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.
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.
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.
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.
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.