Gorillas in films such as King Kong and Tarzan are depicted aggressively beating their chests when under threat. While the behavior is observed in male gorillas in the wild and researchers have speculated about what the behavior might mean, there hasn't been enough research to establish a consensus. Researchers suspect the gorillas exhibit this behavior not to instigate fights, but to prevent them—and chest-pummeling could be used to advertise their body size to other gorillas, reports Jason Bittel for National Geographic. The study was published last week in Scientific Reports.
Scientists observed 25 wild male mountain gorillas for over 3,000 hours at Volcanoes National Park in Rwanda between 2014 and 2016, National Geographic reports. The research team used audio equipment to record the sound frequency, rate, duration, and amount of chest pounds. To determine each gorilla's size, the researchers used cameras with lasers to photograph and measure each gorilla's back, reports Nicola Davis for the Guardian.
While the rate, duration, and amount of beats did not correlate with the size of the gorillas, but sound frequency did, the Guardian reports. The team also noticed that larger gorillas produced deeper-toned chest drumming. Previous research has shown that a gorilla's larger body size is linked to reproductive success and social rank, the Guardian reports. The chest-beating could be another way for the gorillas to convey their size to others and, in turn, avoid fights that could result in serious injury or death.
"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'," says co-author Edward Wright from the Max Planck Institute for Evolutionary Anthropology to the Guardian.
While gorillas can obviously observe size just by getting a good look at their peers, the researchers suggest chest-beating is more reliable when trying to communicate through a dense forest habitat, reports Bryan Lawver for Inverse. The mighty percussions gorillas create by cupping their hands over their chests is powerful enough to travel long distances through thick forests and signal to others their mating status, size, and fighting ability, reports National Geographic.
Researchers are unsure why larger gorillas' chests produce a lower frequency but suspect that it may be because air sacs near their larynx are also larger, Inverse reports.
Primate expert Anna Nekaris of Oxford University, who was not part of the study, told the Guardian that the study showed humans are not the only mammals that make use of body language. Nekaris notes it would be interesting to see if smaller gorillas can mimic the deep tones of larger ones in future studies.
"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," Nekaris tells the Guardian.
For now, the researchers will continue to study chest-beating to see if the action can convey other information, such as dominance rank, sex, age, and individual identity, to nearby gorillas, Inverse reports.