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"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.

Rowe and Goran found that this arms race—technically known as "sexually antagonistic co-evolution"—is difficult to observe in nature because males and females are continuously evolving weapons and counter weapons, and thus the race always appears at a standstill. Also, in most cases scientists are not aware of the different weaponry used by each species.

Each of the 15 species of water strider the team examined was at different stage of the arms race. Some have developed heavy artillery to keep the other sex at bay, whereas others were barely armed at all. But in most cases the male and female adaptations were balanced.

The researchers found that in the case of water striders, when a female evolves a better weapon and gains the upper hand, the mating rate tends to fall. If the male evolves a better weapon—a stronger grasping hook, for example—the mating rate skyrockets.

Rowe and Arnqvist's study is the first direct evidence for "sexually antagonistic co-evolution," which has long been predicted but never proven.

The study is published in the February 14 issue of the British journal Nature.

Prior to this study scientists suggested that the female water striders develop weaponry because they want to choose the male with the best genetic traits. But Rowe and Goran's study found that "the females simply do not want to mate," said Rowe.

When a female water strider has mated, she rejects all subsequent males, both high and low quality. "When she hasn't mated, she accepts the duds at the same rate she accepts the studs," said Rowe.

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