Scientists have now discovered that the drumming sounds change according to the gorillas’ body size – larger males emit lower audio frequencies than smaller males, which they say could be attributed to larger air sacs near the larynx.
Converging evidence suggests this might help females choose mates and rivals assess their competitor’s fighting ability, according to Edward Wright, from Germany’s Max Planck Institute, and co-authors of a paper published in the journal Scientific Reports.
“The chest beat is one of the most emblematic sounds of the animal kingdom, so there has been a lot of speculation of what information it may convey,” says Wright. “It is really nice to be able to now demonstrate that body size is conveyed in these impressive displays.”
Many species have been found to communicate body size acoustically, including rhesus macaques, red and fallow deer, koalas, giant pandas, southern elephant seals, and alligators.
The new study suggests non-vocal signals such as the gorilla chest beat can also transmit important information, which may have evolved to overcome the poor visibility in tropical forests.
The researchers observed and recorded 25 wild, adult male silverback gorillas (Gorilla beringei beringei) monitored by the Dian Fossey Gorilla Fund in Volcanoes National Park, Rwanda, between 2014 and 2016.
They used a parallel laser technique to measure the gorillas’ body size, which involves projecting two small lasers onto the gorilla separated by a known distance. This is used as a scale to measure body parts of interest on photographs.
Chest drumming was recorded using a directional microphone and recorder. “This was a challenge,” says Wright, “as gorillas do not chest beat very often and you need to be at the right place at the right time to capture the beats which have a relatively small duration.”
They managed to measure the duration, number and sound frequency of 36 chest beats made by six of the males. As well as discovering the beats are “an honest signal of body size,” they found variations in the duration and number of beats by different gorillas. These weren’t related to body size, so they might help individuals to be identified.
Wright says future research will need to show that other gorillas actually respond to the body size information transmitted by the chest beats, but he thinks it’s likely.