of the eastern chipmunk include foxes, owls, hawks, snakes, weasels, coyotes, cats, dogs, and even bears (Morgan et al 1997). They are very clean animals able to reach every part of their bodies reducing the chance of external parasites. The average weight of an eastern chipmunk is ~100 grams (de Silva et al. 2002). It is a small, diurnal animal living on average 1.3 years (because of high juvenile mortality), a maximum of twelve years, that burrows in the ground solitarily, but is capable of climbing trees and using the ground for quick transportation. Their tails are not bushy but can be long, short, broad thin, dark, light, blunt, or pointed. In contrast, the least chipmunk is identified as having a longer tail with back body stripes extending down to the tails base and an extra set of premolars in the upper jaw. The eastern chipmunk has a total of twenty teeth consisting of one set of incisors, one set of premolars and three sets of molars. Characteristic of rodents, they have five digits on back paws four on the front paws. There are no distinct differences between the males and females except during mating season. They molt in early summer and sometimes in fall. The overall skeletal shape of the body is a combination between fossorial animals (e.g. woodchuck) and arboreal animals (e.g. red squirrel). An adaptive feature that allows the eastern chipmunk to hoard food is the internal cheek pouches used for storage during transport to the burrow. These pouches are also used to carry dirt away from the burrow during construction to avoid leaving noticeable piles (Morgan et al. 1997). When the pouches of the eastern chipmunk are full they are each nearly the size of their head (de Silva et al. 2002). The least chipmunk has a smaller version of cheek pouches inside their mouths.
The origin of the Tamias striatus goes back to the Miocene epoch almost twenty million years ago and is thought to have remained unchanged. The Tamias is thought to be one of the most primitive living squirrels. The versatility to use trees, ground and burrows is a reflection of the possible adaptations evolved from its closest ancestors (Wishner 1982).
Distribution in Wisconsin:
The eastern chipmunk is concentrated on the eastern half of North American not far past the Mississippi River. Their range extends over the entire state of Wisconsin. Fossil records indicate that at one time they were abundant across the entire continent.
The chipmunk has been described as a short-term hibernator. They stockpile food for the winter and take cover in their burrows. Females become sexually mature at approximately three months of age while the males take nine to ten months to mature. Upon emerging from hibernation in spring the males testicles descend to the scrotal sac making them distinguishable from the females. The sac is covered with whitish-gray fur. The darker the fur the more mature the chipmunk is (Wishner 1982). The male and female cycles are independent from one another. The females exhibit synchronous estrus. They can also be identified due to enlargement of the vaginal opening. The females chose mates. This begins before they come into estrus with the male daily coming onto the female’s territory chasing and acknowledging her. The female will chase and wrestle with the males to see who can keep up with her (Wishner 1982). The day that the female comes into estrus (the end of February, beginning of March) she will choose her mate. The male initiates copulation by flicking his tail vertically and trilling (a distinct scream made by the eastern chipmunk). The copulation is no longer than five minutes, but on occasion couples have been observed staying together for up to two hours grooming and eating afterward. The female will then chase the male away (Wishner 1982). After the first mating season the testicles ascend until the second mating season in mid summer. Eastern chipmunks in Wisconsin mate only in the summer due to the cold weather March.
After the mating the female begins to prepare her burrow for the upcoming litter. To do this, she will dig new entrances into her burrow although never using them. The female also gathers leaves and bedding for a nest and stockpiles food. The females have been noted to eat more animal matter during gestation and have been observed stealing goods from other burrows. Towards the end of gestation the female becomes more territorial of her burrow and the surrounding area. The gestation period is approximately thirty-one days. The litter size ranges from one to nine, but is at average four to five. The young are born helpless, naked with their eyes and ears closed. At forty day old they are seventy percent of their total adult size and are ready to emerge from the burrow. The mother watches and directs the young for two or three days close to the burrow opening. Interaction between the mother and child is decreased while sibling –sibling interactions increase simulating aggression and mating (Wishner 1982). Once the young are able to climb they are then chased away by the mother to set up a burrow of their own. Mostly the young find an unoccupied burrow and make it a home or hide under sticks, in treeholes or hollowed trees until able to construct a burrow of their own (Clarke and Kramer 1994). Some of the offspring, usually females, are burrowed close to their mother’s home range, while others, mainly males, go outside of the area.
