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A biologist at Washington University in St. Louis has shown that for some fish species, females prefer males with larger sexual organs, and actually choose them for mating. That does not exclude males with an average-sized sex organ, called a gonopodium. These fish out-compete the larger-endowed males in a predator-laden environment because they have a faster burst speed than the males with larger genitalia, who lose out because the size of their organ slows them down, making them ripe for capture by larger fish.

Brian Langerhans, Washington University biology graduate student in Arts & Sciences, has performed studies on mosquitofish (guppy-like fish, about an inch long) and found that female mosquitofish spend 80 percent more time with males who have a large gonopodium.

"A male with a larger gonopodium has a higher chance of mating, but in a predator environment he has a higher probability of dying," Langerhans said. "That's the cost, the tradeoff. On the other hand, we found that in predator-free environments gonopodia size was larger, as there is minimal cost for large genitalia in that environment. Bigger is better for mating, but smaller is better for avoiding predation."

Langerhans and colleagues reported their findings in the May 9 on-line issue of Proceedings of the National Academy of Sciences.

Langerhans specializes in the study of ecological factors that shape the evolution of body forms. Male genitalia are more variable than just about any other body form studied, and there is a significant cadre of evolutionary biologists studying this field because genital shape ?morphology ?is one of the chief characters that taxonomists use to distinguish between closely related species.

Striking diversity

Researchers have thought for about 20 years that the striking diversity of genital form results from post-mating sexual selection, such as sperm competition or cryptic female choice. That is, most evolutionists hav
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