The mating system of these animals has not been well described. Males emerge from hibernation earlier than females, and apparently engage in some level of competition for mates. It is likely, therefore, that the species is either polygynous or polygynandrous.
Individuals become sexually mature at 10 months of age (Kurta, 1995). Most mating occurs in April when females first emerge from hibernation. Gestation lasts approximately 30 days (Hamilton and Whitaker, 1979). Litter size varies from 2 to 6 young. There is normally a single litter during the breeding season, although females may produce a second litter if their first litter is lost (Burt, 1946). Newborns are naked and pink in color, measuring 50 mm in length and weighing an average of 2.25 g (Banfield, 1974). Eyes open at 28 days and fur is fully grown in by 40 days (Baker, 1983). Lactation lasts approximately 60 days and offspring remain with the mother for six weeks or longer (Kurta, 1995).
Breeding interval: These chipmunks usually breed once per year.
Breeding season: Breeding occurs in early April when these chipmunks awake from hibernation.
Range number of offspring: 2 to 6.
Average gestation period: 30 days.
Average weaning age: 60 days.
Average time to independence: 60 days.
Average age at sexual or reproductive maturity (female): 10 months.
Average age at sexual or reproductive maturity (male): 10 months.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous
Average birth mass: 2.3 g.
Average gestation period: 30 days.
Average number of offspring: 4.9.
Parental care in least chipmunks is extensive. Young are altricial, and are not even fully furred until they reach about 40 days of age.
Females chose nursery nests while they are pregnant. These nests are located in stumps, under logs, in brush piles, or rock piles. They are generally connected to chambers filled with cached food supplies. A female positions her nursery nest so that it is protected from rainfall and runoff, to ensure the comfort and health of her offspring when they arrive. Nests are often lines with grass.
Mothers take care of their young until they are weaned, sometime after 60 days of age. They provide food, shelter, grooming, and other care for the pups.
The role of males in the care of offspring is not certain. There are some indications that males may help to defend the home range of female's whose young they have sired. They may even help to maintain the nursery nest, and bring food to the young.
Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Male, Female); pre-weaning/fledging (Provisioning: Female, Protecting: Male, Female); pre-independence (Provisioning: Male, Female, Protecting: Male, Female)