section_path: Behavior/Social systems
section_name: Social systems
paragraph_text: Social systems
  
  Richard Wrangham stated that social systems of primates are best classified by the amount of movement by females occurring between groups. He proposed four categories:
  
  Other systems are known to occur as well. For example, with howler monkeys and gorillas both the males and females typically transfer from their natal group on reaching sexual maturity, resulting in groups in which neither the males nor females are typically related. Some prosimians, colobine monkeys and callitrichid monkeys also use this system.
  
  The transfer of females or males from their native group is likely an adaptation for avoiding inbreeding. An analysis of breeding records of captive primate colonies representing numerous different species indicates that the infant mortality of inbred young is generally higher than that of non-inbred young. This effect of inbreeding on infant mortality is probably largely a result of increased expression of deleterious recessive alleles (see Inbreeding depression).
  
  Primatologist Jane Goodall, who studied in the Gombe Stream National Park, noted fission-fusion societies in chimpanzees. There is fission when the main group splits up to forage during the day, then fusion when the group returns at night to sleep as a group. This social structure can also be observed in the hamadryas baboon, spider monkeys and the bonobo. The gelada has a similar social structure in which many smaller groups come together to form temporary herds of up to 600 monkeys. Humans also form fission-fusion societies. In hunter-gatherer societies, humans form groups which are made up of several individuals that may split up to obtain different resources.
  
  These social systems are affected by three main ecological factors: distribution of resources, group size, and predation. Within a social group there is a balance between cooperation and competition. Cooperative behaviors in many primates species include social grooming (removing skin parasites and cleaning wounds), food sharing, and collective defense against predators or of a territory. Aggressive behaviors often signal competition for food, sleeping sites or mates. Aggression is also used in establishing dominance hierarchies.
  
  In November 2023, scientists reported, for the first time, evidence that groups of primates, particularly bonobos, are capable of cooperating with each other.