Thirteen-lined ground squirrels are sexually mature at 9 or 10 months of age. They mate within 2 weeks after emerging from hibernation. Both sexes are sexually active for about 2 weeks. After a gestation period of 28 days, 3 to 14 (average 10) blind, naked, toothless young are born. Only 1 litter is produced per female each year. The young weigh about 1/10 ounce at birth. Their stripes begin to appear after about 12 days and their eyes open after 28 to 30 days. Young are weaned after 6 to 12 weeks.
Eastern chipmunks are sexually mature within a year and mate twice per year, during early spring and summer through early fall. They have a 31-day gestation period. Two to 5 young are born in April or May, and again in August or October.
Least chipmunks mate over a period of 4 to 6 weeks from April to mid-July. They produce a litter of 2 to 7 young. Occasionally, a second litter is produced in the fall. Pups appear above ground when they are 4 to 6 weeks old at about 2/3 the size of an adult. Young leave the burrow at 6 to 8 weeks.
The inconspicuous, 2-inch diameter openings to burrows of 13-lined ground squirrels (Figure 4) often are concealed by vegetation and rarely have soil scattered in front.
The main entrance plunges down 6 inches or more before angling off into a complex system of galleries and side entrances. The chamber for rearing young is about 9 inches in diameter, lined with fine dry grass, and is located somewhat deeper than the main burrow system. Natural enemies include most predator species, especially hawks, badgers, weasels, foxes, coyotes, bull snakes, and black snakes.
Chipmunk burrows often are well-hidden near objects or buildings (e.g., stumps, wood piles, brush piles, basements, and garages). The burrow entrance usually is about 2 inches in diameter. No obvious mounds of soil exist around the entrance, because chipmunks carry the soil in their cheek pouches and scatter it away from the burrow, making the entrance less conspicuous. In most cases, the main tunnel is 20 to 30 feet in length, but more complex burrow systems occur where cover is sparse. Burrow systems normally include a nesting chamber, 1 or 2 food storage chambers, various side pockets connected to the main tunnel, and separate escape tunnels.
Thirteen-lined ground squirrels are diurnal, coming above ground when the sun is high and the temperature is above 80°F, and returning to the burrow long before sundown. They rarely venture out of the burrow on damp, dark, or overcast days. They often stand upright, with front paws held close to the chest, surveying their territory. If danger threatens, they run with tail held horizontally to the nearest burrow.
Thirteen-lined ground squirrels begin hibernation in September or early October and emerge between late March and early May in the northern portions of their range. Males usually begin hibernation earlier in fall and emerge earlier in spring than females. When they hibernate, their body temperature generally is within a few degrees of the ambient air temperature.
With the onset of cold weather, chipmunks enter torpor and are relatively inactive from late fall through the winter. Chipmunks do not enter a deep hibernation and rely on the cache of food they have established in their burrow. Some individuals become active on warm, sunny days during the winter. Most chipmunks emerge from torpor by early March.
Thirteen-lined ground squirrels prefer grassy areas that are cut low. They avoid wet areas with heavy clay soils. Eastern chipmunks typically inhabit mature woodlands and edges of woodlots, but they also inhabit areas in and around homes. Home ranges of 13-lined ground squirrel vary between 3 to 12 acres. Thirteen-lined ground squirrels do not defend home ranges but will defend dens. Densities of adults can reach 10 animals per acre, and increase dramatically when young emerge.
Chipmunks generally are solitary, except during courtship or when rearing young. Least chipmunks inhabit low sagebrush deserts, high mountain conifer forests, and northern mixed hardwood forests. Home ranges of chipmunks may be up to ½ acre, but adults only defend a territory about 50 feet around the entrance of their burrow. Densities of chipmunks typically are 2 to 4 animals per acre, but densities of eastern chipmunks may be as high as 10 animals per acre if sufficient food and cover are available. Home ranges often overlap among individuals.
Thirteen-lined ground squirrels eat vegetation, the eggs and young of ground-nesting birds, earthworms, lizards, mice, and insects. During summer, insects constitute up to ½ of their diet. Thirteen-lined ground squirrels prefer grasshoppers and beetle and moth larvae. They also consume seeds, green shoots, flower heads, roots, vegetables, fruits, and cereal grains. They rarely drink water, depending on moisture contained in their food. They cache large quantities of seeds and grass. The cached food may be eaten during periods of bad weather or when other food is scarce.
Chipmunks primarily eat grains, nuts, berries, seeds, mushrooms, insects, and carrion. Although chipmunks mostly are ground-dwelling rodents, they regularly climb trees to gather nuts, fruits, and seeds. Chipmunks cache food in their burrows throughout the year. By storing and scattering seeds, they promote the growth of various plants. Chipmunks also prey on young birds and bird eggs.