Being Around Predators Changes the Shape of This Fish’s Penis

Armadillos have bony plates, stick bugs evolved camouflage, and the Bahamas mosquitofish evolved a bigger penis

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Animals have evolved all sorts of mechanisms to fight back against their predators. Armadillos have bony plates, stick bugs evolved camouflage and the Bahamas mosquitofish evolved a bigger penis.

According to a new study in the Journal of Evolutionary Biology, male fish that live around predators have longer and bonier penises—or “gonopodia” as they’re called. The North Carolina State University press release gives a little more detail:

Longer, bonier and more elongated gonopodium tips are, of course, relative; in small fish, these organ tips are generally only 1 millimeter long. Yet the findings suggest that male fish under constant threat of serving as a predator’s snack have evolved better ways to impregnate females under these conditions.

The idea behind the bigger gonopodium is that males can transfer as much sperm as quickly as possible each time they mate. When there are lots of predators around, that’s important, because you’re never sure how many times you’ll get to get it on. It’s not that males change their penises when they see more predators, but rather that fish who have lived in areas with lots of predation wind up evolving longer penises than those who live in safer waters.

More from Smithsonian.com:

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