A Closer Look at Our Penises

            Compared to the other male primates, men have very different looking penises.  For starters, a man’s has a bulbous tip (the glans) that most other species lack.  Why is it there?  A better way to frame the question is: what evolutionary function did it serve?  Can answering this question give some sort of insight into human sexuality?

            Let’s try to address the issue of what purpose this mushroom shape serves by noting another important attribute of a man’s member: its size.  The male reproductive organ of a human is much larger than any of the other primates’.  Even the average 450-pound male gorilla has an erect length of only two inches, approximately three times smaller than the average man’s.  This comparative largeness coupled with the explosiveness of ejaculation (up to two feet) suggests that during sexual intercourse, the man is trying to reach the uppermost parts of the vagina.  This would not only maximize the coverage of fertilization grounds in the woman’s body but also fight off semen that other men may have left behind.  After all, most sperm cells are not for fertilizing the egg but for fighting off other sperm.  (Note that this would not have been necessary if women weren’t promiscuous to some degree; there’s not much we can do about that now although countless societies have harshly tried.)

            Now, we can get to the glans’ function: to scoop out whatever semen might be left in there.   Evolutionary psychologists have conducted experiments with artificial penises, vaginas, and semen that show the mechanical sexual (ergo evolutionary) advantage of having a glans as opposed to a straight shaft.  The fake penises (commonly known as dildos) with the glans scooped out over 90% of the “semen” in the “vaginas,” much more than the ones without one (less than 40%).

            How would the man make sure to not remove his own?  Well, all of this information suggests that the refractory period precisely remedies this possibility.  As soon as he ejaculates, the penis decreases in size and hardness and becomes very sensitive, making further stimulation uncomfortable.  This temporary impotence prevents him from undoing his doing.

            There are many fascinating insights into human behavior from combining evolutionary logic with human physiology.  Why are our brains so large, particularly our cerebral cortexes?  Why are men’s testes larger than gorillas’ but smaller than chimpanzees’, despite the fact that we smaller than the former but larger than the latter?  Why do most women have unnecessarily large breasts?  (The size is mostly made up by fat tissue that doesn’t provide any more space for milk production or storage.)  In the future, I will post more topics that will explore the study of evolutionary psychology, the science of why we behave the way we do by tracing back to human nature’s origins.

            For more information about this interesting aspect of nature, check out this great article from the Scientific American: http://www.scientificamerican.com/article.cfm?id=secrets-of-the-phallus