An argument without a fight
Why Does a Gorilla Beat Its Chest?
It is a way of saying how big you are, loudly, from far enough away that nobody has to get hurt.
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Step 1 of 5
Fighting is expensive even when you win.
A gorilla that wins a fight can still finish it with a torn hand or a bitten arm. Anything that settles the matter without touching is worth having.
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Step 2 of 5
So it makes a sound that carries.
The hands are cupped, not clenched, and the chest behind them is a big air-filled box. Cupped hands on a hollow chest make a deep pock-pock that travels through forest for about a kilometre.
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Step 3 of 5
The sound honestly reports how big the drummer is.
Bigger gorillas have bigger air spaces, and bigger air spaces make a deeper sound. A small gorilla cannot fake the low notes, however hard it hits.
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Step 4 of 5
So the listener can decide without coming to look.
A rival hearing deep drumming from a nearby valley learns what it needed to know and moves the other way. The argument was won at a distance.
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Step 5 of 5
It is not only for rivals.
Females listen too, and young gorillas of both sexes drum in play, badly, for years before it means anything.
A chest beat lasts under a second, and a gorilla usually gets in a run of five to twenty.
The short version
A gorilla beats its chest to broadcast its size honestly, so rivals can back down from a distance instead of fighting.
Try it yourself
Knock on a full cupboard and then an empty one. The empty one sounds deeper because there is more air inside — which is the whole trick.