10.9 What is the evidence for sexual selection in humans?
Evolution by sexual selection has been invoked to explain a number of human anatomical features, which appear useless or detrimental to human survival. These include hairlessness, male facial hair, rounded breasts, pubic hair, and penis size. In the next few pages we will talk about some of the more interesting examples of traits that some scientists have argued are products of sexual selection: primary sexual traits (e.g. male penis size), female orgasms, large brains, our extensive vocabularies, and many aspects of our cultural behaviors.
Why do human men have nipples?
Early evolutionary biologists were keen to observe that humans, especially compared to nearly all other mammals, were much less hairy. Many hypothesized that because human females were much less hairy than their male counterparts, the loss of hair was due to selection by pre-historic human males for less hairy female partners. But if this were the case, and men were selecting for less hairy women, than why would hairlessness be present in human males, and not just human females? The answer will show up often as we discuss evolution- by-products of similar biological pathways in male and female animals can give rise to traits that are only selected for in one sex, but will often show up in the other sex. An interesting example of this is the presence of nipples in mammals. In most mammals, including human females, the nipple is important for transporting milk from inside to outside of the body. Given that human women are solely responsible for breast feeding, why do male animals, like human men, have nipples? Maybe this is not a question you have ever thought about; after all, it would be quite startling to suddenly see a handful of animals, including humans, without nipples. But evolutionarily, this question is quite interesting and the answer will come up a lot in our discussions of human evolution, so it’s worth spending some time on it.
Have you ever noticed most animals have a similar body plan? For example, most animals have a set of eyes somewhere on their head, the head rests atop a thorax or torso, and legs protrude downward from the thorax. While aberrations from this body plan can happen, often as the result of mutation, most animals have this body plan. The reason is that all animals, in fact, all living things, evolved from a common ancestor. As such, the developmental pathways that give rise to the traits we see in animals are highly conserved. This means that across a wide variety of animal species, pathways sharing a common function, also share a common origin in the biological past. Evolution typically takes the path of least resistance and which organisms may not be completely optimized, we are often just “good enough” to survive and reproduce.
This same logic applies to the case of nipples in humans. The developmental pathway from embryo to fetus is highly conserved, with little variation in the early stages of embryonic development. Human fetuses do not develop sex-specific characteristics, like a penis or vagina, until certain genes are turned-on around week 7 of development. Because nipples are not a trait that is determined by male or female-specific genes, all human fetuses will develop them. While they do not serve the function of nourishing offspring in males, they also do not affect survival and reproduction. Because they are not costly, nipples persist in men as it is not an evolutionary priority to get rid of them, and a re-working of the entire embryonic developmental plan would be extremely difficult.
Fun Fact: Why do humans have two nipples?
A good rule of thumb for mammals is to have twice as many nipples as offspring that you produce at one time. A female cow typically has two offspring at a time, and has four nipples; a small dog has eight nipples and a large dog has ten nipples because typical litter size is four to five pups at once. Thus, human females, who typically gestate one fetus at a time, have two nipples.
Large penises and breasts
Human penises are quite unique! Despite common slang terms that imply otherwise (e.g., “boner”), the human penis contains no bones. Unlike most of our closest evolutionary relatives, like chimpanzees and bonobos, human males do not have a penis bone, or baculum. Instead, human males must maintain an erection by pumping blood into the penis. Evolutionary biologists have speculated that the loss of the baculum in humans may be due to sexual selection by human females. Because the human erection relies on a type of hydraulic pumping system, erection failure can be an early warning of certain health conditions. Thus, human females are able to use the male erection as a clear sign of good health in potential partners.
Interestingly, the human penis is also much thicker than the penises of other great apes. Some have suggested that the evolution of the human penis towards a large size, both in length and diameter, has been the result of sperm competition. However, sperm competition typically favors larger testicles, not larger penises. Others have suggested that it is the result of mate competition, because a larger penis will be more efficient in displacing sperm from rival males during sexual intercourse. Support for this idea is limited. In fact, one study found that the amount of semen displacement during sexual intercourse was related not to penis size, but to the depth of pelvic thrusting. However, these researchers also stated that a longer penis would be more capable of leaving semen in less-accessible parts of the vagina, making it more difficult for subsequent males to remove or displace the semen.
Similarly, human females have much larger breasts than other primates. Because the additional fatty tissue in human breasts does not contribute to milk production, many think breast size evolved as a courtship signal. Many scientists think that large, round breasts and larger penises may once have served as signals of health and fertility, but many of these traits are now the product of “runaway selection”- a positive-feedback loop in which strong mate choice leads to the further exaggeration of a sexual trait.