Chest beating by mountain gorillas - rapidly beating their chests with their hands to produce a drumming sound - may convey information about their body size and allow identification of individuals, a study published in Scientific Reports suggests. These findings demonstrate how non-vocal behaviours may contribute to mountain gorilla communication.
Although it had previously been suggested that gorillas may beat their chests to convey information, the exact nature of that information was unclear. Edward Wright and colleagues observed and recorded 25 wild, adult male silverback gorillas monitored by the Dian Fossey Gorilla Fund in Volcanoes National Park, Rwanda, between January 2014 and July 2016. Body size was determined from photographs by measuring the distance between the gorillas' shoulder blades. Using sound recordings, the authors measured the duration, number and audio frequencies of 36 chest beats made by six of the males.
The authors found that the audio frequencies of chest beats made by larger males were significantly lower than those made by smaller males. Larger males may have larger air sacs near their larynx, which could lower the frequencies of sound they produce while chest beating, according to the authors. Variations were also observed in the duration and number of chest beats made by different gorillas. These were unrelated to body size but may allow chest-beating individuals to be identified.
The authors suggest that the sound of chest beating may allow mountain gorillas to communicate across the dense, tropical forests in which they live, where it is often difficult for them to see one another. They speculate that mountain gorillas may use the information conveyed through chest beats to inform mate choice and to assess the fighting ability of competitors.
Chest beats as an honest signal of body size in male mountain gorillas (Gorilla beringei beringei)
Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany
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