Sexual competition among ducks wreaks havoc on penis size
When forced to compete for mates, some birds develop longer penises and others almost nothing at all.
By Amy Maxmen
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.
However, evolutionary changes in the size of body parts are generally thought to happen over generations, not within an individual’s lifetime. Brennan wondered whether ducks might buck this trend because some species’ penises emerge anew every breeding season and degenerate afterwards. Similarly, acorn barnacles (Semibalanus balanoides) — hermaphroditic, shelled sea creatures cemented to rocks — generate their penises only when it's time to mate. Because they use their penises to grope for other barnacles to inseminate, the organ’s length depends on the proximity of a barnacle’s neighbours.
Brennan and her colleagues fenced off habitats so that ducks would live either in pairs or in groups with almost twice as many males as females for two breeding seasons over the course of two years. The lesser scaups grew longer penises when they were forced to compete for females than when they were coupled up. A larger reproductive organ likely improves their chances of fertilizing an egg.
http://www.nature.com/news/sexual-competition-among-ducks-wreaks-havoc-on-penis-size-1.22648
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