Teasing in Young Chimpanzees

Juveniles and infants in the large chimpanzee colony in Arnhem Zoo (The Netherlands) regularly direct seemingly annoying behaviour towards senior group-members. It is not clear whether this annoying behaviour is best regarded as a form of play or as a form of or whether it falls into an intermediate category. An inventarisation of these annoying behaviour patterns, provisionally called initiatives, is presented. Systematical observations were carried out, recording the performance by youngsters of quasi-aggressive initiatives directed towards senior individuals. The data indicate that such initiatives were directly responsible for an important part of the aggressive actions of seniors, although many of the initiatives did not result in aggressive responses. Aggressive and submissive reactions by seniors had a reinforcing effect : juniors continued their actions relatively often. However, when the target animals ignored the junior (the most common reaction) or made friendly contact the opposite occurred : juniors stopped their actions. Quasi-aggressive initiatives were predominantly directed towards individuals outside a junior's own mother-aunt subgroup. The actions were performed irrespective of the rank of the target animals: almost 40 % of all initiatives were directed towards individuals ranking higher than mother or aunt (whichever ranked higher). The possibility that quasi-aggressive initiatives were instigated by one or more of several causal contexts is considered. The initiatives for the most part appeared to be performed spontaneously: no possible reason for their performance could be deduced from the situation preceding the initiative. Moreover, quasi-aggressive initiatives occurred almost always unexpectedly: no warning was given. While performing an initiative a junior's body posture was tense rather than relaxed: he or she seemed ready to dash away at the first sign of a countermove, indicating that the risk of a punishing retaliation was taken into account. Several alternative hypotheses are discussed, each with their causal and functional aspects. The explanation of quasi-aggressive behaviour in terms of aggression is the one which best fits the data. According to this hypothesis the behaviour is caused by exploratory impulses and functions as a mechanism for youngsters to learn and expand their social limits. Additional observations and analyses are suggested to specify and test this hypothesis on quasi-aggressive behaviour and the role quasi-aggressive behaviour plays in development.