When they strike their torso, mountain gorillas produce a drumming sound echoing through the forest, which they use to convey information about their size to their fellows without even seeing each other, according to a study published Thursday.
These chest beats are very peculiar because, unlike the croak of a frog or the roar of a lion, it is a non-vocal behavior that can be both seen and heard.
Practiced mostly by dominant gorilla males, they are seen as a way to attract females, to intimidate their potential rivals. But researchers wanted to know if the beating of drums, which can resonate for up to a kilometer through the thick rainforest, were not also a way for drumming primates to convey information about their own physique.
A team observed and recorded 25 “silverback” gorillas (a sign of maturity), monitored by the Dian Fossey foundation in the Volcanoes National Park in Rwanda, between January 2014 and July 2016.
They measured the duration, number and frequencies of 36 torso beats performed by six of the males. Their work, published in the journal Scientific Reports, found that larger gorillas produced chest beats at lower frequencies than smaller ones.
According to the authors, larger males have larger air sacs located near their larynx, which could reduce the frequencies of the sounds emitted.
“Chest beats are a reliable signal of body size in mountain gorillas,” said Edward Wright, of the Max Planck Institute for Evolutionary Anthropology, who led the study.
The information would allow partners, or potential rivals, to judge their size from a distance, in a dense forest where it is often difficult for them to see each other.
“As a male gorilla, if you want to assess the competitive ability of a rival male, it may be safer to do it from a distance,” says Wright.
To study the relationship between the size of wild gorillas and the resonance of their thoracic drums, the researchers had to measure them without getting too close, using lasers, and by taking photos assessing the distance between the shoulder blades of the gorillas. primates.
They also had to be patient in order to register the beats, emitted in short bursts, barely every five hours.
“You had to be in the right place at the right time,” says the researcher. But once picked up, the sound was impressive. “As a human, you are perfectly aware of its power.”