Women's nipples at odds with evolutionary biology
The nipples of women are far more varied than those of men, according to an unusual study published this week that challenges a widely held view among evolutionary biologists.
An axiom in the field says the more important a body part, the less it will differ in essential features from person to person.
We can't really live without a gall bladder or brain stem, and there's not much variation between yours and mine, the thinking goes.
Noses and ears are extremely useful too and work roughly the same way, though core purpose is not compromised by odd shapes or sizes.
Evolutionary pressure, in other words, won't allow truly critical features to stray too far from the functional template.
But with non-critical attributes, Nature or more precisely random change can get creative, according to the theory.
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 non-functional 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 teats adorning the male chest.
That would be consistent with earlier research that found greater variation in the relative length of clitorises compared to penises, and concluded that the female orgasm is a non-functional by-product of the male orgasm.
To find out if the "function-first" rule holds for nipples, Kelly's team scanned and measured the teats and nipples of 63 undergraduate volunteers.
Height and chest circumference were also registered, along with BMI (body-mass index) and the temperature of the room.
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."