Large testicles mean greater infidelity, research finds - Telegraph
Prof Bøckman said bonobos have particularly large testicles and mate in large groups whereas gorillas have small testicles.
He said: "There is an abundant flow of semen. Those who leave the greatest amount of sperm have the largest chance of fathering offspring.
"In gorilla troops there is only one male. Even though the gorilla has a small harem, he has no need for large testicles – his balls are tiny.”