Ducks are among the small minority of birds that has penises.
Adding up to that odd physiology is their ability to develop new penises every year. Their genitals begin to grow when the amount of daylight changes as this signal mating season. After such season, they shrink. Also, did I mention they're corkscrew-shaped?--An evolution to help the males get around the challenges of complicated duck vaginas.
Now, for our last duck penis fact, let's hear it from Patricia Brennan of the Mount Holyoke College in South Hadley, Massachusetts: "If you know exactly where to press, you can [actually easily] pop the penis out," she explains. "They're quite cooperative. The males get used to being handled."
Too much dicks-cussion? Sorry, we're not done yet.
Brennan and colleagues wondered about the effect of male competition to penis development. So, they started sorting two species of ducks with very different mating systems: ruddy ducks and lesser scaups. They housed them in naturalistic outdoor aviaries by pairs or groups of five females and eight males. The highly promiscuous ruddy ducks turned out to almost always force copulation and never formed pair bonds. Meanwhile, lesser scaups forced copulations less often and do from pair bonds.
Results also showed that, as expected, those lesser scaups living in groups developed longer penises that just those living in pairs. Rubby ducks, however, already have "monster" penises that are almost the same length as their bodies. But when housed in groups, their penises hardly grew at all!
Researchers explained that the latter species has dominance hierarchies: There's a top male that develops an enormous penis althroughout the breeding season, so, subordinate ducks had to make a different strategy. "Everybody else grows a penis very quickly, trying to sneak in some copulations before the [dominant] male starts beating them up," says Brennan. And as mention, all their penises shrink back. If one's lucky enough, they can let this slip past boss duck's "radar" and happily get away.
This implies that, indeed, the social milieu can greatly affect these duck's penis development. "It's when the conditions vary year to year that plasticity itself becomes advantageous," he says. "If you don't need it, maybe don't grow it."
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