"Sexually Antagonistic" Bugs Evolve New Weapons

Bijal P. Trivedi
National Geographic Today
February 14, 2002

Forget Valentine's Day. Love is a battlefield. And in the bug world the battle of the sexes has led to an evolutionary arms race—and the weapons to fend off unwanted suitors are getting nasty.

As Locke Rowe, of the University of Toronto in Canada, points out, what is good for one sex is rarely good for the other.

Males want to spread their genes far and wide and mate with as many females as possible. Females would rather keep mating to the minimum.

Water striders—beetle-like creatures with long slender legs that walk on water—have developed biological weaponry to defend their gender-specific interests.

Male water striders have evolved grasping hooks that grip the female in place as he tries to mate with her. To foil the males, females have evolved spines to dislodge unwelcome lovers.

"Mating is a pretty risky thing for any species, but males have a lot to gain," said Rowe. "But females can store sperm and a lifetime's worth of eggs can be fertilized in a single mating session. She doesn't need to waste time on superfluous mating."

Males have everything to gain from mating. A male water strider can dislodge the sperm of the previous male and thus tries to mate with many females. Females, on the other hand, have a lot to lose since mating takes place on the water and leaves them vulnerable to predation.

The Insidious Black Swimmer

An insidious bug called a Back Swimmer swims upside-down just beneath the water and attacks striders from below. It snatches bugs on the water surface and drags them under and devours them. When water striders mate the female floats on the water surface with the male on her back. If a Back Swimmer passes underneath it is usually the female that gets nabbed.

The risk of contracting sexually transmitted diseases is an issue for both sexes.

Hence, the pre-mating "ritual" is a violent struggle between two armed parties.

Locke Rowe and colleague Goran Arnqvist, of the University of Uppsala in Sweden, demonstrated the evolutionary arms race in progress through a study of 15 species of water striders from the United States, Canada, and Europe.

Continued on Next Page >>


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