Ermine mate in late spring to early summer. Implantation of fertilized ova is delayed until early in the following spring, and once this happens there is a gestation period of about four weeks. One litter of four to 13 (average of six) young is born usually in April or May in North America. Females care for young alone. Lactation may continue until seven to 12 weeks, although young begin to venture from the nest at six to eight weeks. The nest is typically lined with rodent hair and is often a former burrow of a rodent, or may be in a rock pile, hollow log or crevice. Females are sexually mature at three to four months, while males likely reach sexual maturity at 12 months. Females may survive for at least two breeding seasons but males generally do not survive this long. Their reproductive success is highly dependent on food availability.