A cooperation experiment in captive white-handed gibbons (Hylobates lar)

Abstract: Cooperative behaviours among individuals play a crucial role in social interactions. There is a special interest in investigating the occurrence of cooperation among apes, because this knowledge could as well shed light on evolutionary processes and help understand the origin and development of cooperation in humans and primates in general. Gibbons are phylogenetically intermediate between the great apes and monkeys, and therefore represent a unique opportunity for comparisons. The aim of the present study was to discover whether or not gibbons (Hylobates lar) show cooperative behaviours among each other. In order to test for the respective behaviours, the gibbons were presented with a commonly used experimental cooperative problem-solving task. Additionally, social behaviours were recorded during behavioural observations. The gibbons in this study did not exhibit cooperative behaviours during the problemsolving task. Behavioural observations revealed that the gibbons spent significantly more time ‘out of arm’s reach to everyone’, suggesting that they are less involved in social interactions than other, more cooperative apes. Both findings combined support the “social brain hypothesis”, which predicts that cognitive abilities are constrained by the complexity of the animals’ social life. Based on previous findings of occurrences of cooperative behaviours in two other primate lineages (i.e. New World monkeys and Old World monkeys) it was suggested that cooperation in primates was a matter of a convergent evolutionary processes rather than a homologous trait.