We investigated how vocalizations may function to reduce anxiety in post-conflict situations in Japanese macaques (Macaca fuscata). Some primate species use soft and quiet vocalizations called grunts or girneys (greeting calls). Such calls are often emitted toward nearby individuals and followed by affiliative interactions such as grooming. We can obtain knowledge of macaque social cognition by investigating how individuals use greeting calls to smoothing their social interactions.

In many animal species, post-conflict situations are associated with increased motivation for opponents and bystanders to engage in affiliative interactions. This is because affiliative interactions can reduce post-conflict anxiety. However, there is individual uncertainty about initiating such interactions. Specifically, both victims and bystanders may be uncertain about whether post-conflict behaviors of former opponents are affiliative or antagonistic. We predicted that greeting calls would be used to reduce uncertainty and anxiety.

We observed aggressors and victims in naturally occurring conflicts and control situations. We recorded whether monkeys emitted greeting calls or not when initiating affiliative interactions. We used self-scratching rate as an indicator of anxiety levels.

We found that, after conflicts, aggressors and victims were more likely to emit greeting calls when initiating affiliative interactions toward former opponents and bystanders than they were in control situations. Both aggressors and victims were more likely to emit calls when interacting with less familiar individuals. These findings indicate that monkeys use greeting calls when uncertainty is high. Self-scratching rates showed a greater decrease after affiliative interactions with greeting calls than after interactions without calls. Greeting calls probably reduced anxiety levels by making subsequent behaviors of callers more predictable. Our findings suggest that greeting calls function to inform other monkeys about the non-hostile intent of callers and that monkeys use calls such that both senders and recipients benefit from information.