Surprisingly, humans are superior to other primates.

Gorilla: Harem structure in which one male monopolizes several females → There is almost no sperm competition → The genitals are small, but competition is achieved with a huge muscular body.

Chimpanzee: Multiple mating system in which females mate with multiple males → **sperm competition** is fierce → testicles are large and sperm count is high.

Humans: polygamous or monogamous mixed societies → Competition through **pair-bonding** and sexual signals rather than sperm competition → Penis size and shape developed as a means of visual and psychological competition.