The sound of a mountain gorilla's chest beat is strongly correlated with the primate's size, suggesting that the well-known behavior likely signals useful information about physical prowess to rivals and potential mates alike.
Drawing on a series of chest beat recordings and gorilla size measurements, an international group of researchers used statistical models to show that larger males produce lower-frequency chest beats than smaller males. They detail their findings in a study published April 8 in Scientific Reports.
"This iconic sound of the animal kingdom that everyone knows about, that even kids sometimes play with and imitate — it's nice that for the first time we can really say it does correlate with body size," said lead author Edward Wright, a postdoctoral researcher at the Max Planck Institute for Evolutionary Anthropology in Germany.
The chest beat is a display in which a male gorilla, typically, rises up on its hind legs and rapidly beats its chest with cupped hands. Ecologically speaking, it is a multimodal sign, which in this case means it features an audible component as well as visual one. A male gorilla may beat its chest once every five hours on an average day, or as often as 10 times in 20 minutes when, for example, it encounters another group of gorillas.
To investigate the relationship between a gorilla's size and the sound of its chest beat, Wright and his colleagues took 3,211 hours of field recordings on 25 adult male mountain gorillas in Rwanda's Volcanoes National Park. From these recordings, they sampled 36 chest beats and analyzed their duration, rate and number, as well as their peak frequency.
Gauging the size of these wild gorillas was slightly trickier. To do so, the team aimed two parallel laser beams at each gorilla's back when the animals were sitting upright. Because they knew how far apart the lasers were set, the researchers could then measure the size of each gorilla's back by using the laser markers as a scale.
Once the researchers acquired the size and sound data, they used linear mixed models to determine how size and chest beating characteristics related to one another. While they could not find any connection between size and any of the temporal characteristics they looked at, the relationship between size and peak frequency was "significant and fairly strong," Wright said.
Primatologists have long suspected that chest beating conveys information about size, power or fighting ability, and this discovery can help answer further questions about the function of this behavior.
"There's a lot of interest in how these signals have evolved," Wright said in an interview with The Academic Times. "I think in respect [to] gorillas, because they live in a dense rainforest, visibility is not optimal, so it enhances the signal. If individuals are close they can see and hear, but if the individuals are far away they'll just rely on the acoustic component."
Females and juveniles sometimes beat their chests as well, though less often than males, and the sound they make is less resonant, more like the sound of slapping skin than the beating of a drum. Still, Wright does not want to give the impression that female gorillas are by any means weak because they generate a "pat pat pat" sound, whereas males make more of a "pock pock pock." Males just happen to be twice the size of females.
"It's an impressive sound, for sure, and kind of humbling in that I'm in the presence of this huge powerful male that could destroy me if he wanted to," Wright said. "That doesn't happen, thankfully. But it's definitely demonstrating power."
While physiological attributes certainly contribute to the auditory frequency of chest beats, Wright and his colleagues cannot definitively say which ones are driving this relationship between size and sound. Chest size is one likely factor, as is hand size, given that male gorillas beat their chests with cupped hands.
Gorillas also have laryngeal air sacs, large spaces inside the chest cavity. Though he says the theory remains unproven, Wright suspects that bigger gorillas with sizable chests may have more spacious air sacs, which could contribute to lower-frequency chest beats. In any case, more research is needed to determine whether chest size, hand size, laryngeal sac volume or some combination of these features governs the sound of a gorilla's chest beat.
Wright has since moved on to conservation work with the United Nations Environment Programme, studying the effects of climate change on mountain gorillas, which only number around 1,000 as of January 2020. But to understand exactly what information gorillas glean from chest beats, Wright acknowledges that further research must be done.
The study, "Chest beats as an honest signal of body size in male mountain gorillas (Gorilla beringei beringei)," published April 8 in Scientific Reports, was authored by Edward Wright and Martha M. Robbins, Max Planck Institute for Evolutionary Anthropology; Sven Grawunder, Max Planck Institute for Evolutionary Anthropology and Goethe Universität Frankfurt; Eric Ndayishimiye and Tara S. Stoinski, Dian Fossey Gorilla Fund International; Jordi Galbany, The George Washington University and University of Barcelona; and Shannon C. McFarlin, The George Washington University.