Evolution 101

Sunday, July 30, 2006

Why Do Men Have Nipples?

Let’s start off with some listener questions.

From Elias: We humans are different since we have control over our environment not much change occurs, so survival of the fittest is no longer true. At the beginning of life was it just a big mess of a puzzle that fell into place over time? For example were there animals who had no reaction to predators hence they died off since they didn’t react to danger.


What I believe he’s trying to ask here is a question about the mechanism of natural selection. Does natural selection mean that you start off with a large group of random organisms, and then end up with only the few that are well-adapted to the environment? Well, sort of, but not exactly. This would make sense if one assumed that the environment was static- it never changed. But the environment is always changing, and so populations that are well-adapted today may be incredibly poorly adapted to it fifty years from now. A predator species that restricts the adaption of another species may go extinct, and allow the prey species to evolve into something different. Or predator species may be introduced to new species, which are not adapted at all to them. This was the situation with the dodo bird, which had adapted to easy island life by growing large and flightless. But when humans arrived, they were easy pickings for food from both humans and the other animals that were introduced by humans to their environment. Survival of the fittest still happens with humans, of course, and even though we do have incredible control over our environment compared to other species, we are not omnipotent.

From Richard: hey, just curious why you didn't mention the arguments against carbon dating in the podcast 122 - what is the evidence against evolution. I have heard this argument from many creationists.


I actually had a separate podcast on radiometric dating methods- the episode just prior to that, 121, titled “How Are Fossils Dated?” But yes, very often evolution deniers will criticize radiometric dating methods as a way of criticizing evolutionary theory. Usually, however, they don’t know what they’re talking about, and one easy way to tell is if they talk about “carbon dating” and “fossils” in the same breath. Radiocarbon dating is really not useful for most fossils, since it can’t be effectively used for artifacts older than about 60,000 years. What evolution deniers are trying to do with this kind of attack is, not to address evolutionary theory on its own merits, or even to question the principles of radiometric dating, but just to try to inject as much uncertainty into the evidence from radiometric dating as possible. Usually they try to make the claim that radiometric dates are made in such a wide range that they can’t possibly be used accurately, or that they have been used in the past to make blatant mistakes. I’ve run into the bulk of these claims made by Kent Hovind and a lot of them are just outright lies or misrepresentations of the actual dating methodology. He makes a claim that living snails were tested by radiocarbon dating and shown to be 2300 years old. What he doesn’t say is that a lot of the carbon in the snails’ shells was derived from dissolved limestone, which was very old, and screwed up the ratio of radioactive to nonradioactive carbon in the water.

From Lee: I just heard and enjoyed your interview on the biota.org podcast, and I thought that you and/or your audience might be interested in an OpEd that I published on evolutionary computation and intelligent design in the Boston Globe last year (August 29, 2005). It's a short, easy read and I've received a lot of feedback from non-scientists saying that the case that it makes is particularly compelling to them because it requires no familiarity with molecular biology, or with the fossil record, or with other areas of science. It is available directly from the Globe here. Alternatively, I have a local copy here.


Thanks, Lee, I had a great time on the Biota.org podcast, and I understand from Tom that it was very well received on his end as well. Digital evolution tools are already and will continue to be great tools both for discovery and education, so anyone who’s interested in that subject can go check out your op-ed.

From Mike: My question is about mammals. How did mammals evolve from Reptiles? Are there transitional fossils of egg laying / lactating species? Reptiles with nipples? I’m sure such a leap had to take thousands if generations. Have any such fossils ever been found to your knowledge?


The short answer is, mammals are distinguished from reptiles partially because of their ability to lactate. So, no, simply by definition, there are no reptiles with nipples, at least no extant species. By looking at the mammal clade as it exists today, it would seem to indicate that the whole nipple-mammary gland structure as it generally understood among mammals today wouldn’t have existed at all in reptiles, although it’s not known precisely when this trait evolved in mammal evolution. Let me just go through the basic steps of mammalian evolution quickly. Mammals and reptiles are both part of the larger clade, the amniotes. Of these, there is a further division into the reptilians, which would include turtles, lizards, snakes, and birds. Yes, that’s right- birds are cladistically reptiles, even though they used to be thought of as similar to mammals, since they’re warm blooded. But the bird singing on your fence is more closely related to the lizard sunning itself on a rock in your garden than to the cat watching both of them at your window. The other division in the amniote clade is the synapsids, which as a group is also very reptilian, but are different from all the other reptiles we know today. The characteristic that made these animals different is primarily the presence of an opening in their skulls just behind their eye. This may seem like a trivial distinction, but it allowed for stronger jaw muscles to develop, and much of an organism’s evolutionary fitness is tied directly to how well it’s able to eat. What makes this especially hard to conceptualize, is that of most of the synapsids, we would recognize nearly all of them as indistinguishable from reptiles, but we would be more closely related to them than we would be to a snapping turtle. However, among this group of synapsid reptiles, a subgroup evolved which we call the therapsids, and it is this group which eventually gave rise to modern mammals. The therapsids are often referred to as “mammal-like” reptiles. It’s within this group that warm-bloodedness evolved, the bones of the reptilian jaw moved into the mammalian ear, scales were lost, and fur eventually began to grow on some of the later therapsids. Eventually, an even further subgroup of the therapsids, the cynodonts, which were small, shrew-like organisms. Finally, from these small organisms, have evolved all living mammals, from whales to elephants to lions and tigers and bears, and humans. So yes, mammals did evolve from reptiles, and we can still see some hints of that in living mammals today- the monotremes. These include the duck-billed platypus and the echidna, both of which lay eggs instead of giving birth to live young, which the other two mammal groups, the marsupials and the eutherians, do today.

