Why do male gorillas beat their chests? New study offers intriguing evidence

King Kong made the behaviour famous, but scientists still know little about why the great apes perform this percussive display.

Published 14 Apr 2021, 08:14 BST
Mountain Gorilla Chest Beating

A male mountain gorilla beats its chest in Rwanda's Volcanoes National Park.

Photograph by Nature Picture Library / Alamy Stock Photo

Since King Kong first appeared on the silver screen in 1933, the fictional giant ape has exposed audiences all over the world to a very real gorilla behaviour—chest-beating.

But it may surprise you to learn that while scientists have speculated about the percussive display’s meanings, actual evidence for why male gorillas sometimes pound their chests has been scarce.

“It is an extremely impressive display,” says Edward Wright, a primatologist at the Max Planck Institute for Evolutionary Anthropology in Germany. “It can be a bit frightening. You don’t want to get in the way.”

Interestingly, while a male mountain gorilla’s hearty chest beat would seem to signal aggression, new research by Wright shows that the behaviour may actually prevent violence between the massive animals, which can top out at nearly 230kg.

Mountain gorillas live in tight-knit family groups led by silverback males, whose authority is constantly being challenged by other males. By advertising their size, mating status, and fighting ability via sounds that can travel long distances through thick rainforests, the silverbacks are signalling would-be challengers that they better think again before starting a ruckus.

To study the behaviour in never-before-seen detail, Wright and his colleagues spent more than 3,000 hours observing endangered mountain gorillas at Volcanoes National Park in Rwanda.

While battling biting insects and navigating the park’s rugged, mountainous terrain, the scientists observed more than 500 chest beats from 25 different males between 2014 and 2016. They stayed at a safe distance from the animals, which have been habituated to the presence of researchers but remain highly vulnerable to human diseases.

The scientists, whose work was funded in part by the National Geographic Society, used audio equipment to record the frequencies of the chest beats, as well as the number of beats and duration of each display. Finally, they searched for relationships between these variables and the gorillas’ sizes. To do so, the team analyzed photographs to measure the maximum breadth of each animal’s shoulders. (See National Geographic’s beautiful photos of gorillas.)

The results showed that the biggest mountain gorillas produced sounds with lower frequencies than smaller gorillas—possibly because bigger gorillas have larger air sacs near their larynx. This means that chest-beating isn’t just a visual display, but what the study calls an “honest signal of competitive ability”—not unlike the alligator’s rumble or bison’s bellow.

Although previous studies showed that a gorilla’s body size is linked to dominance and reproductive success, the idea that chest-beating also communicates some of that information was speculative, according to the study, published in Scientific Reports.

Watch Koko the Gorilla Use Sign Language in This 1981 Film
This is Koko the signing gorilla. Koko died in her sleep on Tuesday, June 19, 2018. Here is a clip from a 1981 National Geographic documentary.

“We thought and suspected, but there was no actual data to support this claim,” says Roberta Salmi, a primatologist and director of the Primate Behavioural Ecology Lab at the University of Georgia, who is unaffiliated with the research. “I was happy to finally see those results.”

Kong’s calling card

Though chest-beating is common in movies and other pop culture portrayals, there’s still plenty we get wrong about the behaviour.

For starters, real-life gorillas don’t strike their chests with balled up fists. Rather, they cup their hands, which amplifies the sounds. They also rise from a sitting to a standing posture, perhaps as another way to ensure that the thumps can be heard more than half a mile away. (Learn about primatologist Dian Fossey’s famous experiences with gorillas.)

Silverbacks beat their chests most often when females under their protection are entering estrus, the time they’re most ready to mate. But it’s not like males are banging on the drum all day, as they’re often portrayed doing in movies.

In fact, Wright found that each male beat its chest an average of just 1.6 times every 10 hours. Lower-level male gorillas, or subordinates, also perform the chest-beating behaviour, as do baby male gorillas when they’re playing.

Wright says there doesn’t seem to be a relationship between how big or dominant a male is and how many times it strikes its chest, or how long its display lasts. But a sequence of chest beats may potentially communicate an animal’s identity, or “individual signature,” to other animals, he says.

Not-so-gentle giants

Though gorillas are equipped with gigantic muscles and long canine teeth, the animals come to blows only rarely. Wright believes this is at least partly because chest-beating allows males to size each other up without getting physical.

“Even if you’re likely to win a fight, there is still quite a high-risk factor,” he says. “These are large, powerful animals that can do a lot of damage.” (Read how Rwanda’s civil war harmed mountain gorillas.)

To smaller males, the sound of a silverback’s chest-beat may discourage them from approaching. Similarly, a silverback may hear the beats made by a smaller male nearby and decide that he’s too puny to bother with.

Because chest-beat peak frequency syncs up with body size—which is itself linked to dominance and reproductive success—female gorillas have plenty of reasons to listen in on the displays, too. Particularly impressive chest-beating might lure females away to a nearby group like a siren’s song, though this has not yet been studied.

Salmi has also studied chest-beating in closely related western lowland gorillas. Intriguingly, this species also performs a hand-clapping behaviour—apparently as a way to alert others to potential danger—that’s so far not been observed in mountain gorillas.

Considering the new study’s results, the next step is to see how other gorillas use the information encoded in chest-beating sounds, Salmi says.

“It will be very interesting,” she says, “to see how hearing chest beats in their environments might affect their movements and decision-making as to which areas of their home range to use.”

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