Female Nipples Call Long-Standing Evolutionary Biology Theory Into Question

Evolutionary biologists think a lot about nipples, but they also find them perplexing. Why are they all different shapes and sizes? This posed such a conundrum that a team at the University of Queensland in Australia compared the nipple size of 33 men and 30 women and published their results in the journal Adaptive Human Behavior and Physiology, suggesting that the variation in size defies a basic evolutionary theory.

For the vast majority of body parts, there is a trend in evolutionary biology: “Use it or lose it.” Many cave-dwelling fish, for example, have independently developed blindness over the course of generations, suggesting that it was an advantage to not waste valuable energy on a useless organ. Conventional evolutionary wisdom suggests that, as this happens, the variation in eye features in a population of those fish would decrease. “Basically, if there is genetically based variation to start with, and natural selection favors one particular variant (increased survival for that one type of trait, for instance), genetic variation will decrease, as the other variants become less common or disappear,” explains Nolan Kane, Ph.D., an assistant professor in ecology and evolutionary biology at the University of Colorado Boulder not associated with this study, tells Inverse. This idea was thought to pertain to nipples, but perhaps not for long.

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By this logic, the female nipple, which has a clear evolutionary purpose (feeding babies), should have very little variation — at least compared to male nipples, which the study authors ruthlessly describe as “non-functional.” So, when the team spent an afternoon measuring female nipple size, they thought they would find less variation — in short, a nipple honed by natural selection. Instead, they found more variation in female nipple size than in the male counterparts. These results implied this genetic variation was still there, even though, according to conventional theory, it shouldn’t have been.

“Female nipples are functional as they are used in breastfeeding,” said lead study author Ashleigh Kelly, Ph.D., in a news release. “Therefore, the finding that females’ nipples are highly variable discredits previous studies that indicate variation in a specific feature indicates a lack of functionality.”

The takeaway of this study actually has less to do with nipple size and more to do with how we interpret a basic theory of evolutionary biology. Lack of variation alone, the researchers find, shouldn’t be how we judge whether a trait is advantageous. This is a potential misuse of a theory that Kane also cautions against. “Just looking at the variation that exists for any trait today is not enough, as the authors demonstrate in this study,” he concludes.