Why do men have nipples?

Question sent by ALONSO SÁNCHEZ POLO. PAU CARAZO answers:

Until the onset of puberty, when feminine hormone release triggers the development of breasts in women, the development of mammary glands happens regardless of the sex an individual belongs to. Nipples are silent witnesses to this physiological fact. Developed in the early embryonic stages, they will decorate men’s bodies without a purpose. The first mystery we might want to solve is why men don’t develop breasts in their puberty. It is obvious that breasts are useful in women because they are used to feed their children but, would it not be even better if both women and men were able to feed their offspring? After all, it happens in other species, as in the case of the Dayak fruit bat (Southeast Asia). Things happen differently for humans due to the existence of sex-specific evolutionary interests.

We must remember that evolution will always try to maximise the number of descendants for each individual. Female humans (like in most animals) bear most of the costs of reproduction, mainly because embryos grow inside their bodies. This means that, for women, the winning formula is to invest a lot in the survival of each one of her descendants. For men, however, reproduction is considerably less costly. A man’s investment in reproduction is reduced to a few minutes (at best!) and to a couple of thousand spermatozoa. As a consequence, the evolutionary winning formula for men is to invest little in the rearing of his offspring and, instead, (try) to mate with as many women possible. For this reason, evolution has not favoured the development of breasts in men.

But, if male nipples serve no purpose, why are they there at all? The answer entails a very important lesson. Men have nipples because, even though they are not useful, evolution does not work by eliminating all dispensable parts in our bodies, but by eliminating only those parts that are costly from an evolutionary perspective. Evolution has not paid attention to male nipples because they have not posed any danger to our survival or handicapped our reproductive success. Evolution is not a directed process, nor does it have any hidden agenda, and therefore does not result in perfect designs, but in designs that will tend to optimise the biological success of each organism in a given series of historic contingencies, limitations in development and sheer chance.

This has caused the appearance of many botched designs in the history of life in our planet. Male nipples are a good example, and yet another piece of evidence against the fallacy known as “intelligent design”. This fallacy, so unfortunately in vogue in the last few years, states that the exquisite and complex designs observed in nature can only be explained through the existence of an intelligent designer. There is compelling scientific evidence to take this explanation to pieces, but one of the most forceful (and amusing) arguments against it is the existence of evolutionary botch jobs such as male nipples.

Why do men have nipples, then? Because women have breasts and God does not exist.

Pau Carazo. Marie Curie researcher at the Department of Zoology at Oxord University.

© Mètode 2012