science

Gorillas beat their chests to size each other up, researchers say


It is a trope used in films from King Kong to Tarzan – a male primate standing upright and beating its chest, sometimes with a yell and often with more than a dash of hubris.

But it seems the pounding action is less about misplaced bravado than Hollywood would suggest: researchers studying adult male mountain gorillas say that while chest-beating might be done to show off, it also provides honest information.

“We found it is definitely a real, reliable signal – males are conveying their true size,” said Edward Wright, co-author of the research from the Max Planck Institute for Evolutionary Anthropology in Germany.

Writing in the journal Scientific Reports, Wright and colleagues report how they studied chest-beating in six adult male mountain gorillas in the Volcanoes national park in Rwanda.

The team used a camera setup involving two parallel green lasers a known distance apart to determine the breadth of each gorilla’s back from a photograph.

They then recorded 36 chest-beating episodes among these six males between November 2015 and July 2016, and analysed the recordings.

The results revealed that the duration of the chest-beating, number of beats and the rate of the beats during an episode were not associated with the size of the gorilla. However, the average peak frequency of the sound produced was – the larger the gorilla, the lower the frequency of the sound produced.

The team noted that larger body size in males had been linked to their reproductive success – both in terms of the number of females and number of offspring a male had – and their social rank among other males, something that suggests body size could be tied to their fighting prowess.

The new study suggests that chest-beating could be a useful way for males to convey their size, something Wright said could help them to both win mates and avoid potentially dangerous fights.

“Quite often it is all about the chest beat and then they don’t fight,” he said , adding that could be because the males size each other up from the chest-beating and gauge which has the physical advantage.

A male gorilla chest-beating as a rival looks on
Researchers found that the larger the gorilla, the lower the frequency of the sound produced by chest-beating. Photograph: Dian Fossey Gorilla Fund/PA

“Then the smaller one presumably says: ‘Right, you are bigger. There is no point in me fighting you because I am likely to lose. I am likely to get injured. This is not good for me and so I am just going to retreat’,” he said.

But quite why larger animals have lower frequency sounds when pummelling their chest remains something of a mystery.

“It could be chest volume or hand size or a combination of the two as they likely correlate,” said Wright, adding that larger males were also likely to have larger laryngeal air sacs in their chest cavity, which may be an additional factor.

Wright said the chest-beating could be particularly useful as the dense vegetation of the gorilla’s natural habitat means that other gorillas may not be able to weigh up a male by sight alone: chest-beating can be audible more than a kilometre away.

Prof Anna Nekaris, an expert in primates from Oxford Brookes University, who was not involved in the work, said the study highlighted that it was not just humans who make use of body language.

“The analyses here that gorilla chest size is an honest signal related to the classic chest-beat sound is not necessarily surprising, but it is a well-conducted study that could lead they way in proving the importance of body language for honest communication in other species,” she said.

“What will be interesting in future is if smaller gorillas with narrower backs or chests would be able to mimic larger ones – and a study like this can lead the way for further [research] to see how animals might be capable of manipulating ‘honest’ signals.”



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