Signals are deemed honest when they provide reliable information about the sender 4.
Acoustic signals can convey information about the sender’s competitive ability, body size and/or condition 5, 6, 7. In particular, acoustic signals have been found to play a crucial role in facilitating assessment of rivals and in the choice of mates 1, 2, 3, 4. One of the many functions of animal communication is to mediate social interactions related to intrasexual competition and intersexual mate choice 1, 2, 3. These results emphasize the potential of non-vocal signals to convey important information in mammal communication. Taken together with earlier findings of positive correlations among male body size, dominance rank and reproductive success, we conclude that the gorilla chest beat is an honest signal of competitive ability.
Using linear mixed models, we found that larger males had significantly lower peak frequencies than smaller ones, but we found no consistent relationship between body size and the temporal characteristics measured. We examined the relationship among body size (back breadth), peak frequency, and three temporal characteristics of the chest beat: duration, number of beats and beat rate from sound recordings of wild adult male mountain gorillas. Among the most emblematic sounds in the animal kingdom is the chest beat of gorillas, a non-vocal signal that is thought to be important in intra and inter-sexual competition, yet it is unclear whether it reliably indicates body size. Whereas body size tends to be negatively associated with formant dispersion in animal vocalizations, non-vocal signals have received little attention. Acoustic signals that reliably indicate body size, which usually determines competitive ability, are of particular interest for understanding how animals assess rivals and choose mates.
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