What the Difference Between Men and Women's Nipples Says About Human Evolution

Ever wondered what the differences in men and women's nipples suggests about how we evolved as humans? Then you're in good company, as experts have delved into this question in a study.

Some evolutionary biologists argue body parts that vary little from person to person could be more important and are the result of strong evolutionary selection, whereas features that vary greatly come from a weak selection process, and have little function. Researchers at the University of Queensland in Australia questioned this theory.

The authors pointed to a 2008 study that suggested the greater variation in the size and length of clitorises when compared to penises—which are largely similar—meant the female orgasm was a non-functional byproduct of the male orgasm.

The team used nipples to test their hypothesis, as male nipples are generally uniform compared to women's nipples. Male nipples are understood to be a non-functional version of the female nipple, because the latter is used to breastfeed.

Evolutionary biologists have asked whether variations in body parts indicate how functional they are. Getty Images

Enlisting the help of 63 undergraduate students as participants, the team scanned and measured their nipples. Variables including BMI, height and chest circumference of participants were also noted. The temperature of the room was documented, too, as this can affect the size of the protruding tissue.

On average, male nipples were found to be 36 percent the size of female nipples. Women also had a greater variation in their nipple area and size.

The results discredit past studies that suggest variability in size means a feature has no function, the authors concluded in the study, published in the journal Adaptive Human Behavior and Physiology. That includes the study dismissing the female orgasm.

Dr. Kim Wallen, author of the 2008 study and professor of psychology and behavioral neuroendocrinology at Emory University, told Newsweek: "This study adds significant data to the debate [on variability and functionality] and suggests that a simple relationship between functionality and increased variation is not likely."

Addressing the authors' argument that the 2008 study should be disregarded because male and female nipples appear to show the opposite effect to penis and clitoris size: "we cannot conclude anything concrete about the relationship between variability and functionality. These new data add important information to the discussion and support the idea that variability doesn't reflect functionality in any consistent manner."

Ashleigh Kelly, of the University of Queensland school of psychology, said in a statement: "We found that female nipples were significantly more variable than male nipples."

"Female nipples are functional as they are used in breastfeeding. Therefore, the finding that females [sic] nipples are highly variable discredits previous studies that indicate variation in a specific feature indicates a lack of functionality."

Mark Pagel, Professor at the University of Reading school of biological sciences, told Newsweek the study adds to a small number of previous works that refute the idea that traits that are under strong selection will tend to be less variable than traits that are not under such strong selection.

"Its principal interest outside of this theoretical question will be to raise the question of why female nipples vary more than men's. Previous studies have shown that traits that are sexually selected (traits that are used to attract mates) often vary more than traits that are not sexually selected," he said.

"The colorful plumage of many male birds is a good example of a sexually selected trait. Human female breast size (of which nipples are a part) is large compared to other great apes (humans are a great ape) and this might be because human female breast size is at least partially a result of sexual selection. If true, this could explain the authors' findings."

This article has been updated with comment from Professor Mark Pagel and Professor Kim Wallen.