Ecology and Behavior:
Communication between eastern chipmunks begins with chattering between the mother and child. There are five different vocalizations recognized from chipmunks (de Silva et al. 2002). The loudest is the chip, which is exclaimed once or repeatedly for up to thirty minutes. The chip is made to express existence. The chuck is a low-pitched chip that expresses anger, annoyance, or fear. Chucks can be a sign of danger until ceased or used intraspecifcally as a threat. Trills are used for play or danger. The trill is a scream used when pouncing or being pounced on. It is also used for mating and playing. The chatter is rapid growling heard during confrontations also being used between the mother and child. Chipmunks also whistle or scream in both happiness and fear (de Silva et al. 2002). A chipmunk chorus has also been observed when all of the chipmunks in the area sing together. This called an “epideictic display.” The purpose of this is unknown. Chipmunks also use both urine and feces to mark where they have been and possess anal scent glands. Females in estrus are the only other chemical communication that they use (Wishner 1982).
The most obvious behavior of the eastern chipmunk is its constant foraging and hoarding of food. The chipmunks hoard food from sunrise to sunset. They eat a wide variety of things including: nuts, seeds (maple, beech, oak), fruit (many types of berries), mushrooms, animal matter, earthworms, slugs, grubs, insects, frogs, salamanders, caterpillars, and ants. The hoard instinct that the chipmunks possess can even outweigh caution. This obsession is useful for females with young and also chipmunks during the winter. They store more than enough food to survive being inactive during the winter, Morgan et al. (1997) added that the chipmunk stores enough food under its grass and leaf mattress that it hits the ceiling of their basketball size sleep chamber. They are not true hibernators and the short period in which they may hibernate is unpredictable. Eastern chipmunks also produce water thru energy metabolism of nutrients when water is scarce.
This intense foraging may also explain the home ranges of the eastern chipmunk. The home range consists of a core area that is approximately seventy-five square feet from the burrow. Depending on the fruiting seasons, home ranges are expanded to acquire as much food as possible (Lacher and Mares 1996). During the mating season males will completely leave their home range in search of females, while females stay closer to their burrow. This core area is very familiar to the chipmunks. They learn routes using landmarks that elevate themselves, along with standing on their hind legs for better vision. This familiarity produces confidence. If they do get frightened they will return to the last elevator to look around. The chipmunks usually stay closer than farther away from their burrows utilizing their ability to quickly scamper over short distances (Morgan et al. 1997). Home ranges do overlap and invasion of their burrow is a strong fear since they hoard food. According to Graybill (1970), home ranges were possibly fixed over time due to chipmunks getting most of their energy from mast, which falls in the autumn when they are most easily observed Lacher and Mares (1996) state that distribution and home range size is related to gender and age class differences taking into account the access to certain mast producing species. They found that there was an association between adult males and white oaks. Also found was an association between juveniles and red or black oaks and/or an exclusion from white oaks. This is consistent with their hypothesis that white oaks are a preferred resource. Furthermore, Lacher and Mare (1996) suggest that females also base home sites on white oak distribution, construction of burrow sites and the presence of dominant males to defend against intruders during the mating season. Males were found in the red and black oaks but females were not. Therefore males may occupy and defend white oaks for both nutrition and access to females.
The name chipmunk has been assumed to come from the American Indian “Adjidaumo” pronounced a-chit’-a-mauk. This means “head-first” which is how squirrels descend trees (Wishner 1982). Scheffel (1991) also notes that Tamias means treasurer illustrating the chipmunk’s hoarding ability and obsession. The eastern chipmunk is often characterized by its alertness and speed. They have also received false accusations of digging up planted bulbs for food. During the mating season females are very aggressive and if approached too aggressively by the male will bite its tail, sometimes removing half of it. This can be easily accomplished due to the tendency of chipmunks to run with their tail aloft making predators more likely to get a mouthful of hair instead of flesh (Morgan et al. 1997). Wernert (1982) describes the eastern chipmunk home as a 12 –30 foot long burrow consisting of a storage chamber, sleeping room (able to hold a half bushel), dump, and latrine with several entrances. These burrows are even constructed under stone/brick buildings due to their tolerance of humans (Kurta1995).
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Reference written by Jacalyn Turner, Biol 378: Edited by Chris Yahnke. Page last updated 4-29-04.