Nipple - in Male Mammals

In Male Mammals

From conception until sexual differentiation, all mammalian fetuses within the same species look the same, regardless of sex. In humans, this lasts for around 6 weeks; after which genetically-male fetuses begin producing male hormones such as testosterone. Usually, males' nipples do not change much past this point; however, some males develop a condition known as gynecomastia, in which the fatty tissue around and under the nipple develops into something similar to a female breast. This may happen whenever the testosterone level drops. Gynecomastia, although not as severe, may occur in pubescent boys undergoing physical changes due to the rapid and uncontrolled release of hormones, including estrogen. The heightened levels of estrogen in pubescent male bodies leads to the swelling of the nipple and surrounding tissue – this can often look similar to a female human nipple – and may cause slight discomfort. Thus, because the "female template" is the default for humans, the question is not why evolution has not selected against male nipples, but why it would be advantageous to select against male nipples to begin with:

The uncoupling of male and female traits occurs if there is selection for it: if the trait is important to the reproductive success of both males and females but the best or "optimal" trait is different for a male and a female. We would not expect such an uncoupling if the attribute is important in both sexes and the "optimal" value is similar in both sexes, nor would we expect uncoupling to evolve if the attribute is important to one sex but unimportant in the other. The latter is the case for nipples. Their advantage in females, in terms of reproductive success, is clear. But because the genetic "default" is for males and females to share characters, the presence of nipples in males is probably best explained as a genetic correlation that persists through lack of selection against them, rather than selection for them. Interestingly, though, it could be argued that the occurrence of problems associated with the male nipple, such as carcinoma, constitutes contemporary selection against them. In a sense, male nipples are analogous to vestigial structures such as the remnants of useless pelvic bones in whales: if they did much harm, they would have disappeared.

In a now-famous paper, Stephen Jay Gould and Richard C. Lewontin emphasize that we should not immediately assume that every trait has an adaptive explanation. Just as the spandrels of St. Mark's domed cathedral in Venice are simply an architectural consequence of the meeting of a vaulted ceiling with its supporting pillars, the presence of nipples in male mammals is a genetic architectural by-product of nipples in females. So, why do men have nipples? Because females do.