Interestingly, and to get back to the nipple question, monotremes do provide milk for their young, but they don’t have nipples. Instead, the milk is secreted sort of non-specifically from their skin, and their young lap it up off of their fur. So nipples as a physiological structure would seem to be a relatively recent evolutionary development in the history of mammals, and not even one that is necessary to be designated as a mammal.

But this brings me to another interesting nipple topic, and one that is commonly brought up in evolutionary discussions- why do men have nipples? It seems pretty obvious that for females, these structures are of obvious utility, both to mother and child. But wouldn’t it be just as useful for the child to have access to milk from both parents? In many species, food is provided for offspring by both male and female parents, to the obvious advantage of the young. This is a common behavior in birds, for example. Why, then, are male mammals so selfish? Why is there no such thing as the milk of male kindness?

As above, I’ll give a short answer and a long answer. The short answer is, men don’t nurse because we don’t have to. It’s not in our interest, evolutionarily speaking. The long answer is there is absolutely no physiological reason why men couldn’t nurse babies- all the equipment is there. The development of mammary glands in embryonic development happens independently of sex- for all intents and purposes, these glands remain indistinguishable between the sexes until puberty, during which exposure to hormones from the ovaries, adrenal glands, and pituitary gland. Pregnancy further enhances the development of these glands, especially the hormone prolactin. But exposure to the same kinds of hormones at the same times would give the same results for men, or, more specifically, removing the masculinizing hormones like testosterone. Mutations of the testosterone pathway can cause males to be born without recognizable masculine genitalia, and they develop usually into extremely beautiful “women”, with long legs and full breasts, simply because they’ve had their masculinization progress impaired. Genetically and genitally they’re still male- they have internal testes, not ovaries, but they sport breasts that could grace a magazine pictorial.

But, even without the full-on breast development that is caused by hormones, men can still lactate. Even human women who have never gone through pregnancy can induce lactation by mechanical manipulation of their nipples, since the nerve stimulation causes expression of the same hormones that induce lactation after pregnancy. These same hormones can be induced in males, making it the case that, if a man really wanted to breastfeed his child, all he’d really have to do is… well, practice.

So, we can see clearly that there’s no big physiological barrier standing between men and the ability to lactate. So what’s the reason why they don’t, especially because they do have nipples, after all? The reason is not physiological, but evolutionary. In the vast majority of mammalian species, offspring are born and raised solely by the female- the male plays no parental role at all. His evolutionary interests are best served by having children with as many females as possible, and so he doesn’t gain any advantage to sticking around to help raise one or two of them. According to the rules of evolutionary selection, whichever organisms pass on the most copies of their DNA are the most successful. I hesitate to use the word “rules,” because that makes it seem as if evolution was a game being played by all creatures, but that’s sort of how it works out.

But what about those species for whom males do play some role in the parenting of their offspring? This would include humans, of course. Well, we see that the male contributions in these species include things like bringing back food for the female, chasing off potential competitors within its species, and looking out for predators from other species. These alternatives to lactation are well-adapted traits for the species in which they’re found, and offset any benefit which male lactation would provide evolutionarily. But this doesn’t preclude the existence of any species for which male lactation would be an evolutionary benefit, and in fact one has been recently discovered, the Dyak fruit bat found in Malaysia. Males of this species were discovered with functioning mammary glands, both full and drained, indicating that they were providing milk to their young. This species hasn’t been studied enough to determine why male lactation was advantageous for development, but as you should know already, the physiological requirements would already have been in place. As bats are closely related to humans (just outside of the primates), this seems to suggest that male lactation in humans is only a few environmental variables away from becoming commonplace. Certainly it’s not outside of the range of personal choice, although cultural pressures may discourage this behavior.

So, to review, males have nipples because the development of mammary glands is independent of sexual development, at least until puberty. And the reason why men don’t use those nipples is because, at least so far, it hasn’t been evolutionarily advantageous for us to do so, although physiologically, we have essentially the same equipment as females and can produce a comparable product.

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