Female mosquitofish give birth to tiny babies, like the guppies you may remember from your third-grade aquarium. That means male mosquitofish don’t spawn: if they’re going to be dads, they need to use a modified fin (called a gonopodium) to inseminate their mate. And sex with a long, stiff fin demands a daunting level of precision.

For these fish, the sex act is blisteringly fast: it takes less than 50 milliseconds for a male to mate. When he does, he twists his body and accelerates toward his intended while simultaneously flipping his gonopodium forward to point towards his head. If he does everything right, he’ll come up alongside her, hook the claws at the tip of the fin into her genital opening, and leave some sperm behind. But it only takes a tiny movement on her part to make him miss the target.

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He could take time to court her, getting her to hold her position while he makes his attempt. But if they’re living in an environment with a lot of fish-eating predators, that extra time might not make him a dad–it might make him lunch instead. The presence of predators affects what the gonopodium looks like in these fish: males living in pools with predators have evolved a longer and bonier fin than males in pools without predators, and they’ll make more fast and furtive stabs at the females that live with them instead of courting them. A new study published in Evolution this month suggests that the presence of predators has also had a complimentary effect on the evolution of the female’s genitalia.

A female mosquitofish’s genitals aren’t much more than a hump and a slit, but because that slit is the target the male aims for, its size and shape define whether insemination is merely challenging or far far more difficult. After measuring and analyzing fish from isolated vertical caves on Andros Island in the Bahamas, North Carolina State University graduate student Christopher Anderson, working with biologist Brian Langerhans, found that in pools with many predators the female’s genital slit is a considerably smaller target than it is in females living in predator-free pools.

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Anderson and Langerhans think that the smaller target may have evolved to give females more choice about which males father their babies. When a females lives in an environment that promotes rapid-fire sex, she may not have much time to evaluate his finer qualities before he’s screwing her. A small target may make it easier for her to deflect oh-so-eager males that she’s not all that into.

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[Anderson and Langerhans 2015]

Image from Anderson and Langerhans 2015


Contact the author at diane@io9.com.