A Better Explanation for Physical Differences Amongst Sexes

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Source: Quanta Magazine

By Sophie Zhu

Often, society has noted the stark physical differences between men and women: men are generally taller, and women often have wider hips. The typical explanation stems from Charles Darwin’s note that, “undoubtedly,” the greater size and strength of a man is due to how men must impress mates, with natural selection choosing time and time again the strongest and most able in the struggle for life. Men had to be aggressive and competitive for wives, whereas women were picky to choose the best mate.

However, biological anthropologist Holly Dunsworth from the University of Rhode Island recently published a paper in Evolutionary Anthropology that suggested a different reason for the clear differences. She argues that the argument for natural selection driving human body size is rather unsupported and baseless, and that a far more reasonable explanation has been shrouded by popular belief today.

The main issue with the general theory is that scientists reason is, if men are larger than women and tend to be more combative, the two actions must be connected. However, there is little data to truly support the claim that the combative trait implies the size difference. With this in mind, another evolutionary anthropologist Louis Barrett pondered on how dominance and competition may in fact be the consequence of the size difference, rather than the cause.

Dunsworth delved into bone biology and development, and she realized that women are generally smaller than men because they have ovaries, which produce far more estrogen than testes do. Following puberty, estrogen tends to stimulate long bone growth, but prior to it, nearly everyone grows at the same rate. So, when girls hit puberty, the ovaries heighten estrogen production, allowing early adolescent girls to be taller than boys. However, these high levels of estrogen make the growth plates in bones fuse, muffling bone growth. Meanwhile, testes allow for bones to grow for years and years before their estrogen peaks, so boys end up taller in general.

This explanation corresponds far better to historical data with human sizes. After the Black Death, men got about 9 cm taller and women got 5.5 cm shorter, creating a 62% increase in male-female height differences. Clearly women didn’t start preferring taller males, so the hormonal explanation alludes to the cause of the increase being that people were presumably healthier following the pandemic.

Dunsworth also takes into consideration hip size. Most characterize female pelvis size with the need to give birth to larger babies. However, there is little evidence hip width affects reproductive success. In particular, chimpanzee newborns’ heads are far smaller than chimpanzee mothers’ birth canals, yet chimpanzee females still have wide hips. So, Dunsworth perceived the difference in pelvis width as a matter of providing enough space for the female reproductive system.

Dunsworth also notes how important getting this sexual selection narrative accurate is: the storyline of how competitiveness in males incites violence gives room for the stereotypes, both good and the bad. However, using bone growth biology, scientists like Dunsworth and Barrett can determine the true, justifiable hypothesis behind sex differences.

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