Social Structure


Chimpanzees live in social groups called communities or unit groups. At Gombe, the number of individuals in the main study community (Kasakela) has ranged between 40 and 60 since 1960. Communities may be larger in other areas, or may be reduced to very small remnant groups.
Chimpanzees' social structure can be categorized as "fusion-fission." This means they travel around in small groups of up to six, the membership of which is always changing as individuals wander off on their own for period of time, or join other groups. At times many of a community's members come together in large excited gatherings, usually when fruit is available in one part of the range, or when a sexually popular female come into estrus. Mothers and dependent young up to age seven or so are always together. And some individuals travel together more often than others - such as siblings and pairs of male friends. Contact is maintained between members of the scattered groups by means of the distance call: the pant hoot.

Within the community a male hierarchy, ordered more or less in linear fashion, establishes social standing, with one male as the alpha. Females have their own, somewhat confused, hierarchy. All adult males dominate all females. Most disputes within a community can, therefore, be solved by threats rather than actual attacks. However, the males of a community regularly patrol their boundaries, and if they encounter individuals of a neighboring community they may attack with extreme brutality. The only individuals who can move freely between communities are adolescent females who have not yet given birth. They may transfer to a new community permanently or, having become pregnant, move back to their own natal group.

When a female is in estrus and sexually attractive and receptive to the males, the skin around her rump swells considerably and is clear pink. Females show their first very small sexual swellings at age eight or nine, but are not sexually attractive to the older males until they reach age 10 or 11. There is usually a two-year period of adolescent sterility before the female finally conceives. Spacing between births, provided the previous infant lives, is about five years.

Some females in estrus are more attractive to the males than others. A popular female may be accompanied by many or all the adult males of her community, with adolescents and juveniles tagging along. Or, the dominant male of the group may show possessive behavior toward her, trying to prevent other males from mating with her. A third observed mating pattern is the consortship, during which a male persuades a female to accompany him to some peripheral part of the community range. If he can keep her there, away from other males, until the time of ovulation, he has a good chance of siring her child. Even low-ranking males can become fathers if they have the skill to lead a female away at a time in her reproductive cycle when she is not interesting to the high-ranking males, and keep her there until her fertile period. At Gombe, chimpanzee males may be capable of reproduction at age 12 or 13, but are not socially mature until a few years later.