Penises of humans are much longer and wider than the penises of other great apes. Human females, too, have larger genitalia and breast size compared with other direct wild relatives.

On the other hand, human testicles are smaller than the balls of bonobos, chimpanzees, orangutans, and gorillas. Why so?

Scientists have noticed that a chimpanzee’s testicles weigh more than a third of its brain while human’s weigh less than 3%. Penises, on the contrary, are smaller with our closest relatives. Even the largest gorillas, which on average are at least twice as heavy as humans, have a penis just two and half inches long when erect.

An article published in the Conversation explains that the proportions of penis and testicles depend on the mating strategies and social relationships within the species.

According to researchers, primates display all sorts of mating behavior, including monogamous, polygynous (where males have multiple mates) and multimale-multifemale. One indicator of behavior in a species is the size difference between males and females, known as sexual dimorphism. The greater this dimorphism is, the more likely the mating is either polygynous or multi-male to multi-female, observations of chimpanzees and gorillas show.

Great ape sexual organs, compared for size (bonobos are flat chested until they get pregnant). Credit: Mark Maslin, "The Cradle of Humanity"

“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,” the study says.

In contrast, male gorillas, also significantly larger than females, practice a polygynous mating system, with one male mating with many females. With minimal intrauterine competition, gorillas have not undergone the same evolutionary pressure for large testes, resulting in relatively smaller ones.

Similarly, modern humans possess modestly sized testes, producing a comparatively smaller volume of sperm. Human sperm count significantly diminishes with frequent ejaculation, unlike chimpanzees (-80%).

Despite possessing larger penises than our primate relatives, particularly chimpanzees and gorillas, human penis size is not exceptional when considering all primates.

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This discrepancy between male size and mating patterns suggests a history of polygynous rather than exclusively monogamous mating among humans. Anthropological data supports this, indicating widespread engagement in polygynous marriage across cultures, albeit typically practiced by high-status or wealthy men.

Despite the prevalence of polygynous marriage, most individuals within these societies remain monogamous, with only a privileged few practicing polygyny. This suggests that ancestral humans likely employed a mating system of monogamy or serial-monogamy, akin to many contemporary hunter-gatherer societies.

Anthropologists propose several theories to explain human monogamy, including the need for long-term parental care, mate guarding, and protection against infanticide by rival males. Monogamy may have evolved as a strategy to ensure offspring survival and reach maturity by providing social and physical protection.

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In complex human societies, monogamy emerges as a social adaptation, requiring significant effort to maintain and protect multiple mates. Access to additional resources and power enables males to safeguard multiple females, often by leveraging social networks. Cultural norms further reinforce monogamous behaviors.

Ultimately, in human evolution, intelligence and sociality have emerged as critical factors influencing access to sexual partners, superseding physical attributes like penis size. Thus, the brain, rather than physical characteristics, becomes the primary sexual organ in navigating complex social landscapes, the authors conclude.

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