Males are generally twice as large as females. Their short, moderately fine fur is white in the winter and the tip of the tail is black. They are specialist predators on small, warmblooded vertebrates, preferably mammals of rabbit size and smaller. When mammalian prey is scarce, ermine eat birds, eggs, frogs, fish, and insects. In severe climates, ermine frequently hunt under snow and survive entirely on small rodents and lemmings.
Daily meals are essential to meet the ermine's exorbitant energy and heat production demands.
Reproduction Ermine are a polygynous-promiscuous species. They 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 8-9 month period of developmental delay. Longer days beginning in March trigger the resumption of fetal development.