Behavioural indicators of anxiety: An empirical test in chimpanzees

Behavioural, pharmacological, and physiological evidence supports an association between displacement activities and anxiety in macaques. Information is scarce in other primate and non-primate species. This study contributes to the understanding of the relation between displacement activities and emotional states by investigating the correspondence between self-directed behaviour and an inherently stressful situation in chimpanzees (Pan troglodytes). Data were collected on 81 captive chimpanzees housed in conditions varying from indoor single caging to indoor/outdoor enclosures containing up to 14 individuals. Observation of gentle and rough scratching, self-grooming, and yawning were used to test predictions concerning the response of individuals to neighbour vocalisation (i.e. the calls and noisy displays produced by neighbouring groups of chimpanzees), which has been previously demonstrated to increase the likelihood of intragroup-agonistic behaviour. In socially-housed chimpanzees rough and gentle scratching and yawning were significantly more common after neighbouring individuals vocalised or displayed than before, but the effect lasted longer for rough-scratching. In addition, during periods characterised by higher levels of neighbour vocalisation, socially-housed chimpanzees performed rough scratching at higher rates. Self-grooming was not affected by the level of neighbour vocalisation. Single-caged chimpanzees, for whom neighbour vocalisation carries no risk of aggression by group members, showed no increase in self-directed behaviour when neighbour vocalisation level was high, suggesting that the risk of intra-group aggression, rather than neighbour vocalisation itself, elicits anxiety. While our study suggests that rough scratching is the most reliable indicator of anxiety in chimpanzees in the context of neighbour vocalisation, it also provides evidence that gentle scratching and yawning can be considered displacement activities in this species.