Last updated June 29, 2018 at 4:50 pm
Large variation debunks previous assumptions.
Male nipples show much less variation in size than women’s. Credit: iStock
Human nipple sizes put the lie to one of the central tenets of evolutionary biology, according to a study conducted by researchers at Australia’s University of Queensland.
Some scientists within the field hold that size variation in any particular body feature is an indication of the degree of evolutionary selection to which it is subject. A feature that shows little variation across populations, the argument runs, is therefore strongly conserved genetically. This, it follows, is the result of it having a common and essential function.
In contrast, features that have few or no essential functions – chins, for instance, or hair colour – are subject to little selection pressure and thus differ a lot between individuals.
A team led by psychologist Ashleigh Kelly wondered just how well this argument applied human nipples. The answer, as she and colleagues reveal in a paper published in the journal Adaptive Human Behaviour and Physiology, is not very well at all.
Indeed, the scientists report, variation in nipple size is so great that it pretty much invalidates the entire proposition.
Weak selection processes
Nipples were a very good target for the investigation by Kelly and her team because both women and men have them, but it’s only in women that they have a biological function.
Male nipples are a classic evolutionary by-product – a non-functional echo of a bunch of genetic instructions that work properly only in women.
To make their finds, the researchers enrolled 63 undergraduate volunteers from both genders and asked them all to get their gear off – in safe, individual and private circumstances, of course.
Measurements were then taken from each participant. They included not just nipple size, but also general height, size, and weight, as well as a couple of specifically relevant additional markers, such as room temperature.
The research revealed that male nipples – which are non-functional and therefore should be subjected to very weak selection processes – showed much less variation than female ones. Male nipples averaged 36% the size of their female equivalents.
Female nipples were not only substantially larger than male ones, but also showed significantly greater variation in size and shape – a variation that remained robust even when differences in height, weight and breast size were taken into account.
“Female nipples are functional as they are used in breastfeeding,” explains Kelly. “Therefore, the finding that female nipples are highly variable discredits previous studies that indicate variation in a specific feature indicates a lack of functionality.”
The finding, say the researchers, is not just a telling test of one of evolutionary biology’s key assertions. It also has implications for the validity of earlier research that suggested that wide size variability in other gender-differentiated features, notably the clitoris, meant that female orgasm was a non-functional by-product of male sexual process.
Such suggestions have sometimes been pressed into service to justify gender inequality in cultural and legal arenas.
