Love is truly a battlefield--no matter what your species. Thanks to natural selection, male and female animals are more interested in passing on their own genes than those of their mates, and as a result they end up in a pitched struggle. In 1996 researchers studying fruit flies discovered just how deadly a struggle it can be.
It’s in the interest of a female fruit fly to mate with as many males as necessary to get the highest-quality sperm; it’s in the male’s interest to cut down the competition for her eggs. To do so he loads up his semen with a cocktail of pheromones that alter the female’s reproductive patterns. Some of them make her ovulate more quickly, so that his sperm will have a better chance of fertilizing an egg; some suppress her sex drive, so that she won’t be inclined to mate again; and some help flush out other males’ sperm. As an inadvertent side-effect that makes no difference whatsoever to the brutish male, the chemicals are so toxic to the female that they often shorten her life span.
The female is not a hapless victim, however; she has various chemical defenses against the sneaky semen. Indeed, researchers have long thought that this kind of sexual competition should take the form of an evolutionary arms race, with males and females continually improving their weaponry. But it was only this past year that flesh was put on the theory by William Rice, an evolutionary biologist from the University of California at Santa Cruz who prefers a different metaphor. You can think of this as an evolutionary dance between males and females--an antagonistic dance, says Rice. When the two dancers are moving, one in response to the other, their relative positions don’t change much--they’re moving in unison. But if you could reach out and hold one of them still, just for a moment, you’d be able to see the motion they have between them.
Rice figured out a way to let male flies evolve while holding the females still. First he manipulated the flies’ genes so that their only offspring were males, and those males’ genes came only from the father. Then he drew a fresh supply of virgin females from a separate population of flies for each new generation of males to mate with. Each batch of females was thus completely unfamiliar with the chemical warfare of Rice’s males, but the males could spend generation after generation perfecting their attack.
Forty-one generations later, Rice had created what he calls supermales. These males not only mated more often and more successfully with the females but their seminal fluid became much more poisonous. The females they mated with died sooner.
What does this say about the war-dance of love in, say, humans? No one knows--but Rice points out that male mammals carry female hormones in their semen. Now, that makes you suspicious that the males are influencing females’ reproductive behavior and physiology, he says, because they’re giving them hormones that we know can have that effect on them.