Saturday, July 7, 2012

Gender 3: Male Nipples and Human Evolution

Note: This program first aired July 7, 2012.

I was asked recently, why men have nipples. As we are currently working our way through the biological issues of gender, I thought this might be a good side track because it takes us from the somewhat silly idea of men’s nipples, to the much deeper and fundamental concepts of evolution and human development.
My initial guess was that men have nipples because nipples are an evolutionary orphan—left over from a time when perhaps both sexes of mammals nursed young.   Another idea was that maybe nipples had some other function in early mammals; regardless, I thought men had them because they once needed them and now don’t, and, like our appendix, they haven’t fully evolved away.
It turns out that, to the best of our knowledge, a small part of my guess was right (which means that large part of it was wrong as well). The part I was right about was the easy part. If an individual has a trait that has a negative impact on its ability to reproduce, that individual’s genes (and the traits they encode for) will disappear from the population. This is the mechanism of natural selection. However, if an individual displays a trait that is neutral to its ability to reproduce, there is no selective pressure that will make that trait disappear.  The bottom line is that once a trait appears in a population, if it isn’t doing any harm, it isn’t likely to disappear. This is the argument for why men still have nipples. While it is possible for men to get breast cancer (not only do they have nipples but mammary tissue as well), it is rare and not affecting the overall reproductive rate of the human population. So, while they are arguably useless, they are likely here to stay.
But this doesn’t really address the primary question, why do they have them in the first place? My thought, that they have them because they once needed them, seems to be all wrong. Almost all male mammals have nipples, and there is no evidence in the geologic record or in current species, that males ever used them for nursing. Mammary tissue originated pre mammal—in a group called synapsids ( first seen about 310 mya—small lizard like animals). They didn’t have nipples per say, but simply nourished their eggs and eventually young with secretions that came out of their skin, through structures related to hair follicles (platypus and other monotremes still lactate this way, picture milk oozing out of the skin of your chest and your infant licking it off you). There is some debate about origin of this tissue, did it arise from sweat glands? sebaceous glands?  or some other related cutaneous gland? Did the secretions nourish live young or as some hypothesize, get absorbed directly through the thin shells of the eggs of these early animals? It is not clear, as this tissue does not fossilize well. But it is clear that lactation predates nipples.
So it seems that the real reason male mammals have nipples is due to the realities of our very early development as embryos. It is commonly said that all human embryos start out “female”; this is an oversimplification that has the ring of urban legend to me, but there is a bit of truth to it. Typically, and in most mammals, individual who carry the XX genotype for their sex chromosomes develop into females, individuals who carry the XY genotype develop into males. All the traits and structures that we think of as human are on the 22 other pairs of chromosomes, including the coding for nipples and mammary tissue. The only difference between males and females is the presence of that Y sex chromosome, and amazingingly, the only thing it does is tell certain cells to make testosterone several weeks into the development of an embryo. That testosterone then tells the sex organ cells to become testes and male genetalia instead of ovaries and female genetalia. Without that testosterone cue, an embryo with an XY genotype will develop in the default mode, as a human who’s development is governed more by estrogen than testosterone, someone we would identify as female. 
Another thing that testosterone pulse does is suppress the development of the mammary tissue (though in humans, not completely). In a few species of mammal, (rats, horses) the testosterone completely suppresses the development of this tissue, and as a result, males of those species have no nipples at all. Again though, nipples on human males don’t seem to cause many problems, so we haven’t evolved such radical measures. In fact, males who experience non typical hormonal patterns or who experience the lower levels of testosterone common in older age can develop more mammary tissue, breasts and even in very rare cases, can lactate.
To recap: Mammals evolved nipples because nipples worked better than sweating milk for feeding young. Men have them because they are in the generic blue print for human being, and there has been no selective pressure to fully get rid of them. And the more we know about the delicate nature of the early development of the human embryo, the more clear it becomes that both native hormones and pollutants that mimic estrogen and testosterone in the body can have a profound effect not only on our health, but on our very identity.

Interesting thoughts from Cecil Adams, the “worlds’ smartest humanbeing” In fact, there are many people blogging about this issue. If you Google “Why do men have nipples?”, you will primarily get blogs as top hits.
“The Mammary Gland and Its Origin During Synapsid Evolution” Olav T. Oftedal,  Journal of Mammary Gland Biology and Neoplasia Vol. 7, No. 3 July 2002