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What If Your Friends’ Penis Sizes Determined Yours?

After mating season, much a male duck’s penis withers away, only to grow anew when it’s times to produce babies again. Sounds dreadful, sure, but it’s also a chance to get a larger dong. But how big their members become is directly tied to the endowment of the ducks they hang around with.

A drake’s penis substantially wastes away at the end of one breeding season and then regrows as the next season begins. Among lesser scaup and ruddy ducks, the regrowth varies in length or timing depending on whether males have to compete with a bunch of other guys, said Patricia Brennan of Yale University. Her new measurements offer the first evidence in vertebrates that social circumstances influence penis growth, she reported July 29 at the annual meeting of the Animal Behavior Society.

In many bird species, males don’t grow specialized organs to deliver sperm. Ducks typically do, their penises sometimes reaching considerable lengths (9.8 inches for a ruddy duck, more than half its body length). That extra length may give a male a competitive advantage in delivering sperm when females have multiple mates. Brennan’s past research has documented strong sexual conflict in ducks, with males forcing copulation and females employing strategies such as corkscrew-shaped vaginas, developed over the course of duck evolution, that apparently thwart male control of reproduction.

So what happens if you’re a guy duck and you’re not facing down other men for lady love? Expect your penis to be 15-25 percent shorter than ducks in a more competitive pond.

See, straight guys? Sometimes it pays to surround yourself with other fellas.