Humans have a much longer and wider penis than the other great apes. Even the largest of gorillas, more than twice as heavy as a human, will have a penis just two and half inches long when erect.
However our testicles are rather small. A chimpanzee’s testes weigh more than a third of its brain while ours weigh in at less than 3%. The relative size of our penis and testes is all down to our mating strategies, and can provide some surprising insights into early human culture.
Primates exhibit all sorts of mating behaviour, including monogamous, polygynous – where males have multiple mates – and multimale-multifemale. One indicator of which behaviour occurs in a species is the size difference between males and females. The greater this sexual dimorphism, the more likely the mating is either polygynous or multi-male to multi-female. This can be shown by observing chimpanzees and gorillas, our closest living relatives.
Male chimpanzees are much larger than females, and they have a multi-male to multi-female mating system. Essentially, male chimps have sex all the time with any female and with any excuse. A female, therefore, may contain sperm from multiple partners at any one time, which puts the sperm itself – and not just the animals that produce it – into direct competition. For this reason, chimpanzees have evolved huge testicles in order to produce massive amounts of sperm, multiple times a day.