Why apes and Tarzan beat their chests

Gorillas and other greater apes can really pack a punch. But bones, including ribs, only stay strong when stressed. Therefore, if you are an alpha ape ready to challenge and be challenged, it behooves you to keep your rib cage stressed and strong. Therefore, you have no choice but to drum on your chest periodically.

The apes who liked drumming on their chests survived and won the territory and the females. Those who didn't lucked out, got their wind knocked out of them, and bore fewer or no offspring.

When I see young guys drumming at random on the table or what not, I wonder if some relic of that chest drumming survives. When I hear about politicians, generals, or other officials sounding the drumbeat of war, or setting their troupes on display in a massed march, I envision an ape drumming on his chest to get ready for a fight.