CChest spanking is very special because, unlike a frog crowing or a lion’s roar, it is a non-vocal behavior that can be seen and heard. It is mostly practiced by the dominant male gorillas, and is seen as a way to attract females, to intimidate their potential competitors. But the researchers wanted to know whether the beating of drums, which can reverberate up to a kilometer through dense rainforests, was also not a means of beating the drums to convey information about their physique.

A team observed and recorded 25 “silverback” (a sign of maturity), spotted by the Dian Fossey Foundation in Rwanda’s National Volcanoes Park, between January 2014 and July 2016. The duration, number and frequency of 36 chest beats made by six males were measured. Their work, published in Scientific Reports, found that larger gorillas produce chest beats at lower frequencies than smaller ones.

According to the authors, larger males have larger air sacs located near the larynx, which may reduce the frequencies of the sounds emitted. “Chest strikes are a reliable indication of body size in mountain gorillas,” said Edward Wright of the Max Planck Institute for Evolutionary Anthropology, who led the study.

The information will allow partners, or potential competitors, 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 competitiveness of a rival man, it may be safer to do so from a distance,” says Wright.

To study the relationship between wild gorilla size and the resonance of their chest drums, researchers had to measure them without getting too close, using a laser, and taking pictures that assessed the distance between the gorilla’s shoulder blades, the primates. They also had to be patient in order to record the emitted beats in short bursts, barely every five hours.

“You had to be in the right place at the right time,” says the researcher. But once captured, the sound was impressive. “As a human being, you are fully aware of its power.”