Loud Call Production in Male Vervet Monkeys (Chlorocebus pygerythrus) Varies with Season and Signaller Rank

Adult males of many nonhuman primate species utter characteristic loud calls that are audible over long distances and serve various functions. In response to large terrestrial predators, male vervet monkeys (Chlorocebus pygerythrus) produce loud 'barks' that alert group members and are assumed to deter predators. It is less well known that male vervet monkeys also produce barks during aggressive interactions within and between groups. Here, we investigated whether barks potentially serve a dual function as alarm calls and quality signals. We used ad-libitum event sampling to record natural barking events from 45 adult males in six free-ranging groups over 24 months at Mawana Game Reserve, KwaZulu-Natal, South Africa. For each barking event in groups with multiple males, we aimed to record whether each adult male in the group barked or not. Using generalised linear mixed models, we investigated whether the number of barking events recorded per observation day and the probability that an individual male barked in each event were associated with male rank, the number of males in the group, the adult sex ratio, and the mating season. The highest-ranking males showed a very high probability of barking, and the number of daily barking events increased during the mating season. Males frequently barked in aggressive interactions, supporting the hypothesis that this signal plays a role in male-male competition. We conclude that, in addition to serving as alarm calls, barks may also advertise male competitive ability or motivation to compete.

The online version contains supplementary material available at 10.1007/s10764-024-00475-x.