Mountain gorillas send messages to rivals with chest beating ritual

Study finds the larger the gorilla, the lower the sound made during chest thumping displays

A male silverback gorilla beating his chest, which researchers believe sends a message about his fighting ability to rivals. Credit: Jordi Galbany/Dian Fossey Gorilla Fund

Mountain gorillas can send messages to their rivals more than a kilometre away about their size and fighting ability through their chest beating rituals, a study has suggested.

Chest beating by the biggest silverback gorillas sound lower in pitch compared with smaller animals and could represent a long-distance warning to rivals, according to research published in Scientific Reports.

The sporadic chest drumming by male gorillas has been linked to the search for a mate and for communication, but it was not clear what message was being conveyed.

But a two-year study of 25 adult male silverbacks in Volcanoes National Park, Rwanda, found a direct link between audio frequency and the size of the animals.

They recorded the duration, number and frequency of 36 chest beats made by six of the males and linked them to measurements taken from photos of their shoulder width.

The researchers from Germany, Spain and the US believe that the bigger males have larger air sacs near their larynx, making the drumming beat sound lower in pitch to the human ear.

Gorillas are highly competitive in the search for a mate, with a single male living among a group of females.

Different gorillas drummed more rapidly and for longer, which could allow them to be identified by sound through the dense tropical forest, according to the paper Chest beats as an honest signal of body size in male mountain gorillas.

Lead author Edward Wright, from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, said the researchers had to keep at least seven metres from the gorillas to ensure no disease was transmitted to protect the population of little more than 1,000.

"Gorillas do not chest beat very often, around once every five hours, so it is a big challenge to record ... as most times you don't know when they are going to happen, and when they do, you need to be at the right place at the right time," he said.

Gorillas are not alone among mammals to signal their size through the noises they make. A study of the eland antelope found that the frequency of “knee clicking” was a reliable gauge of body size.