Patricia Brennan strolls through an outdoor aviary with a net in hand. She scoops up a duck, turns it onto its back, and applies pressure to its belly. "If you know exactly where to press, you can pop the penis out," she explains. "They're quite cooperative. The males get used to being handled."

The biologist didn't plan to spend more than a decade coaxing ducks to reveal their genitals to her. It wasn't until the end of graduate school that she realized birds could have penises at all. And with good reason: the vast majority of birds – 97 percent – don't. Ducks are among the small minority that does.

Owing to an odd quirk in their physiology, ducks develop their penises anew each year. When the changing amount of daylight signals the start of breeding season, their genitals begin to grow. At the end of the breeding season, they shrink. But Brennan, who is based at Mount Holyoke College in South Hadley, Massachusetts, and her team have found that ducks’ social scene can have an effect too.

Duck penises evolved their corkscrew shapes as a result of conflict with females. Brennan wondered whether competition among males could also influence their penis development. To find out, she and her team sorted two species with very different mating systems, ruddy ducks and lesser scaups, either into pairs or into groups of five females and eight males, all housed in naturalistic outdoor aviaries.

The highly promiscuous ruddy ducks never form pair bonds, and almost all copulations are forced. Lesser scaups do form pair bonds, and attempt forced copulations with other females somewhat less often.

View Images "They're quite cooperative," biologist Patricia Brennan says of her research subjects. "The males get used to being handled." Photograby by Patricia Brennan

As the researchers expected, the lesser scaups housed in groups grew longer penises than those housed in pairs. The results were published Wednesday in the journal The Auk.

Meanwhile, ruddy ducks already have enormous penises that stretch nearly as long as the rest of their bodies. Brennan didn't see how the ruddy ducks could possibly grow them even longer. In one way, she was right: most group-living ruddy ducks did not develop longer penises. Instead, she was surprised to find, they hardly grew penises at all.

These ducks formed dominance hierarchies. One male at the top grew a really long penis and kept it throughout the breeding season. Subordinate ducks, unable to grow longer penises, demonstrated 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. After breeding, their penises then shrink back to a non-reproductive state just as rapidly. If they're lucky, they can get away without incurring the wrath of the boss duck.

The impacts of communal life on each species reveal a unique aspect of duck biology: the social milieu has a stark effect on penis growth. The only other animal known to modify its genitals according to the social environment is the hermaphroditic acorn barnacle, a crustacean that grows a longer penis when its colony is sparsely populated so it can better reach its nearest neighbor.

These patterns are only possible because these animals have the unique ability to regrow their genitals each year, says University of Southern California biologist Matt Dean, who wasn't involved in the study. Other vertebrates do annually grow and shrink their testes, but it's very rare for animals to enlarge and reduce the penis itself.

"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." Or at least, don't grow it quite as long.