Body size dimorphism and relative testes size in primates.
In single-male species such as Gorillas, where a male must physically prevent other males from mating with females, males tend to be much larger than females, resulting in body size dimorphism between the sexes. In multi-male species such as Chimpanzees, where promiscuous mating behavior leads to high sperm competition, the relative testes size of males is larger than in a single-male species.