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Key points
- Sexual dimorphism refers to physical sex differences within a species.
- Species with more sexual competition are more sexually dimorphic.
- Species that pair bond are less sexually dimorphic.
Humans are sexually dimorphic, which means that males and females are, on average, different sizes. In humans, like most sexually dimorphic species, males are typically larger than females. Why does this sex difference not go in the opposite direction, and why are all species not equally sexually dimorphic? The explanation is the same as most things in nature: energy.
Sex Differences in Parental Investment
In some species, fathers contribute nothing more to their offspring than sperm. In peacocks, for example, males do not parent and instead invest their energy into growing beautiful feathers to attract mates. The females are drab by comparison, out of practical necessity: she must invest her energy in laying eggs and feeding chicks. In other species, it is the male who is drab: he instead devotes his energy to building nests and feeding the mother and her young. These species are most likely to pair bond monogamously.
In general, the more sexually dimorphic a species is, the less the males contribute to parenting, the more sexual competition there is, and the less likely they are to be monogamous. We can look to our primate relatives to see where humans fall on this spectrum.
Sexual Dimorphism in Primates
Gorillas are extremely sexually dimorphic and competitive. Males weigh more than twice as much as females and will fight with other males to monopolize the females. The alpha silverback, after proving his dominance, will form a harem with multiple females all to himself. Bonobos, by contrast, are highly sexually free. Everyone has sex with everyone, even same-sex sex, as a form of social bonding. They are not sexually dimorphic but have evolved huge testicles to deal with intense sperm competition. Gorillas, meanwhile, have tiny testicles. Their huge muscles and canines ensure that all sexual competition happens outside the womb; they only need the bare minimum amount of sperm.
Sexual Dimorphism in Humans
Humans fall somewhere between these two extremes. Men are roughly 15 percent larger than women and are, on average, more aggressive. This indicates that over the course of human evolution, men were selected based on their fighting capability. This does not necessarily mean that men were constantly fighting over women, though this is certainly a popular trope in both classic literature and modern media. Women themselves may have sexually selected formidable men for protection and hunting ability; research indicates that women find taller and more muscular men more attractive, no doubt a preference shaped by evolution.
Polygyny in Our DNA?
Our sexual dimorphism means that our ancestors were far from monogamous throughout history. In fact, DNA evidence tells us that we have roughly twice as many female ancestors as male ancestors. That makes our ancestors polygynous, the technical term for polygamy where one man partners with multiple women.
To understand how this could be true, imagine a population of 100 males and 100 pregnant females. It is possible that each man who successfully mated had two partners, and only fifty men impregnated all women. The other half of the men did not successfully reproduce, and this generation has twice as many female ancestors as male ancestors.
Now imagine that this pattern repeated itself over many generations. Over time, we became more sexually dimorphic. This does not mean that sexual selection was only acting on males. For example, human females have much larger breasts than our ape relatives who can nurse just fine. Large breasts have no obvious survival or reproductive benefits other than signaling attractiveness and genetic fitness. They likely evolved through sexual selection.
While our bodies and ancestry show that there has been substantial polygyny throughout human history, there is a reason that we are not as sexually dimorphic as gorillas or as promiscuous as bonobos. There are strong evolutionary pressures–biological and cultural–toward pair bonding. These pressures promote monogamy and fatherhood and keep sexual dimorphism at bay.
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