DOWN THERE (Part 1): Making Sense of Amazing Genitalia

DOWN THERE (PART 4): Sex As War By Other Means?

A bedbug’s instrument of traumatic insemination (Photo: © Power & Syred, reprinted by permission)

 On the opposite side from Eberhard, Swedish biologist Göran Arnqvist says it’s actually about conflict and coercion: Male and female interests often clash.  She may do better, for instance, by mating with lots of different males, for greater genetic variety in her offspring.  He does better, on the other hand, if he can get her to produce only his offspring, even if it means reducing her total reproductive output or, in some species, dramatically shortening her life.  So male genitalia change to exploit the female, and female genitalia change to resist, in an endless co-evolutionary arms race.  He evolves claspers to seize hold of her.  She evolves spines she can lift up to push him away.  Then he evolves to hang onto her spines for better traction during sex. 

It’s a world in which genitalia come equipped with snippers, levers, syringes, and even a sort of built-in “date rape drug.” In their recent book Sexual Conflict, Arnqvist and co-author Locke Rowe cite the case of a funnel-web spider from the American Southwest.  The male sneaks up on the female and sprays her with a chemical, possibly a component of his semen.  She collapses into a trance and he proceeds to have sex with her. 

Why be such a brute?  The female in this species is not only bigger than the male, she’s also waiting in her web for dinner to show up.  So without the drug, the male stands a high risk of being accidentally cannibalized.  It’s hardly a tender little romance, on either side.

The sex-as-conflict camp often focuses on genitalia that seem to be built for something other than pleasure.  In some blowflies, for instance, the male has sharp saw-like edges on his penis for piercing the lining of the female reproductive tract.  This allows substances in his semen to find their way to her brain, where they make her less interested in mating with other males.  Other species resort to damage alone to sideline the female (that’s one possible explanation for the penile claws of woolly lemurs and some other primates).  Some males even damage themselves.  At the peak moment of honeybee sex, for instance, the male’s genitalia rip off with an audible crack (“Uh-oh”) and remain inside the female as a plug to keep out other males.

When Cole Porter wrote his song about how birds do it and bees do it, his view of sex wasn’t entirely romantic either. His Cape Cod clams did it against their wishes, for instance, and his electric eels did it despite the shock.  Even so, the refrain, “Let’s fall in love,” reflected the traditional belief that male and female were ultimately in this together. 

We live in harsher times now.  Instead of Fred Astaire and Ginger Rogers sweeping across the ballroom, tender and graceful in each other’s arms, sex sometimes seems like a tango of the selfish genes. Male and female tug and twist against each other, each seeking to exploit the other for maximum advantage in the Darwinian struggle. 

So where do humans fit into the strange evolving picture of genitalia in the natural world? In some ways, it may seem that we don’t.  Human genitalia are relatively simple, compared to some other species. (And suddenly that sounds just fine, doesn’t it?)  On the scale of sexual behaviors from total promiscuity to strict monogamy, humans turn up somewhere in the middle.  Women don’t generally mate with three or four men in rapid succession.  So men haven’t faced the kind of intense competitive pressure that might have caused us to sprout feather dusters or french ticklers.  We also have agile fingers and lips, making penile gadgetry perhaps a little less mandatory. 

It’s possible, in addition, that male and female genitalia have adapted to each other in subtler ways we simply don’t recognize in the heat of the moment. Human semen, for instance, is a rapidly evolving chemical soup of proteins known to affect a woman’s hormonal and immunoregulatory functions.  Many of these proteins can pass through the vaginal wall into a woman’s bloodstream, with unknown effect.  Some seminal proteins seem to be geared for cooperation, helping to synchronize reproductive events.  Other reproductive chemicals are clearly geared for conflict.  She sends out antibodies against his sperm.  He floods her with the zinc in his semen to suppress her immune response.  Both sexes send out chemicals that chop up rival proteins.  “Could it be,” Arnqvist and Rowe ask, “that male and female humans are involved in chemical warfare?”