Monkey rituals: When gorillas drum their chests, the pitch is important

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Are there two cars at the traffic lights, one driver lightly presses the accelerator and looks over, well, shall we?

This may or may not result in an illegal race, in any case it is ostentatious.

Men stand at the bar and hastily tip their beers.

If the glass is empty, it happens that someone presses his fist on his chest and belches loudly.

What does he mean by that?

We could now talk about the mating initiation and courtship behavior of young men in swimming pools and on beaches, about the display of strength and broad shoulders, but it is actually clear that this is about well-known rituals.

What follows is a bit tricky, and we don't want to draw any clear parallels.

But there is news about the gorilla breast pounding.

The researchers studied the behavior of mountain gorillas in Rwanda

Source: Jordi Galbany / Dian Fossey Gorill

An international team of researchers headed by the Max Planck Institute for Evolutionary Anthropology in Leipzig reports that drumming in a sophisticated way probably provides information about the body size of the primates.

An important piece of information aimed at both rival males and females who are interested in mate selection.

Up until now it was not entirely clear which information the animals conveyed with it, as a rule it was understood as a threatening signal or as an act of showing off, which is why Tarzan, formerly known as the master of the jungle, imitated it in a dramatic pose.

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Breast drumming is one of the best-known forms of communication for gorillas: the animals stand upright and hit their chests with their hands in quick succession.

Sometimes accompanied by characteristic calling or a rumbling.

The signal is quite loud: even in the dense, often foggy forests of the monkeys' ancestral home, it can be heard over a radius of up to a kilometer.

Edward Wright's researchers write in the journal "Scientific Reports": "The gorillas' chest pounding has both an acoustic and a visual component and is therefore a multimodal signal."

For more than two years, the team observed 25 adult male mountain gorillas (Gorilla beringei beringei) from ten groups in the Volcanoes National Park in northern Rwanda.

Using photos, the researchers determined the chest size and height of the males;

it is about 1.70 meters on average.

Then they recorded the duration, number and tone frequencies of the drum noises produced by the silverback - that is, the adult males.

That turned out to be not that easy.

The duration of the chest drumming is relatively short, the monkeys drumming at the longest for almost three seconds, on average only 0.65 seconds.

So the researchers had to be in the right place at the right time, the technology had to work, and then there was an understandable need for security.

Co-author Eric Ndayishimiye says, "We also had to stay away from these big, powerful animals."

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It was found that the drumbeats of larger males had lower frequency maxima than those of smaller males.

The larger animals might have larger air sacs near their larynx, which lowers the sound frequencies.

In this way, the chest beats would reliably provide information about the body size of the respective drummer.

Size does matter

.

Famine is a major cause of gorilla extinction

The people of the Congo are starving.

Many resort to gorilla meat in their need.

In 2017 there were only about 3800 gorillas left.

However, many former hunters are now risking their lives to protect the remaining animals.

Source: WELT / Stefan Wittmann

For many animals, height is considered a key attribute as it often reflects the ability to fight or compete, which in turn has consequences.

Larger male gorillas are more socially dominant and more successful in terms of reproduction than smaller males.

Real or at least signaled strength is attractive to female gorillas.

In fact, when the females are ready to mate, the males are more likely to hit each other on the chest.

The researchers also measured the frequency; some gorillas hit their chest five times in ten hours, most less than once;

on average 0.5 times.

The scientists did not establish a connection between body size and duration, number or beat frequency of the drumming.

Differences between individual animals might have a different meaning: "That could indicate that the breast drumming can have individual signatures," says Wright.

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Rival males are likely to pay attention to the height information transmitted while drumming.

Does the competitor sound terrifyingly big and strong or rather puny?

This could help decide whether a combative argument is promising.

For females, the information conveyed by drumming is of other interest.

"We believe that chest strokes also play a crucial role in partner choice, providing females with information about the size of males in their own and neighboring groups," the authors write.

"That can influence your decision to move to another group."

The male primates call out to potential partners: Come on, bum-bum, on our side, bum-bum, it's better here. You don't need to be a field-experienced anthropologist to suspect that there are equivalents for this in the human realm.