Despite our fascination with our own sexuality, we tend to take the shape and workings of our reproductive organs for granted. We also tend to consider them uniquely human. But as an evolutionary biologist, I take little for granted and see parallels in the animal world.
This is no surprise. In almost all animals there is a conflict of interest whenever a female mates with a male. For her, it's usually best to keep her options open and select the best fathers for her offspring from among the available suitors. For him, on the other hand, it would be best if a female were to use his sperm to fertilize all her eggs. So evolution favors different things on either side of the sexual divide--in humans as much as in other creatures great and small.
What does this mean? Two things. First, that genitals evolve at lightning speed. It's a law of nature that an animal's nether regions are where its most typical distinguishing features lie. Pick up any insect or flatworm identification manual, and you'll find hundreds of species that look near-identical on the outside, but have wildly differently shaped genitals, both male and female. Second, that genitals are not just mundane, utilitarian sperm injectors and receptacles. Rather, they are the stage where evolution's greatest plays are set.
In my new book Nature's Nether Regions [Viking, $28.95], I explore what the study of animal sex can teach us about our own. And here's what I found:
1. Antidepressant semen and chemical warfare
You may think semen is just a kind of watery slush to cushion sperm cells on their way to eggs. But the liquid part of semen in humans and animals is actually a noxious mix of hundreds of manipulative substances. In fruit flies, some proteins in the semen slip into the female's blood stream and latch onto receptors in her brain, turning off her sex drive and making her give the cold shoulder to other males for as long as needed for the female to fertilize and lay her eggs. In other words, semen wages a kind of chemical warfare, evolved by male fruit flies to counter any promiscuous inclinations of the female (which would not benefit his chances on fatherhood). And not just in fruit flies. Manipulative proteins have been found in lots of animals. In humans too? Well, we don't know for sure, but there are some indications that semen plays tricks on a woman's physiology, too. Medical research found that exposure to semen protects pregnant women against preeclampsia, a condition in which the female's body launches an allergic reaction to her fetus. Possibly, proteins in a man's semen take control of the woman's immune system to protect the fruit of his loins.
2. Female orgasm and the spotted cucumber beetle
Mating spotted cucumber beetles are like clowns in love. In their oversized yellow coats with the large black polka-dots, they are the sweetest things. The male, perched on top of the female, his penis inside her, strokes her rhythmically with his antennae. But the female prevents him from penetrating her inner sanctum deep enough to fertilize her eggs, by keeping her vaginal muscles taut. Only when she decides that the male has stroked long and fast enough will she relax and, simultaneously, will he ejaculate. Sounds familiar? Well, research by two English scientists has shown that the female orgasm may indeed accomplish the same in humans. They conducted an experiment involving condoms, colleagues, and questionnaires, and found out that if a woman climaxes during or immediately after the man, she sucks a lot of his sperm into her uterus. If she does not have an orgasm, or if the orgasm comes before the man, then after intercourse, most of his semen is dumped from the vagina. It seems that human females may make the same choices as cucumber beetles when it comes to allowing or disallowing a male to fertilize her eggs.
3. Traumatic insemination and non-communicating uterine horns
In bed and other bugs, a gruesome practice called "traumatic insemination" is commonplace. In these animals, the male ignores the female's vagina, and injects his sperm cells directly into the female's body by sticking a dagger-like penis into her skin. The sperm then squeeze right through her internal organs, reaching her ovaries from within. This may seem outlandish, but human sperm cells can do the same--albeit accidentally. Women with the congenital condition of "non-communicating uterine horns," in which one of the ovaries is inaccessible from the vagina, still often get pregnant in that locked-off part of their uteri--meaning that sperm cells must travel up her vagina, up the one oviduct that can be reached, out the far end, into her belly and into the other oviduct by traveling cross-country through her internal organs.
Finding common threads that tie our reproductive features with those of the rest of the living world can makes us better understand our place in the fabric of life. But for that we are going to need evolutionary biologists to pay more attention to human sex. With modern scientific methods and techniques, a latter-day Alfred Kinsey could do wonders. It is time for biologists, psychologists, and medical scientists to unite and figure out what the birds and the bees can really tell us.