Sexual competition among ducks wreaks havoc on penis size

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Male ducks respond to sexual competition by growing either an extra-long penis or a nub of flesh, a new study finds.

The unusual phenomena occurred in two species studied: the lesser scaup (Aythya affinis) and the ruddy duck (Oxyura jamaicensis). It suggests that penis size — in line with many traits and behaviours meant to impress or allow impregnation of the opposite sex — involves a trade-off between the potential to reproduce and to survive.

Patricia Brennan, an evolutionary biologist at Mount Holyoke College in South Hadley, Massachusetts, compared the penises of ducks kept in male–female pairs to those housed with multiple males per female. The findings are published in a study on 20 September in The Auk: Ornithological Advances1.

“If they were alone with a female, the males just grew a normal-sized penis, but if there were other males around, they had the ability to change dramatically,” Brennan says. “So evolution must be acting on the ability to be plastic — the ability to invest only in what is needed in your current circumstance.”

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