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.
Langerhans also plans to examine whether other species of livebearing fish also exhibit female mating preference for males with large genitalia. If true, he said, the evolution of this mating preference might help explain the evolution of swords in male swordtail fishes. Swords are conspicuous, elongate projections of the tail fin and are known to be subject to female mate choice.
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