Solitary living has been regarded as a primitive ancestral state in mammals (1). Thus, behavioral ecologists studied for decades pair- and group-living species, while solitary species have been ignored (1, 2). However, we cannot understand the ecological and evolutionary reasons of group-living if we do not understand the costs and benefits of the alternative, which is solitary living (1). So far, it has been assumed that individuals of solitary species are aggressive toward each other, only interacting during mating and territorial disputes (1, 2). In a study published in the current issue of PNAS, Twining and a consortium of 38 coworkers present evidence for seven species of solitary mustelids that their social life is much more complex than previously believed (3). This study adds to growing evidence that solitary living mammals are not unsocial per se (1). Here, we argue that solitary living in mammals is not a primitive ancestral stage but an adaptation to local environments.