Common Chimpanzee - Behavior - Group Structure

Group Structure

Common chimpanzees live in communities that typically range from 20 to more than 150 members, but spend most of their time travelling in small, temporary groups consisting of a few individuals, "which may consist of any combination of age and sex classes." Both males and females will sometimes travel alone. The common chimpanzee lives in a fission-fusion society and may be found in groups of the following types: all-male, adult females and offspring, consisting of both sexes, or one female and her offspring. Chimpanzees have complex social relationships and spend a large amount of time grooming each other.

At the core of social structures are males, which roam around, protect group members, and search for food. Males remain in their natal communities, while females generally emigrate at adolescence. As such, males in a community are more likely to be related to one another than females are to each other. Among males, there is generally a dominance hierarchy and males are dominant over females. However, this unusual fission-fusion social structure, "in which portions of the parent group may on a regular basis separate from and then rejoin the rest," is highly variable in terms of which particular individual chimpanzees congregate at a given time. This is mainly due to chimpanzees having a high level of individual autonomy within their fission-fusion social groups. Also, communities have large ranges that overlap with those of other groups.

As a result, individual chimpanzees often forage for food alone, or in smaller groups (as opposed to the much larger "parent" group, which encompasses all the chimpanzees which regularly come into contact and congregate into parties in a particular area). As stated, these smaller groups also emerge in a variety of types, for a variety of purposes. For example, an all-male troop may be organized to hunt for meat, while a group consisting of lactating females serves to act as a "nursery group" for the young. An individual may encounter certain individuals quite frequently, but have run-ins with others almost never or only in large-scale gatherings. Due to the varying frequency at which chimpanzees associate, the structure of their societies is highly complicated.

Male chimpanzees exist in a linear dominance hierarchy. Top-ranking males tend to be aggressive even during dominance stability. This is likely due to the chimp’s fission-fusion society, with male chimps leaving groups and returning after extended periods of time. With this, a dominant male is unsure if there has been any "political maneuvering" and must re-establish his dominance. Thus, a large amount of aggression occurs 5–15 minutes after a reunion. During aggressive encounters, displays are preferred over attacks.

Males maintain and improve their social ranks by forming coalitions. These coalitions have been characterized as "exploitive" and are based on an individual’s influence in agonistic interactions. Being in a coalition allows males to dominate a third individual when they could not by themselves, as politically apt chimps can exert power over aggressive interactions regardless of their rank. Coalitions can also give an individual male the confidence to challenge a dominant male. The more allies a male has, the better his chance of becoming dominant. However, most changes in hierarchical rank are caused by dyadic interactions. Chimpanzee alliances can be very fickle and one member may turn on another if it serves him.

Low ranking males commonly switch sides in disputes between more dominant individuals. Low ranking males benefit from an unstable hierarchy and have increased sexual opportunities. In addition, conflicts between dominant males causes them to focus on each other rather than the lower ranking males. Social hierarchies among adult females tend to be weaker. Nevertheless, the status of an adult female may be important for her offspring. Females in Taï have also been recorded to form alliances. Social grooming appears to be important in the formation and maintenance of coalitions. It is more common among adult males than adult females.

Chimpanzees have been described as highly territorial and are known to kill other chimps, although Margaret Power wrote in her 1991 book The Egalitarians that the field studies from which the aggressive data came, Gombe and Mahale, use artificial feeding systems that increased aggression in the chimpanzee populations studied, so might not reflect innate characteristics of the species as a whole. In the years following her artificial feeding conditions at Gombe, Jane Goodall described groups of male chimps patrolling the borders of their territory brutally attacking chimps which had split off from the Gombe group. A study published in 2010 found that the chimpanzees wage wars over land, not mates. Patrol parties from smaller groups are more likely to avoid contact with their neighbors. Patrol parties from large groups will even take over a smaller group's territory, gaining access to more resources, food and females.