The basic unit of beaver social organization is the family, which is composed of an adult male and an adult female in a monogamous pair and their offspring, both from the current and previous years' litters. Beaver families can have as many as ten members besides the monogamous pair. Groups about this size build multiple lodges, while smaller families usually need only one. However, large families have been recorded living in one lodge. Mutual grooming and play fighting reinforces bonds between family members, and aggressive behavior amongst them is uncommon.