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A gorilla with a typical chest-drum pose.
Photo: hypergurl / iStockphoto / Getty Images
If a gorilla pounds his chest, he wants to impress others and threaten his rivals.
This is how the human world has interpreted the peculiar behavior of animals during courtship.
At least that's only half the story, as an international team of researchers led by the Max Planck Institute for Evolutionary Anthropology in Leipzig has now discovered.
According to the scientists, the gorillas are not interested in making the "fat Maxen".
Rather, the breast drumming is a communication with the community: "We can prove that gorillas transmit information about their own height in this spectacular way," explained first author Edward Wright.
Rival males would probably pay attention to the body size information transmitted while drumming in order to better assess the competitiveness of the drummer.
This could help to decide whether a combative dispute is promising or not, write Wright's authors in the journal Scientific Reports.
The scientists did not establish a connection between body size and duration, number or beat frequency of the drumming.
The differences between individual animals might have a different meaning: "That could indicate that the breast drumming can have individual signatures," says Wright.
For many animals, height is considered a key attribute as it often reflects the ability to fight or compete.
Previously, the same team had shown that larger male gorillas were more socially dominant and more successful in terms of reproduction than smaller males.
A message for the females
For females, the information imparted by drumming could be useful in other ways.
"We assume that chest strokes also play a crucial role in partner choice, providing the females with information about the size of the males in their own and in neighboring groups," the authors write.
"That can influence your decision to move to another group."
For the study, the team observed 25 adult male mountain gorillas from ten groups in Rwanda's Volcanoes National Park.
Using photos, the researchers determined the size of the males by measuring the distance between the animals' shoulder blades.
In addition, the researchers recorded the duration, number and tone frequencies of the drum noises produced by the silverback - that's what researchers call the adult males.
During the evaluation, the biologists discovered that the drum beats of larger males had lower frequency maxima than those of smaller males.
Perhaps this is because larger males can also have larger air sacs near their larynx, which lowers the sound frequencies.
In this way, the chest beats would provide reliable information about the height of the respective drummer.
sug / dpa