Do she-monkeys have orgasms? Why does the spider Harpactea sadistica wield its two penises to sew a tidy zigzag pattern in his mate’s belly? Do crane flies really use vibrators? And how often do women get pregnant from sperm cells aimlessly traversing their abdominal organs? These and many other titillating questions about the organs that humans and other animals use for sex are being answered in the up-and-coming field of biology that investigates genital evolution.
We humans find genitals endlessly fascinating—our own, but by extension those of other animals as well. Millennia of bathroom graffiti attest to that. Over the past 25 years, scientists all over the world have channeled this innate obsession into a new field of research that is exciting in more than one way. For it is nature’s nether regions that evolution has picked out for her greatest feats. Private parts, be they of ducks, damselflies or dung beetles, turn out to have evolved novel forms at breakneck speeds.
This explains how arrays of, say, insects that seem identical on the outside, have penises that often look like a page from a mail-order catalog of outlandish kitchen appliances. But the question is, why would this be so? How come organs with such mundane functions as injecting and absorbing a droplet of sperm have evolved all those seemingly useless ornamentations? Two schools of thought are at loggerheads over this. Are penises perhaps “internal courtship devices,” tactile versions of the peacock’s tail? Or are they used in an evolutionary battle of the sexes over who has the last say in fertilization?
Smithsonian Institution scientist Bill Eberhard, founding father of the whole field, favors the internal courtship idea. In a long series of publications over the past decades he has piled up evidence that it is the males who wield the outlandish genitals, and it is the females who employ their reproductive organs to either store and use, or dump and destroy their sperm. For example, he and his colleagues discovered that the better male spiders are able to make sexy prying movements with their penises (yes, they have two) during mating, the less likely it is that the female, after the male has said goodbye, will resolutely eject his sperm. And, in another experiment, Eberhard showed that female beetles were less keen to have their larvae fathered by males of whom he had snipped off a particular whip-like adornment of the penis. The same kind of internal sperm selection may even go on in humans, too. Women who orgasm within a few minutes of the man’s climax absorb much more of his semen that those who don’t.
But the battle of the sexes school has a different take on the matter. They say that the other camp is misled by a woeful lack of knowledge of female genitals. Where Eberhard’s team claim that genitals are like bird plumage (diverse and bizarre in the displaying males, mundane and similar in the choosy females), their opponents state that their careful dissections of vaginas throughout the animal kingdom show that the same smorgasbord exists on the feminine side as well. And that what we see is an arms race, rather than gently genital persuasion.
Take bedbugs for example. Copulation in these insects is “brutal in every sense of the word,” says British entomologist Mike Siva-Jothy, who studies these critters: A male simply impales a female with his dagger-like penis and injects his sperm directly into her blood, where the sperm cells wiggle their way through her organs to land eventually in her ovaries and fertilize her eggs. Not to be outdone, however, the females of some species have evolved a second vagina (in addition to her original one, which the male ignores), in the spot where the male normally likes to sink his penis into her (midway along the right-hand flank of her belly), and thus manage to gain the upper hand by reducing the damage and intercepting sperm before they begin their cross-country hike through the female body.
So which is it? Genital courtship or all-out four-letter-word war? Fittingly, some rapprochement seems to be taking place between both schools of thought. For many behaviors that begin as sexual aggression seem to evolve into a more ritualized, courtshippy version of their former brutal selves. Perhaps, says Eberhard, even in species where battles of the sexes may seem to be fought, female discretion is key. Who would have thought that penis and vagina dialogues could be so Shakespearean?
Menno Schilthuizen is an evolutionary biologist and science writer at Naturalis Biodiversity Center and Leiden University in The Netherlands. This article is excerpted from his new book Nature’s Nether Regions in which he explains what the sex lives of bugs, birds, and beasts tell us about evolution, biodiversity, and ourselves.
This excerpt is published with the permission of the author and the publisher, Viking Press.