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For ducks, penis length depends on the other guys

Genitals grow longer with more male competition

WILLIAMSBURG, Va. — New measurements find that the maximum length of a duck’s penis depends on the company he keeps. And in this case, it’s his fellow males who make the difference.

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.

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