Describing the most highly developed form of animal societies, such as those of colonial ants, termites, wasps, and bees (see hymenoptera). Typically there is extensive division of labour and cooperation, with various castes each performing particular tasks, such as food-gathering, defence, or tending to the young. Reproduction is undertaken by an elite of fertile individuals, assisted by sterile workers. Moreover, there is an overlap of different generations within the colony at any one time, and continuity of the society is maintained from year to year. Some vertebrate animals may display equivalent levels of social organization. For example, colonies of naked mole rats have a caste system, with nonbreeding adult members that forage, maintain the burrow, or simply keep the other group members warm.