Chemical Warfare of the Sexes

The struggle for sexual success is a continually shifting battle, one that carries on with every generation. It’s hard to catch the struggle in action, but thanks to some brilliant experiments, scientists have been able to see a few glimpses. William Rice, a biologist at the University of California at Santa Barbara, has studied the chemical warfare that male fruit flies use to help their sperm compete against other sperm.

Not only does a male fruit fly’s semen disable the sperm of other males and wipe out a female’s libido, it even speeds up her egg‑laying schedule. By making her lay her eggs sooner after mating, he reduces the time in which she might mate with other males. The chemicals that male fruit flies use to alter their mates are poisonous to the female. They don’t kill her immediately, but the more often she mates, the shorter her life span becomes. It doesn’t matter to the male fruit fly that his mate dies young. Since male fruit flies don’t take care of their offspring, his only evolutionary interest is to produce more fertilized eggs.

The arsenal that male fruit flies use has the same effect on females as the pesticides farmers use to kill them: it makes them evolve. Just as pesticides trigger the evolution of resistance, female fruit flies have evolved ways to neutralize the poisons in semen. And the evolution of female defenses has encouraged the semen of the male flies to become more toxic.

In 1996 Rice was able to document this deadly pas de deux. He maintained a large breeding population of fruit flies, and thanks to some peculiarities in fruit fly genetics, he was able to manipulate the flies so that their descendants were all males and only inherited the genes of their fathers. These new male clones then mated with a fresh supply of females from Rice’s breeding population, producing the next generation of males.

Each new batch of females was unfamiliar with the chemical warfare of Rice’s males, so they had no opportunity to evolve defensive adaptations. Meanwhile, the males that produced more toxic semen could manipulate the females more effectively and father more flies. Forty‑one generations later, Rice had created a race of supermales who mated more often and more successfully with females than their ancestors. Their success cost the females dearly: as the males’ semen became more poisonous, their mates died at a much younger age.

Rice found still more evidence for this sexual arms race by forcing the flies to declare a cease‑fire. In 1999 he ran an experiment in which he paired off males and females into monogamous couples. Instead of competing with other males, these males could mate only with the partner Rice gave them. When their eggs hatched, Rice again sorted the new flies into monogamous pairs. Since the monogamous males faced no competition, the poisonous chemicals in their semen no longer offered any evolutionary benefit. And once the males abandoned their poisons, the females had no incentive to evolve antidotes to them. After 47 generations, Rice found that the monogamous males had become significantly less harmful to their mates and the females were less resistant to toxins in semen.

Rice’s fruit flies were able to enjoy a more tranquil life, but only because he forced them into it. On their own, they couldn’t have found their way to a truce. Any male fly that can evict another male’s sperm will spread his genes more successfully than a one‑mate male. And any female who can defend herself will be favored as well. Evolution does not have the foresight of a biologist in a lab, so love among the fruit flies is truly blind.