Chest-Beating Gorillas Don’t Bluff: Honest Signal of True Body Size

Male Gorilla Chest Beating

A male gorilla chest beating. Credit: Jordi Galbany / Dian Fossey Gorilla Fund

The chest beats given by adult male gorillas reliably indicate their body size.

Gorillas usually stand bipedally and rapidly beat their chests with cupped hands in rapid succession. Chest beating is a unique sound because is it not a vocalization, like frogs croaking, but rather it is a form of gestural communication that can be both heard and seen. The emanating drumming sound can be heard over one kilometer away. The presumed function of gorilla chest beats is to attract females and intimidate rival males.

Researchers recorded chest beats and used a technique called photogrammetry to non-invasively measured body size of adult male wild mountain gorillas monitored by the Dian Fossey Gorilla Fund in Volcanoes National Park, Rwanda. They found that larger males emitted chest beats with lower peak frequencies than smaller ones. In other words, chest beats conveyed information regarding the body size of the chest beater.

Silverback chest beating. Credit: Dian Fossey Gorilla Fund

“The gorilla chest beat is one of those iconic sounds from the animal kingdom, so it is great that we have been able to show that body size is encoded in these spectacular displays,” says Edward Wright, the first author of the study from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

Body size is a key attribute in many animals as it often reflects fighting or competitive ability. Previous research by this team showed that larger males were more socially dominant and more successful in terms of reproduction than smaller males.

“Conducting this study was challenging because the chest beats are relatively short in duration and we needed to be in the right place at the right time to obtain the sound recordings, as well as staying clear from these large powerful animals,” says co-author Eric Ndayishimiye, research assistant with the Dian Fossey Gorilla Fund.

Silverback chest beating during an inter-group interaction. Credit: Dian Fossey Gorilla Fund

Rival males are likely to attend to the body size information transmitted in chest beats as it allows them to assess the competitive ability of the chest beater, this will help them decide whether to initiate, escalate or retreat in aggressive contests with them. Females on the other hand, are likely to use this information in their choice of potential mates.

Interestingly, the researchers also found a great deal of variation among males in both the number of beats comprised in a chest beat as well as the duration of the chest beat. “This hints at the possibility that chest beats may have individual signatures, but further study is needed to test this,” says Wright.

Reference: “Chest beats as an honest signal of body size in male mountain gorillas (Gorilla beringei beringei)” by Edward Wright, Sven Grawunder, Eric Ndayishimiye, Jordi Galbany, Shannon C. McFarlin, Tara S. Stoinski and Martha M. Robbins, 8 April 2021, Scientific Reports.
DOI: 10.1038/s41598-021-86261-8

Max Planck Institute for Evolutionary Anthropology

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