Breeding season shorter in north and at high elevations than elsewhere. In the northeastern U.S., reproduction is curtailed in fall and winter. See Kirkland and Layne (1989) for information on breeding seasons in different areas. Gestation 23 days. Litter size averages 5-6 in north, 4.5 in south, multiple litters/year (1-2 in north, more in south). Young independent in about 16-25 days (varies geographically). Young of year may attain sexual maturity by 2 months, or may not breed in some areas. Some litters fathered by more than 1 male; mating system ranges from promiscuity to facultative monogamy (Kirkland and Layne 1989).