Life in the group
In a chimpanzee community, several adult males live together with females and their offspring. Such groups can have between 12 and 150 members. However, the animals do not stay together all day long but instead prowl around in smaller groups, called parties.
These consist of between 2 and 20 individuals of varying composition. The male chimpanzees from different parties remain in touch with one another by drumming on large tree roots. The sound produced can be heard within a one-kilometre radius. The entire group, however, can meet up again in the evening to collectively build sleeping nests in the trees.
The males are the dominant sex in a chimpanzee group. A rigid hierarchy exists among them. Once they reach adulthood at the age of 15, they must assert themselves within the group. The highest ranking male is known as the alpha male. There is also a hierarchy among the females, albeit a less well defined one.
Social rank is an important factor for reproductive success. In the Taï National Park in the Ivory Coast, Christophe Boesch and his team from the Max Planck Institute for Evolutionary Anthropology in Leipzig have shown that on average half of all offspring are sired by the highest ranking male. The decisive factor in the paternity rate was the number of competitors: If there were less than four males in the group, two-thirds of all offspring were fathered by the alpha male. However, where there were more than four competitors, the paternity rate fell to one-third. Even when there were several fertile females in the group, this reduced the success of the highest ranking male. If there are numerous females in the group, the second- or third-ranking males also have the opportunity to mate. Using paternity tests, the researchers also proved that the females look beyond their own community when it comes to selecting partners. Of 19 chimpanzee offspring tested, two had a father from another group.
While male chimpanzees spend their entire lives in the same community, the females leave their birth group as soon as they reach adulthood. They join other communities to start families. They have their first child at the age of 13 on average. Individual infants are usually only born every five years. They initially cling to their mother’s belly and are later carried around on the back. They are breastfed for up to five years. After six to twelve months, they begin to explore their environment and to meet playmates. Even after the infants have been weaned, they still stay close to their mother for many years. She teaches them everything they need to know in order to survive.
However, as Christophe Boesch has discovered, maternal care is not always distributed equally. Among the Taï chimpanzees, for example, high-ranking females spend longer caring for their sons than their daughters. If the offspring is male, the gap to the next birth is two years longer in comparison with low-ranking females. The explanation for this discrepancy may lie in evolutionary biology, which suggests that females invest more time in the sex that promises more grandchildren. This is the most efficient way for them to pass on their genes to future generations. By caring intensively for their sons, the mothers increase their own probability of survival. They later also help their sons to achieve senior positions in the group. A high-ranking position means more offspring - and thus more grandchildren for the mother.
Christophe Boesch has not observed this phenomenon in other regions. This may be due to differences in social structure. Whereas 88% of the females leave their birth group in the Taï National Park as soon as they reach adulthood, the figure stands at just 13% in the Gombe National Park in Tanzania. Mothers and daughters are therefore highly likely to remain together forever - an incentive to care intensively for daughters, as they will subsequently become important cooperation partners.
Image: Sonja Metzger