A Duck’s Penis Length Depends on The Male Ducks Around Him

Daffy DuckYou are really screwed, Daffy. All that time you spent arguing with that rabbit … Susan Milius writes on Wired Science:

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.

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.

To see whether competition among males influences penis growth, Brennan housed some of her drakes in groups of seven to eight males with just five or six females. Other males lived with just one female.

Among the scaup, males competing in groups grew penises 15 percent longer, and sometimes up to 25 percent longer, than drakes with no mating rivals, Brennan reported.

Read More: Wired Science

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