Ermine are a polygynous-promiscuous species, with males and females mating opportunistically.
Mating System: polygynandrous (promiscuous)
Ermine mate in late spring to early summer. Females are polyestrous, but produce only 1 litter per year. Young are born in April or May after an average gestation period of 280 days, which includes an 8 to 9 month period of developmental delay. Longer days beginning in March trigger the resumption of fetal development. Litter size ranges from 3 to 18 offspring and averages 4 to 9. The sex ratio is unequal. Young are blind and helpless. They are covered with fine white hair, and a prominent dark mane of dense fur develops around the neck by the third week (function unknown). The young grow quickly and are able to hunt with their mother by their eighth week. Although females do not reach adult size until a least 6 weeks after birth, they are able to mate when they are 60 to 70 days old, often before they are weaned. Males do not breed or gain adult dimensions until their second summer.
Females in nature may survive for at least 2 breeding seasons, while males generally do not survive this long. Reproductive success is highly dependent on food availability.
Breeding interval: Ermine generally breed once yearly.
Breeding season: Ermine mate in late spring to early summer.
Range number of offspring: 3 to 18.
Average number of offspring: 4-9.
Average gestation period: 280 days.
Average weaning age: 8-10 weeks.
Average age at sexual or reproductive maturity (female): 60-70 days.
Average age at sexual or reproductive maturity (male): 2 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous ; delayed implantation ; embryonic diapause
Average birth mass: 2.3 g.
Average gestation period: 43 days.
Average number of offspring: 6.77.
Average age at sexual or reproductive maturity (male)
Sex: male: 365 days.
Average age at sexual or reproductive maturity (female)
Sex: female: 95 days.
Females exclusively care for their offspring, nursing and protecting them until they become independent. The young are born blind and helpless.
Parental Investment: no parental involvement; altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female)