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Do not bluff a gorilla when it strikes its chest: honest signals indicate body size

The gorilla usually stands on two legs and quickly strikes its chest with arched hands in quick succession. Chest spanking is unique because it is not vocalizing, like a frog’s cawing, but rather a form of gestural communication that can be heard and seen. The sound of drums can be heard from a kilometer away. The supposed function of gorillas’ chest strikes is to attract females and intimidate rival males.

The researchers recorded chest beats and used a technique called imaging to non-invasively measure the body size of the male wild mountain gorillas spotted by the Dian Fossey Gorilla Fund in Rwanda’s Volcanoes National Park. They found that the larger males emit pectoral beats at lower peak frequencies than smaller beats. In other words, the chest beats convey the information about the body size of the beater on the chest.

Says Edward Wright, first author of the study from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

Body size is a key feature in many animals as it often reflects competitive or fighting 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 chest beats are relatively short in duration and we needed to be in the right place at the right time to get the sound recordings, as well as to distance themselves from these large, powerful animals,” says fellow author Eric Ndayishimiye, research assistant at Dian Fossey. Gorilla Fund.

Competing males are more likely to bring the body size information conveyed in chest strikes as it allows them to assess the competitiveness of chest strikes, and this will help them determine whether to start, step up, or decline in aggressive contests with them. On the other hand, females are more likely to use this information in selecting potential mates.

Interestingly, the researchers also found a large amount of variation among males in both the number of strikes constituting the chest strokes as well as the duration of the chest strokes. “This hints at the possibility that breaststrokes may have individual signatures,” Wright says, “but further study is needed to test this.”

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Original post:

Edward Wright, Sven Grounder, Eric Ndayishimi, Jordi Galbani, Shannon C. McFarlane, Tara S Stoinsky and Martha M Robins

Chest beats as an honest indication of body size in a male mountain gorilla (Gorilla beringei beringei)

Scientific Reports08 April 2021

Contact:

Dr. Edward Wright

Max Planck Institute for Evolutionary Anthropology, Leipzig

+49 341 3550-232

[email protected]

Dr. Martha Robins

Max Planck Institute for Evolutionary Anthropology, Leipzig

+49 341 3550-210

[email protected]

Dr. Tara Stowinski

[email protected]

Dian Fossey Gorilla Fund, Atlanta, USA

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