"Sexually Antagonistic" Bugs Evolve New Weapons

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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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