Hypothesis, Sex

Why Did Humans Evolve Big Penises But Small Testicles?

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.

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. 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. Their testes, therefore, are relatively small. This is similar to modern humans, whose testes are also of very modest size and produce a relatively small amount of sperm. In fact human sperm count reduces by more than 80% if men ejaculate more than about two times a day.

Chimps have huge testicles for their size.

The human penis is large when compared with those of our closest relatives: chimpanzees, gorillas, and orangutans. 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.

Comparative measurements show the human penis is not exceptionally long. 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.

Some of the complex penises found in multi-male to multi-female mating primates such as chimpanzees (h), brown lemurs (a) or macaques (d, e, f). Alan F. Dixson, Primate Sexuality

The human penis is in fact extremely dull — it does not have lumps, ridges, flanges, kinks or any other exciting feature that other primates have. In primates, this lack of penis complexity is usually found in monogamous species.

Monogamy mystery

This observation clashes with the fact that men are significantly larger than women. This suggests our evolutionary background involved a significant degree of polygynous, rather than exclusively monogamous, mating. This is supported by anthropological data showing that most modern human populations engage in polygynous marriage. Anthropologists Clellan Ford and Frank Beach in their book Patterns of Sexual Behaviour suggested that 84% of the 185 human cultures they had data on engaged in polygyny.

Primates with simpler penises tend to be monogamous like cotton top tamarins (a) or polygynous like gorillas (g). Alan F. Dixson, Primate Sexuality

However, even in these societies most people remain monogamous. Polygynous marriages are usually a privilege reserved only for high status or wealthy men. It is worth noting that hunter-gathers around the world practice only monogamy or serial-monogamy which suggests that our ancestors may have used this mating system.

At first sight, however, it would seem sensible for males to reproduce with as many females as possible. Human monogamy has long puzzled anthropologists, and lots of effort has gone in to working out what keeps males hanging around.

Three main theories have been put forward. First is the need for long-term parental care and teaching, as our children take a long time to mature. Second, males need to guard their female from other males. Third, our children are vulnerable for a long time and infanticide could be a risk from other males. So to ensure that children are able to reach maturity the male is likely to stay to protect them, both socially and physically. This may be why males have maintained their larger relative size.

Hamadryas baboons have unusually long penises.

If we view the evolution of monogamy mating systems in humans through the lens of human society it is clear that it takes a huge amount of social effort to maintain and protect more than one mate at a time. It is only when males have access to additional resources and power that they can protect multiple females, usually by ensuring other males protect them. So monogamy seems to be an adaptation to protect one’s mate and children from other males. This monogamy is reinforced by the high social cost and stress of attempting to do this for multiple partners, and it has become supported by cultural norms.

So when living in complex human societies the largest and most important sexual organ is the brain. Somewhere in our evolutionary past how smart and social we are became the major control on our access to sexual partners — not how big or fancy a male’s penis is.


Mark Maslin is a Professor of Palaeoclimatology at University College London.

This article was originally published on The Conversation.

Filed under: Hypothesis, Sex


Mark Maslin is a Professor of Palaeoclimatology at University College London.


  1. Ed Joyce says

    One important fact left out of this article: Unlike the great apes, humans don’t have a baculum — the penis bone. Why?

  2. Scott Reilly says

    I have only a passing interest in this subject, but there are a couple of glaring errors here. Firstly, chimps and gorillas are not our closest living relatives. Chimps and bonobos are. The common ancestor we share with gorillas is further back in our ancestral past than the one we share with chimps and bonobos.

    Secondly, the author claims that large body size diamorphism exists between male and female chimps. This is simply not true. Male chimps (and bonobos) are only slightly larger on average than females (comparable to human male-female diamorphism), whereas male gorilla are fully twice the size of females. As stated by the author, gorillas operate in a harem-based mating structure, with the males having evolved much larger and stronger bodies to be able to compete for the rights to mate with all of the females. Chimps live in multi-male multi-female structures, where there is little competition among males (and none exists among bonobos). This undercuts what the author claims in the third paragraph when saying that the greater the body size diamorphism, the more likely the society is either polygynous or multi-male multifemale. This is true for polygynous societies, but makes no sense in multi-males multi-female ones, as there it wouldn’t be advantageous for males to evolve to much larger sizes than females as there is little to no competition between males for mates.

    • Guille says

      Scott Reilly

      Actually human sexual dimorphism is significantly larger than in chimpanzees and bonobos.and closer to that of the gorillas once you control for the fact that humans have large sex differences in muscle to fat ratio which are unique among primates. When you measure only muscle mass men have 60% more total lean muscle mass than women which is fully comparable to that of gorillas.

  3. Simon says

    “At first sight, however, it would seem sensible for males to reproduce with as many females as possible. Human monogamy has long puzzled anthropologists, and lots of effort has gone in to working out what keeps males hanging around.”

    I would say that social/cultural factors are a significant contributor here. When human societies moved from states of generalized skills (i.e. hunter gatherer) to specialist skills (i.e. agricultural), the effects of culture and in particular the ability of parents to pass down cultural knowledge that is specific to these specialist skills became a prime factor in the ability of offspring to successfully mate.

    Essentially fathers have to stick around to ensure that their offspring become useful members of society. This at least partly explains the ‘fitness’ bestowed by cultural conventions such as marriage etc.

  4. Raymond Hames says

    The author’s claim that “It is worth noting that hunter-gathers around the world practice only monogamy or serial-monogamy which suggests that our ancestors may have used this mating system.” is widely incorrect. 90% of hunter-gatherers societies permit polygyny. See Marlowe, F. W. (2005). Hunter-gatherers and human evolution. Evolutionary Anthropology, 14(2), 54-67.

  5. @Simon

    “Essentially fathers have to stick around to ensure that their offspring become useful members of society.”

    The advantage is to the offspring, not (necessarily) to the society. The offspring would be learning whatever skills were advantageous to the father, which could be detrimental to the society (e.g. the father is a skilled fraud, a self-serving religious authority, or a vainglorious and ignorant politician)

    • Simon says

      Such behaviors are generally not advantageous to the individual as they are typically subject to punishment by the society. i.e. as society became more complex and co-operative, rules and norms and punishments for non-adherence to these rules had to be developed to ensure ‘buy-in’ to that co-operation. This is the basic function of justice, religious and moral systems..

  6. Howard Nelson says

    This is erudite madness masquerading as meaningful mentations.
    Darwinically, humans have ‘large’ penises due the simple fact that we evolved to have large, accommodating penal colonies.
    Our relatively smaller testicles are due to the declining need and use for testing icles. Thus, the testers cannot find a sufficient number of testees to pay the cost of examination.
    For the cognoscenti, this argument is incontrovertible.
    I hope I’ve not pricked the pride bubble of the banal babblers.
    As they said in the ancient Middle East, ‘Babylon, if you must.’

  7. @Simon

    You don’t think societies, however advanced, are not open to constant abuse? You don’t think individuals exploit other individuals e.g. relationship cheats? You don’t think justice systems are relentlessly challenged by successful frauds (you think a majority of fraud is punished?). You think religious systems are not hotbeds of exploitation, enforcers of primitive and primal instincts, and authoritarian bulwarks against progress? You think “moral systems” are not hotly contested and not repositories of self-interest and self-righteous sanctimony? I could go on. And on.

    Yes, societies can make advances (and reversals) against the shitty parts of our nature, but it’s an endless all-hands-to-the-pump effort, and one without a destiny that’s necessarily better. A world of injustice (though with great advances over time) testifies to this.

    I admire your optimism in human nature and progress.

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