World / Science & Health

Variation in women's nipples challenges evolutionary theory


The nipples of women are far more varied than those of men, according to a study that challenges conventional theory.

An axiom in evolutionary biology says the more important a body part, the less it will differ from person to person.

We can’t live without a gall bladder or brain stem, and there is not much variation among them.

Similarly, noses and ears work roughly the same way despite sometimes having odd external shapes or sizes.

Evolutionary pressure, in other words, won’t allow truly critical features to stray too far from the functional template. But with noncritical attributes, nature — more precisely, random change — can get creative.

Which brings us to the study on nipples, led by Ashleigh Kelly at the University of Queensland in Australia and published in the journal Adaptive Human Behavior and Physiology.

“Male nipples are regarded as a prototypical evolutionary by-product, a nonfunctional version of the functional female nipple,” the researchers note.

Female nipples, by contrast, are designed with a fundamental purpose: nursing newborns.

If the theory is right, then there should be less variation among female nipples than the pointless spots on the male chest.

That would be consistent with earlier research that found greater variation in the lengths of clitorises than penises, and concluded that the female orgasm is a nonfunctional by-product of the male orgasm.

To find out if the “function-first” rule holds for nipples, Kelly’s team scanned and measured the breasts and nipples of 63 undergraduate volunteers. Height and chest circumference were also registered, along with body-mass index and the room temperature.

As Kelly suspected, the results did not conform to theory.

“We found that female nipples were significantly more variable than male nipples,” said Kelly.

“This finding discredits previous studies that indicate variation in a specific feature indicates a lack of functionality.”