In general, male gorillas likely beat their chests to attract estrous females and to intimidate rivals, a Scientific Reports paper on mountain gorillas says. Its authors conclude that the chest beat is an honest signal of competitive ability. They also found that chest beats from larger males had significantly lower peak frequencies than those from smaller males.
Male body size, the authors say, is thought to track fighting ability: it correlates strongly with dominance rank in mountain gorilla groups that have several males, and with reproductive success in both gorilla species.
How gorillas beat their chests
In the paper’s description, an individual usually rises onto two legs and quickly drums on the chest with cupped hands, making an impressive drumming sound. Sometimes the chest beat begins with a hooting call and comes with slurred growling, though not every time.
Smithsonian’s National Zoo, on its western lowland gorilla page, says an adult male defending his group may try to intimidate an aggressor by standing on his legs and slapping his chest with cupped or flat hands, roaring and screaming as he does. The San Diego Zoo Wildlife Alliance describes a male gorilla scaring unwanted gorillas away by beating his chest with cupped hands and making a loud noise, then screaming, baring his teeth and charging forward.
The paper reports a mean peak frequency of 638.9 Hz per male, within a range of 459.0 to 1003.0 Hz, from 36 data points covering 6 males. The authors found no support for body size influencing how long chest beats last, how many beats there are or how fast they come.
Why sight and sound both matter
The paper calls the chest beat a multimodal non-vocal signal, since it has both acoustic and visual components. Gorillas live in tropical forests with dense vegetation, which the authors say often makes it hard to see other gorillas even when they are close by.
Individuals close enough to see the gorilla can both see and hear the chest beat, whereas those farther away rely on the sound, and chest beating, the paper adds, can be heard over 1 km away. The authors argue that the chest beat’s evolution as a multimodal signal serves, at least in part, to improve how well the signal carries where visibility is limited.
Competition between males and choice by females
The paper says gorillas live mostly in social groups with a single male and several females, which results in high competition between males. Intense contact aggression between males is infrequent in gorillas, and this is presumed to reflect the high costs of aggression.
Smithsonian’s National Zoo says on its western lowland gorilla page that size is thought to play an important role in deciding who wins an encounter between males, the larger male winning.
The paper says silverbacks beat their chests more often on days when females are in estrous. It adds that groups led by larger alpha males contain more adult females than groups led by smaller males, which strongly suggests that females actively chose to move into groups headed by large alpha males. Females may also disperse among social groups, thereby showing female choice for mates.
Who beats their chests
Chest beating is most commonly performed by adult males (silverbacks), the paper says, and both dominant and subordinate male gorillas emitted chest beats. Younger subordinate males may also beat their chests to fine tune the signal and get social feedback from other gorillas.
The paper adds that infants commonly start chest beating during social play at as young as one year of age. The authors observed 503 chest beats, an average of 1.6 for every ten observation hours, and say further study is needed to find out whether chest beats carry individual signatures.