Wellness under the forest canopy
As scientists from the Max Planck Institute for Evolutionary Anthropology observed in chimpanzees in Ivory Coast’s Taï National Park, the partners check each other for lice every seven days on average. And in fact, the amount of time spent grooming each other is very balanced over a lengthy period of time. Thus, in the long term, the group members each pay all their partners back for grooming them, and can remember well over one week which animal they still owe a favour. Very much like us humans, chimpanzees can recall encounters they’ve had in the past.
However, grooming is not the only area where give and take prevails in chimpanzee society. Faced with aggression, the Taï chimpanzees also back up any partners who’ve helped them in the past. And a favour does not necessarily have to be paid back in kind, which leads to some brisk trading: the animals use meat to "buy" the support of others in conflicts, or they swap meat for sex. Both males and females trade in a variety of goods or services. In the long term, the accounts all balance, with each animal getting something out of it. That creates stable relationships within the community.
In a study of wild chimpanzees in the Budongo Forest Reserve in Uganda, the Max Planck researchers were keen to find out how mutual grooming affects the release of the hormone oxytocin - a substance that strengthens social ties. To this effect, the scientists took advantage of the fact that oxytocin can be measured in the urine of humans and apes alike. They found that a few minutes of grooming actually does increase the concentration of oxytocin in the urine. However, the effect is strongly dependent on the identity of the grooming partner. If the partner is an animal with whom a close relationship is maintained, the hormone level rises to a greater extent, regardless of whether the two are related or not. The closer the animals are to each other, the more oxytocin is released.