WILLIAMSBURG, VIRGINIA—Some animals bulk up to compete against their rivals; ducks just grow larger penises. At a presentation here this week at the 47th annual meeting of the Animal Behavior Society, researchers described their observations of two types of duck: Lesser Scaups and Ruddy Ducks (pictured). During the normal mating season, the penises of Lesser Scaup ducks grow about 4 centimeters longer than average, and those of Ruddy Ducks grow about 15 centimeters longer. But when the team made several Lesser Scaup males compete for just a few females, their penises grew up to 25% longer than normal. In the same experiment with the Ruddy Ducks, the males' penises didn't grow any longer than they usually do during mating season, but one of the males maintained his length for twice as long as his rivals did. The researchers say this is the first example of social competition driving genitalia size in vertebrates. Why the males get or stay bigger is unclear, but the scientists say they are looking at whether it helps them father more ducklings.
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