This is the hidden meaning of the blows that gorillas give themselves in the chest

Until now, it was thought that the blows were associated with competition between males and the process of group selection by females, but the type of information communicated was still unknown.

A file photo of an endangered silvery mountain gorilla inside Virunga National Park.

Male gorillas beat their chests like acoustic indicator of your body size and their competitive capacity against rivals and females, according to an international study published by the magazine Scientific Reports.

The work, in which Jordi Galbany, professor at the Faculty of Psychology at the University of Barcelona, ​​Spain (UB) has participated, has revealed the hidden meaning of iconic punches on the chest of mountain gorillas, whose larger and more competitive specimens emit lower sound frequencies, that is, more serious sounds, so the size of the gorilla is encoded in the sound message.

In the investigation, led by Edward Wright and Martha Robbinsfrom the Max Planck Institute for Evolutionary Anthropology, experts from the Dian Fossey Foundation, the George Washington University and the Goethe University of Frankfurt have also participated.

The researchers have discovered a correlation between the body size of gorillas and the frequency of the sounds characteristic of their blows to the thorax. According to the study, this visual and acoustic signal it reliably indicates the body dimensions of gorillas to their social group—males and females—and also to other gorillas in neighboring groups.

The work has studied a dozen social groups of gorillas of mountain in the National Park of the Volcanoes of Rwanda, a reduced habitat and well known thanks to the American primatologist Dian Fossey.

The mountain gorilla (Gorilla beringei beringei, one of the great African apes that inhabits the volcanic slopes of the Albertine fault, is an endangered subspecies of which there are less than 1,000 copies in the Bwindi Impenetrable Forest National Park (Uganda) and the Virunga Mountains, between the Democratic Republic of the Congo, Uganda and Rwanda.

Until now, blows were thought to be associated with competition between males and to the process of group choice by females, but the type of information communicated was still unknown.

The work has verified that the strategy of the blows in the thorax is a reliable indicator of body size of gorillas and reveals their competitive capacity with respect to members of both their social group and others.

show of force

According to the researchers, the anatomy close to the larynx of the largest males lowers the sound frequency that these apes produce while beating their chests. Thus, rival males could feel intimidated by the sound of those blows —which can be heard up to a kilometer away— and they would choose to avoid fighting with the emitting male, while the females could use the information to choose a mate.

“The chest-thumping behavior is typical of adult male gorillas and had always been described as a show of force related to social status and threatening behaviours”, explained Jordi Galbany, Professor of Psychobiology at the UB.

“This behavior -he adds- is the climax of a demonstration: the gorilla begins to do vocalizations similar to short howlsstands up and runs on his legs while striking his chest strongly with the palms of both hands alternately, producing an impressive sound, like the roll of a drum.

“Male gorillas beat their chests once every twenty hours of observation, but they can do it every few minutes when they interact between two groups”, Galbany has detailed. Researchers have also found that this behavior is more frequent on days when a female is in heat.

The team recorded the gorillas’ chest beats to analyze various sound parameters (duration, number of blows, frequency of emission) and studied the width of the back of each animal using non-invasive photogrammetric techniques, a task in which Galbany directly participated during four years of field work in the Volcanoes National Park. .

“Other species of great apes can also reach communicate remotely through non-vocal acoustic signals, like chimpanzees, which use the buttresses of trees as a drum and emit a wide range of communicative signals with different durations and characteristics”, Galbany pointed out.

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