WILLIAMSBURG, Virginia — 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.

The scaup species doesn’t show many signs of conflict between males and females, though, she said. Scaup drakes, for example, rarely force themselves on a resistant female. In contrast, ruddy duck relations seem rife with conflict, with males often forcing themselves on females in chaotic mating scenes.

Among ruddies, penis length did not differ overall between males in competitive crowds and those in lucky privacy. What did differ was timing.

In the competitive groups, a few big males grew prodigious organs as if dominating the group. Other males grew more moderate penises, which started wasting away weeks earlier than those of dominant males or males with no competition.

Thus, Brennan said, male ducks are “prudent.” In a crowd, a ho-hum male apparently doesn’t bother sustaining a big investment in tissue that’s not going to pay off.

The results shed light on how ducks became so well-endowed compared with other birds, Brennan said. “It’s really likely that having a longer penis evolved in male-male competition.” Guy-versus-guy battles then could have started playing a role in battles between the sexes.

“Elegant,” says evolutionary ecologist Maydianne Andrade of the University of Toronto Scarborough, who has studied sexually cannibalistic spiders. The experiment shows that ducks “are essentially engineering their own phallus in response to social challenges.” Now she’d like to know more about just how this duck physiology works and whether any other species respond to sexual competition the same way.

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Image: 1) Flickr/milesizz. 2) Patricia Brennan.