The Hamadryas baboon, for instance, a native of the Horn of Africa, has an erect penis that is five and half inches long – slightly shorter than an average human male, but they weigh only a third of our weight.Ĭhimpanzees are seen during feeding time at the Los Angeles Zoo in Los Angeles, California on Mark Ralston/AFP However, primatologist Alan Dixson in his wonderfully detailed book, Primate Sexuality, suggests that if we look at all primates, including monkeys, this is just wishful thinking.Ĭomparative measurements show the human penis is not exceptionally long. The human penis is large when compared with those of our closest relatives: chimpanzees, gorillas, and orangutans. In fact human sperm count reduces by more than 80% if men ejaculate more than about two times a day. This is similar to modern humans, whose testes are also of very modest size and produce a relatively small amount of sperm. Their testes, therefore, are relatively small. With little or no competition actually inside the uterus, gorillas have had no need for a testicular arms race to facilitate the production of more and more sperm. Male gorilla are also much larger than females, but they have a polygynous or harem-style mating system where many females live with a single male. Great ape sexual organs, compared for size (bonobos are flat chested until they get pregnant) Mark Maslin, The Cradle of Humanity, Author provided For this reason, chimpanzees have evolved huge testicles in order to produce massive amounts of sperm, multiple times a day. ![]() 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. Essentially, male chimps have sex all the time with any female and with any excuse. Male chimpanzees are much larger than females, and they have a multi-male to multi-female mating system. This can be shown by observing chimpanzees and gorillas, our closest living relatives. ![]() The greater this sexual dimorphism, the more likely the mating is either polygynous or multi-male to multi-female. One indicator of which behaviour occurs in a species is the size difference between males and females. Primates exhibit all sorts of mating behaviour, including monogamous, polygynous – where males have multiple mates – and multimale-multifemale. 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. A chimpanzee's testes weigh more than a third of its brain while ours weigh in at less than 3%. Bigger penises make female fish evolve bigger brains – so they can avoid males
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