The nest is often lined with mouse fur. The number of young born and the number that survive until weaning depend in large part on the abundance of food in the mother's home range. Young weasels remain in the den for 30 to 45 days, at which time they are about two-thirds grown. After emerging, they stay near the home den for a week or two before beginning to accompany their mother on foraging trips. At this time, they disperse away from their mother's home range.
Sexual maturity is reached by the following spring.
Weasels must have almost daily access to rodents in order to survive. They prefer mice. When mice are not abundant, weasels will also take shrews, pikas, birds, fish, and insects. Short-tailed weasels can also kill young snowshoe hares. Weasels have a high metabolic rate which drives them in search of prey.