Richard Wrangham¹ stated that social systems of non-human primates are best classified by the amount of movement by females occurring between groups. He proposed four categories:

• Female transfer systems – females move away from the group in which they were born. Females of a group will not be closely related whereas males will have remained with their natal groups, and this close association may be influential in social behavior. The groups formed are generally quite small. This organization can be seen in chimpanzees, where the males, who are typically related, will cooperate in defense of the group's territory. Among New World Monkeys, spider monkeys and muriquis use this system.
   

• Male transfer systems – while the females remain in their natal groups, the males will emigrate as adolescents. Polygynous and multi-male societies are classed in this category. Group sizes are usually larger. This system is common among the Ring-tailed Lemur, capuchin monkeys and cercopithecine monkeys.
   
• Monogamous species – a male–female bond, sometimes accompanied by a juvenile offspring. There is shared responsibility of parental care and territorial defense. The offspring leaves the parents' territory during adolescence. Gibbons essentially use this system, although "monogamy" in this context does not necessarily mean absolute sexual fidelity.
   
• Solitary species – often males who defend territories that include the home ranges of several females. This type of organization is found in the prosimians. Orangutans do not defend their territory but effectively have this organization.

Photo: Japanese Macaques bathe together in Jigokudani Hot Spring