Why Do Gorillas Beat Their Chests?

Much like fighting cocks or even adrenaline-filled human males who puff their chests to try to show dominance, the male gorilla also has a multifaceted purpose for beating his chest. But what's his purpose?

Researchers at Max Planck Institute for Evolutionary Anthropology think they know why. They published their findings in a study in the journal Scientific Reports.

No doubt you've seen this posturing and behavior — a male gorilla standing on two feet and using his cupped hands — not fists — in an alternating pattern beating his pecs. Scientists believe gorillas use these chest beats as a nonvocal communication to both attract females and intimidate potential rivals.

With both acoustic and visual elements, this long-distance signal is most commonly performed by adult males (silverbacks) and can be heard more than 0.62 miles (1 kilometer) away. However, younger subordinate males may also chest beat to practice and gain social feedback. It's also been observed in infants as young as 1-year-old during social play.

Researchers came to this conclusion while using a technique called photogrammetry, which allowed them to measure body size of adult male wild mountain gorillas monitored by the Dian Fossey Gorilla Fund in Volcanoes National Park, Rwanda in a noninvasive way.

The scientists found that larger males emitted chest beats with lower peak frequencies than the smaller males. King Kong aside, it seems the beating conveys information about body size, with larger males having a higher success rate in mating, as well as social dominance.

Although intense contact aggression between males is infrequent, rival males use the information to determine the competitive ability of the chest beater and whether to initiate, escalate or retreat from aggressive behaviors with the other. Not surprisingly, females most likely use the sound to determine whether this is a good candidate for a mate.

Because gorillas live in predominantly one-male, multifemale social groups there's high male-to-male competition. Females, however, are not locked into one group and may wander among social groups, thereby presenting females with the option to choose mates.

The researchers also found a lot of variation among males in both the number and duration of the chest beats, suggesting that like human fingerprints, they may have individual signatures. However, more research is needed to confirm that theory.