Social Organization

Fission-Fusion Society

Bonobos live in a classic fission-fusion society. Basically this means that bonobos live in large communities and travel in smaller parties during the day (de Waal 2001). The communities are usually made up of 25-120 individuals, while the groups that they travel in by day are much smaller. Members in one community are known to mix together, but individuals rarely ever change communities once they have established their membership. The separate communities avoid each other as much as possible, however when they do encounter each other there is not, usually, aggressive conflict (de Waal 2001). The parties during the day usually contain about 7- 20 individuals, both male and female (de Waal 1997). These groups are often called foraging groups. The foraging group composition is constantly changing (on an hourly and daily basis) and often depends on availability of food. According to a study done by Richard Malenky and Frances White, if bonobos have access to a larger "fruiting tree" there will be more bonobos feeding together (de Waal 1997). At night the entire community nests together in the trees for protection from predators (see Predators). Adult bonobos have been known to sleep in the same nest, which is extremely rare in primates (de Waal 2001).

Males

Bonobos are a male-philopatric society, in which males stay in their "natal" groups and women travel to different groups when they are about 7 years old (de Waal 1997). Although the males are bonded through kinship, they do not like to associate with each other. For example, bonobo have been known to kick another male out of a tree when a female comes along, whereas in other chimps, males usually kick the females out (Gibbons 1992). There is a male dominance hierarchy within these natal groups. This dominance hierarchy is determined through competition for females (aggressive chases) and is usually settled with sexual contact (penis rubbing) (de Waal 2001). Male dominance rank is positively correlated with mating success since the females tend to mate with the males that are higher on the dominance hierarchy (Dixson 1998). The male dominance hierarchy is loose and probably developed in order for the females to determine which individual has the best genes.

"The Artificial Sisterhood" (de Waal 1997)

Although bonobo society is male-philopatric, females are dominant over the males. This is most likely due to the strong social bonds that are formed between the females (de Waal 1989). The strong social bonds between the females are often referred to as the "artificial sisterhood". The females within the group are not related at all, but they demonstrate an incredibly close social bond. Hashimoto et al. discovered that female bonobos transfer to different communities of bonobos because the DNA of the females in the group was unique to each individual. They determined that females that are related do not tend to migrate to the same groups. Therefore, sisters, cousins, etc. do not migrate to a particular group because of kinship. The bond between female bonobos is facilitated by sexual contact and grooming behavior. Females travel to different communities in order to avoid incest (which would occur if they remained in their natal groups with the rest of their family). When a new female (usually an adolescent) is trying to join a group she will seek out an older female in the group and try to befriend her. In order to befriend the older female, a young female will groom her or invite sexual contact. This behavior elicits acceptance by the older/higher ranking female, and therefore the lower-ranking females in the group will accept the new member as well. The grooming behavior of the female bonobos also helps solidify and maintain the "aritificial sisterhood" (Di Fiore and Rendall 1994). The "artificial sisterhood" is extremely important because it allows the female bonobos to exploit the richest patches of food and keep the male bonobos away while they are eating (de Waal 1989).

There is no true dominance hierarchy for females; rather they are called "influential" females. This social ranking is most likely due to age and length of membership in that particular group. There are some benefits to being a high ranking female. Alpha females are dominant when it comes to food (de Waal 1997). They eat first and receive the best patches of food. They also receive sexual solicitation and grooming from females trying to become new members in the group. Finally, as mentioned above, a female's social ranking can influence the dominance rank of her son. Therefore it is extremely beneficial for a female's direct fitness if she is a high ranking female (de Waal 1997).

Photo courtesy of Dave Liggett (www.daveliggett.com)

It should be noted that there is a debate in the literature as to whether bonobos engage in reconciliation or tension reduction in order to avoid large amounts of aggression in their society.

