Chimpanzee quiet hoo variants differ according to context

Abstract

In comparative studies of evolution of communication, the function and use of animal quiet calls have typically been understudied, despite that these signals are presumably under selection like other vocalizations, such as alarm calls. Here, we examine vocalization diversification of chimpanzee quiet 'hoos' produced in three contexts-travel, rest and alert-and potential pressures promoting diversification. Previous playback and observational studies have suggested that the overarching function of chimpanzee hoos is to stay in contact with others, particularly bond partners. We conducted an acoustic analysis of hoos using audio recordings from wild chimpanzees (Pan troglodytes schweinfurthii) of Budongo Forest, Uganda. We identified three acoustically distinguishable, context-specific hoo variants. Each call variant requires specific responses from receivers to avoid breaking up the social unit. We propose that callers may achieve coordination by using acoustically distinguishable calls, advertising their own behavioural intentions. We conclude that natural selection has acted towards acoustically diversifying an inconspicuous, quiet vocalization, the chimpanzee hoo. This evolutionary process may have been favoured by the fact that signallers and recipients share the same goal, to maintain social cohesion, particularly among those who regularly cooperate, suggesting that call diversification has been favoured by the demands of cooperative activities.

Keywords: animal communication; call diversification; chimpanzee; cooperation.

Figure 1.

Spectrograms of the three hoo variants, including hoo sequences. (a) Alert hoos: two hoo sequences emitted by an adult male and female, respectively. (b) Travel hoos: four hoo sequences, emitted by two adult males and two adult females, respectively. (c) Five rest hoos, emitted by two adult males, two adult females and finally one adult male, respectively. Time and frequency scales are equivalent across spectrograms. Audio recordings are included in the electronic supplementary material.

Figure 2.

Classification of three hoo variants emitted in different behavioural contexts, delineated by two acoustic variables highly influential in permuted discriminant function analysis: call duration and maximum fundamental frequency (table 2). Group centroids with 95% confidence interval are shown.

Figure 3.

Influence of context on call interval in travel and alert contexts.