How do chimpanzees choose their partner for a coordinated cooperation problem?

One widely spread definition of cooperation is when two or more individuals work together to achieve a common goal. This behaviour can be observed throughout the animal kingdom and is especially complex in human beings. To understand the evolution of cooperation, it is useful to study one of our closest relatives: chimpanzees. Chimpanzees are known to cooperate in groups when hunting monkeys or patrolling their territories. They also cooperate in small coalitions for the obtention of social benefits. In this thesis, we want to understand how chimpanzees choose their partner for a coordinated cooperation problem. We want to know if they choose their partner based on social tolerance, rank difference, age difference, relationship quality, sex, maternal relatedness, or an interaction of these parameters. At the Wolfgang Köhler Primate Research Centre (WKPRC), Leipzig, Germany, we tested a new experimental design: the “Cooperation Box”. It consists of a metal cage with a tray and an automated peanut dispenser. The peanuts can only be accessed when the tray is lifted to the top of the cage. To lift the tray, two chimpanzees must, at the same time, lift two handles, which are too far apart to be held by just one. The chimpanzees were free to access the box whenever and with whom they wanted to. The experiment was not successful. We hypothesize that because the chimpanzees received food every 2 hours, they might not have been motivated enough to try to solve the task. Next, we tried to answer our research questions using part of the data collected by Nolte, Sterck and van Leeuwen (under review). They reproduced the experiment by Hare et al. (2007) where social tolerance of chimpanzees and bonobos was compared, using the loose-string paradigm by Hirata and Fuwa (2007). We confirm the conclusion of Hare et al. (2007) that chimpanzees only cooperate when food is not monopolizable. Furthermore, we observed that chimpanzees chose to cooperate with partners with whom they shared high social tolerance, but only when rank difference between the two was above the group average. Relationship quality seemed to follow the same trend, but the interaction was strongly influenced by one outlier. We could not conclude on sex and maternal relatedness because of a small number of male and related dyads, albeit both seemed to cooperate more often than female and non-related dyads. Age difference did not influence cooperation. We conclude that studying chimpanzees in a dyadic experimental setting can give us some insight into how chimpanzees cooperate, but it is indispensable to do experiments in social settings to fully understand how chimpanzees choose their partner for a coordinated cooperation problem.