Reconciliation

The rate of violence is extremely lower in the bonobos than in chimpanzees. This is demonstrated by the lack of evidence for intercommunity raiding, infanticide, or cooperative hunting. Reconciliation is a large contributor to the nonviolence in bonobos. Reconciliation is defined as a friendly post conflict reunion (Aureli, Cords, and Van Schaik 2002). Although even the presence of reconciliation hints towards, the social cohesiveness of the bonobos is so strong that reconciliation is utilized more than aggression (de Waal 2001). Reconciliation occurs most around food and mating. When there is competition over mating, one male will chase away another and then after he has mated he will reunite and engage in scrotal rubbing with the chased male (de Waal 2001). Similarly, with food, there will be a short aggressive conflict over a patch of food which is quickly resolved with sexual contact. Aureli et al. (2002) studied reconciliation (friendly post conflict reunions) in gregarious animals. They were looking to determine the costs and benefits of friendly post conflict reunions, both for the individuals and their relationship to each other. They predicted that they would find reconciliation between individuals in stable social groups, those who have individualized relationships who experience post conflict hostility, and those individuals in which aggression disrupts important relationships. They were also interested in whether aggressive conflicts occur between cooperative partners and whether that aggression was enough to jeopardize the relationship between these two individuals. They hypothesized that reconciliation between individuals could serve to relieve the negative consequences that resulted from the conflict. This reconciliation could be used as reassurance or appeasement of tension-reduction. According to Aureli et al. (2002) previous studies have shown that friendly interactions between former opponents occur sooner after an aggressive conflict than in control periods. Reconciliation in this case is most likely used to reduce the probability of further attacks and decreases the anxiety associated with agonistic encounters or tension. This is beneficial because the lower risk of the same opponent attacking again solidifies social dominance as well as decreases the energy necessary to defend against those attacks. Aureli et al. (2002) determined that some possible causes for the increase in the frequency of reconciliation could be due to the intensity of the conflict, the decidedness of the outcome, the presence of infants, and being in the reproductive season. They also determined that the degree of kinship is usually the most important factor in reconciliation (Aureli et al 2002, Hohmann and Fruth 2000). This is most likely due to the "Valuable Relationship Hypothesis" which is based on the concept that, "social relationships can be explained at the ultimate level as valuable tools to increase the reproductive success of each partner." If post conflict reunions restore the relationship between former opponents, they should occur more often when the opponents are mutually valued social partners, because disturbance of a more valuable relationship entails a larger loss of benefits for both opponents. This is true for the bonobos because males do have a kinship bond since they are brothers. Therefore it is beneficial to maintain an amiable relationship with each other in order to maintain the dominance hierarchy's balance. Female bonobos also use reconciliation because they need to maintain an incredibly strong social bond in order to remain dominant in bonobo society. Without their "artificial sisterhood" they would not be able to control the best patches of food, which solidifies their dominance.

Tension Reduction

Unlike reconciliation, tension reduction occurs before an activity when aggression could be present between individuals. Tension reduction activities consist of sexual contact, usually when food is present (de Waal 1997). Female-female sexual contact is used to establish submission and to reduce tension of competition for food source within the "artificial sisterhood" (de Waal 1997, Sagan and Druyan 1992). Male-Male sexual contact is used to reduce tensions about competition for food and mates. Female-male sexual contact is used to establish access to food, which is definitely the source for the most aggression within bonobo society (de Waal 1997). Females have been known to mate with males in order to steal their food (de Waal, 1997)

Reciprocal Altruism

Although there have been no studies examining reciprocal altruism in bonobos, there is some evidence for this behavior. The most common activity that is most likely reciprocally altruistic is food sharing. According to de Waal (1989) bonobos often co-feed (or share food) between intimate individuals (like offspring, mates, etc). The female bonobos are most likely to engage in this behavior because of their strong social bonds. They will often forage together, which allows them to easily share the best food sources with the other females in the group. They will also share food with their offspring in order to ensure their survival.

Photo courtesy of Dave Liggett (www.daveliggett.com)