Social Organization

Family Groups Gibbons are organized into family groups, consisting of a mated adult pair and their direct offspring. The offspring usually consist of an infant, a juvenile, an adolescent, and a subadult (Leighton 1986). The infant is newborn to two years old and still clings to the mother, while the juvenile (two to four years old) is able to move around independently (Leighton 1986). The adolescent is not yet full grown at four to six years old and the subadult looks like an adult but does not have a mate (Leighton 1986). Offspring stay with the family group until they reach sexual maturity around the ages of 8 to10, but some may be allowed to stay up to two years after reaching sexual maturity (Brockelman, et al. 1998). The maturing subadult finally leaves its natal territory as a result of growing aggressiveness from the same-sexed parent (Tilson 1981). This is a result of sexual competition between the sexually mature offspring and parent.

Photograph courtesy of S.P. Flannery <http://members.tripod.com/cacajao/index.html>

Usually by the time the mature offspring is forced away by its parent, it has located a mate of its own from a neighboring territory (Brockelman, et al. 1998). If a subadult or mature offspring is forced to leave the natal territory before finding a mate, it has a very small chance of surviving on its own, because ‘floaters,’ or individuals without a group, have a very high mortality rate (Leighton 1986). As a result, most dispersing subadults try to find a mate and territory near their natal territories (Leighton 1986). Without a territory or group, an individual is unlikely to obtain enough food or to be as aware of predators and will probably not survive on its own.

Nuclear Families

The gibbon family groups are typically thought of as nuclear groups, in that the pair members have not previously been paired with other gibbons and the non-adult members of the group are the offspring of the parents (Palombit 1994). In order for gibbons to live in nuclear families, there must be very little turnover in mated pairs, no extra-pair copulations, and pairs must be formed between two previously unmated, newly matured subadults (Palombit 1994). There is evidence of all of these occurrences, so while gibbon groups are typically nuclear, they may also be non-nuclear. In certain gibbon populations, it is common for a subadult leaving its natal group to replace a previously-paired adult rather than mate with another unpaired subadult (Brockelman, et al. 1998).

Photograph courtesy of S.P. Flannery <http://members.tripod.com/cacajao/index.html>

Why Stay Together?

Individual gibbons stay close together, creating a cohesive group (Reichard 1995). They perform most activities together, most often eating when others eat and following the group while foraging (Reichard 1995). Gibbons on their own are more prone to predation, so they prefer to stay close to their family members during the day.

Young gibbons stay in family groups to give them time to mature, learn social behaviors, and stay safe while waiting for an available mate or territory. Gibbon families typically stay together until it is time for subadults to disperse because leaving the group is unnecessary and would increase the risk of predation.

They also remain in their own family groups because helping behaviors are exhibited between subadults and younger offspring, as well as between parents and subadults (see Parents and Offspring). This helping behavior can be extremely beneficial to those receiving the help, but it is a result of direct parental care and indirect fitness and would likely not be exhibited between individuals who are not related. Thus, it benefits the offspring to remain with the family group as long as possible in order to obtain these benefits.