Cheating and Large Testicles

There is a new article making the rounds on the science pages of many online publications. The seemingly shocking claim is that, “Large testicles mean greater infidelity in primates.” In fact, Petter Bøckman, one of the authors of the original article claimed that, “We can determine the degree of fidelity in the female by looking at the size of the male’s testicles.” Unfortunately for the authors of both the scientific and popular news articles, this claim is not accurate, and the idea of male testicular anatomy being correlated with mating behavior is not news to anyone who studies sexual reproduction, or has read any of the primary or secondary literature in this area over the past 30-plus years.

The understanding of testicle size, sperm production and mating behavior among animals in general stretches back to at least 1970 with Parker’s work on sperm competition in insects. Since Parker’s seminal (pun intended) work, the number of studies incorporating, focusing on and illuminating the impact of mating behavior on testicle size and sperm production in primates and other animals has been nearly as overwhelming as the number of sperm in a typical bonobo ejaculation.

In 1981, Harcourt et al published a pioneering paper in Nature, that laid the foundation for sperm competition studies in primates. Harcourt and his colleagues have, subsequently followed up on that original paper extensively. Others built on Harcourt's work in primates, expanding on the species considered and adding to the general pattern that was emerging. For example, Dr. Peter Kappeler found that among lemurs (distant primate cousins), “…multi-male species had significantly larger testes than pair-living ones.” From Alan Dixson to Karen Strier to Jane Goodall, researchers have understood the connection between mating and testicle size among primates for decades.

It appears that Dr. Bøckman tried to get around the predictability and redundancy of his research by focusing on female, instead of male behavior. Unfortunately, Dr. Bøckman used the word “fidelity” when referring to female sexual behavior, calling the accuracy of his claims into question. Fidelity is a difficult term to apply to those few primate species that are regularly sexually monogamous, much less the rest of the 300 plus species that are not monogamous. Fidelity refers to faithfulness, and sexual infidelity implies cheating, betrayal and all of the psychological ramifications involved, which we can only really apply to humans. To put it a different way, female bonobos cannot “cheat” on individual males in their communities because they are not “committed” to those males in the first place. Female bonobos can be promiscuous though, and they tend to be highly promiscuous, having sex with multiple males (and some females) regularly.

This terminology gap may be the result of native language differences, and the terms regularly used for different behaviors, but it’s important here. When media outlets run titles with the terms “cheating” and “infidelity” in them, they are gunning for increased readership, hits, pings, etc. When scientists use those terms, incorrectly, it provides legitimacy to the sensational headlines chosen by editors.

So, while larger testicles do not result in greater infidelity in female primates, larger testicles do correlate with high levels of promiscuity in female primates. Again, we have known this for decades. While correlation does not automatically equal causation, in this case it appears that males of some species have evolved large testicles in response to female choice. Females, in some primate species have chosen to mate with multiple males in an effort to maximize their own fitness, and males in those species have had to adapt accordingly. As a result, males have developed large (in some cases huge) testicles, which allow them to produce and store sperm in massive quantities in an effort to use the law of large numbers to defeat their rivals.

What does all of this tell us about human mating? It may tell us quite a bit, but one thing it won't tell us is whether or not females have been unfaithful. While our mating systems have become more nuanced, complex and legalistic, our current adaptations are the result of ancient selective forces, and the most base of those forces still act on us today. Human males do not have the relatively massive testicles found in highly promiscuous species with intensive sperm competition, but also do not have the minute testicles found in species in which males can monopolize female mating opportunities. Ours is a mixed lot. Men are built to be competitive with sexual rivals by producing and storing substantial amounts of sperm on a daily basis, but we lack the equipment needed to be successful in intensive sperm competition. Contrary to what Dr. Bøckman concluded, our anatomy does not provide, “…evidence that our females are cheating.” Instead, human testicular anatomy shows the results of millions of years of female choice and male response in a mating system that had nothing to do with “cheating,” and everything to do with both sexes trying to be successful in the mating game.