House mice have two forms of social behaviour, the expression of which depends on the environmental context. House mice in buildings and other urbanized areas with close proximity to humans are known as commensal. Commensal mice populations often have an excessive food source resulting in high population densities and small home ranges. This causes a switch from territorial behaviour to a hierarchy of individuals. When populations have an excess of food, there is less female-female aggression, which usually occurs to gain access to food or to prevent infanticide. Male-male aggression occurs in commensal populations, mainly to defend female mates and protect a small territory. The high level of male-male aggression, with a low female-female aggression level is common in polygamous populations. The social unit of commensal house mouse populations generally consists of one male and two or more females, usually related. These groups breed cooperatively, with the females communally nursing. This cooperative breeding and rearing by related females helps increase reproductive success. When no related females are present, breeding groups can form from non-related females.