Female territoriality (defined here as defense of exclusive space with respect to conspecific females) among mammals typically has been considered a mechanism to defend food resources. We propose an alternative hypothesis that the primary function of territoriality in solitary and semi-social female mammals is to protect vulnerable young from infanticidal conspecific females. Female territoriality is prominent among rodents, carnivores, insectivores, and some lagomorphs, but occurs rarely in bats, ungulates, marine mammals, marsupials, and most primates. A common trait of species with female territoriality is that they have nonmobile altricial young that are deposited in a burrow or protected den site. In contrast, female territoriality does not occur in species with precocial young or species with altricial young that are carried with the mother or that are reared communally. The timing of female aggression and territoriality appears to be more closely associated with lactation than with food habits or the distribution and abundance of food. Hence, we hypothesize that vulnerability of altricial young to infanticide, is a pervasive factor in the evolution of territoriality in female mammals. Territoriality for defense of food appears to be relatively uncommon and restricted to those situations in which food is centrally located or can be stored in a defensible cache.