The breeding habits are known in detail for only a few species. In the ermine and the long-tailed weasel, implantation of the fertilized egg is delayed, but in other species apparently not. The young are born blind and thinly furred, in a secure nest, often borrowed from a prey species and lined with fur from previous meals. Both sexes of the European common and least weasel may mature at 3-4 months if food is abundant. Weasels hunt largely underground or under snow, and are active day and night.
Some larger species are more often nocturnal.
Home ranges are smaller in habitats rich with prey, in the nonbreeding season, and in the smallest species. However, four grisons observed together in captivity showed no evidence of the mutual grooming characteristic of social animals. Most mustelids, sociable or not, may be seen in parties of females and young just before the dispersal of the litters. Some people persecute small mustelids as pests of game and poultry. The extinction of the European polecat in England was probably due to intensive gamekeeping.