Battle of the Sexes, Water-Strider Style

Not tonight, honey. When it comes to mating, the dominant sex sets the pace.

As many lovers celebrating Valentine's Day can tell you, mating doesn't come free, and all participants want what's best for their genes. But the costs of this "battle of the sexes" have been impossible to document in real life. Now, by examining the body armament of insects, researchers have shown how males evolve new weapons in their attempts to mate with females, while the females counterevolve ways to avoid too much love.

In nonmonogamous species, males want to spread their genes as often and as widely as possible. Females, on the other hand, want to choose the best mate. But once a female has her offspring, mating (in some species, anyway) becomes superfluous. These opposing interests ought to create conflict between the sexes, which should appear as an escalating arms race: adaptations in one sex followed by counteradaptations in the other. Researchers have seen evidence for this in the lab, where male fruit flies have evolved devious tricks, such as compounds in their semen that made females less sexually attractive to other males (ScienceNOW, 22 June 1998).

But how well does what happens in the lab reflect what goes on in nature? Evolutionary biologist Göran Arnqvist of the University of Uppsala, Sweden, and Locke Rowe of the University of Toronto answered this question by examining 15 species of water striders. The researchers compared the bugs' armament--such as a male's ability to grasp a female, or a female's ability to wriggle away from a male--with the number of times the bugs mated. The armament carried by each sex in each species predicted which sex's mating preferences dominated, they report in the 14 February issue of Nature. Species in which the males had flat stomachs or strong forelegs--the better to grasp the gals with--mated more frequently than species in which the females had an advantage, such as spines sticking out of their backends. Mating rates ranged from once every 2 days when the females were in charge, to 20 times a day, when the males had their way.

Evolutionary geneticist William Rice of the University of California, Santa Barbara, says this study exposes nature's evolutionary battle of the sexes. He says there's no reason the findings wouldn't apply to other animals: "No one species tells us what happens in general in nature. But this is all the more convincing." It turns out, lovers, that Cupid isn't the only one carrying a weapon.

Related sites
Göran Arnqvist's Web site
Uppsala University's Evolutionary Biology Centre
Uppsala University's Department of Animal Ecology