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Adapted from Paleofantasy: What Evolution Really Tells Us About Sex, Diet, and How We Live by Marlene Zuke. We can also draw conclusions about how natural and sexual selection have acted on the sexes in the past, simply by looking at the kind and magnitude of differences between male and female bodies today, as well as by comparing those differences across species.

For instance, we are reasonably sure that some dinosaurs cared for their young, because of the discoveries of eggs or young dinosaurs associated with an adult.

Usually, male animals produce enormous numbers of sperm cells (human ejaculates are 1.5–5.0 milliliters and contain anywhere from 20–150 million sperm per milliliter), but ejaculates need to be replenished, and might not be sufficient to fertilize the available females if a male is mating very frequently.

In addition, if a female mates with more than one male in a short period of time, the sperm in her reproductive tract can compete with each other, in which case the male supplying the most competitors is at an advantage. Human testes are smaller relative to body size than those of chimpanzees and bonobos, but larger than those of either the monogamous gibbons or the gorillas.

Whether or not a society was polygynous made no difference, although the authors caution that their sample might not have allowed detection of a contribution by the mating system.

The last piece of evidence about our mating history that we can glean from our bodies is a bit more personal than height.

More recently, detailed examination of the stretches of DNA that are present in our ape relatives but absent in modern humans revealed a loss that women, at least, have cause to celebrate: the genes coding for “genital tubercles,” or more graphically, penis spines.

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Our behavior is slippery stuff, with men and women acting differently in different societies and under different circumstances.These structures are absent in humans because we lack the genes responsible for the hormone signals that would cause them to develop.The relative smoothness of the human penis is thought to be linked to a reduced frequency of sperm competition.Along with the rest of his focus on sex, Darwin was extremely interested in these differences, and he drew a distinction between what he called primary and secondary sexual characteristics.The primary sexual characteristics are what define us as male or female—the plumbing, so to speak, with human (and other mammal) males having testes and females having ovaries, for example.