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Male and Female Nipples as a Test Case for the Assumption that Functional Features Vary Less than Nonfunctional Byproducts

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Evolutionary researchers have sometimes taken findings of low variation in the size or shape of a biological feature to indicate that it is functional and under strong evolutionary selection, and have assumed that high variation implies weak or absent selection and therefore lack of function.


To test this assumption we compared the size variation (using a mean-adjusted measure of absolute variability) of the functional human female nipple (defined as the nipple-areola complex) with that of the non-functional human male nipple.


We found that female nipples were significantly more variable than male nipples, even after controlling for body mass index, testing-room temperature, bust size in women, and chest size in men.


Morphological variation in a feature should not be used by itself to infer whether or not the feature is functional or under selection.

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Correspondence to Brendan P. Zietsch.

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All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

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The authors declare that they have no conflict of interest.

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Kelly, A.J., Dubbs, S.L., Barlow, F.K. et al. Male and Female Nipples as a Test Case for the Assumption that Functional Features Vary Less than Nonfunctional Byproducts. Adaptive Human Behavior and Physiology 4, 344–353 (2018).

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