Health Care

Social environment matters for duck penis size

Social environment matters for duck penis size”

There are only a few species of birds that have penises.

Most species of birds do not have penises, but ducks are a noted exception, possessing substantial members.

While other species have not evolved to wield quite such improbable appendages, all male ducks sport sizeable willies. New research shows that the social environment in which the male duck finds himself in has a pronounced effect on the length of his penis, a finding that may finally put the "size matters" debate to rest. The study found that social environment does play a part in how long these birds' penises grow, suggesting that some can grow larger dicks to compete with other males.

The variable nature of genital growth in the ducks seems to come down to social factors. The male duck thrusts it outwards whenever he feels the moment is right.

A male and female Lesser Scaup. "So [the females] have evolved this anatomy that allows them to prevent males, essentially, from being successful during these forced inseminations".

Patricia Brennan of Mount Holyoke College and her colleagues tested their hypothesis in two species: Ruddy Ducks, which are very promiscuous, do not form pair bonds, and have relatively long penises, and Lesser Scaup, which form seasonal pair bonds and have relatively short penises. Scientists estimate that a third of all duck matings are forced, leading to a genital arms in race in which the the twisted configuration of the female reproductive tracts, or oviducts, restore female choice.

Male lesser scaups form seasonal partnerships and so have relatively short penises. They have been found to grow their penises larger when surrounded by other males to give themselves a competitive edge during mating season

"However", she said, "to do this study, we would have to sacrifice the females to dissect their vaginas, whereas with the males, we can evert the penis, measure it, and then the males go on happily". In the experiment, many males did not reach sexual maturity till the second year. In that case, it would be more about males showboating and showing off.

"The question now is whether the observed increase in penis size in Lesser Scaup under the threat of sperm competition actually gives males a competitive advantage", said Montgomerie.

Brennan separated both species of duck into several different groups. While the "lesser" lesser scaups grew smaller penises, in the first year of the experiment, the smaller ruddy ducks grew next to nothing at all.

The paper is titled "Evidence of phenotypic plasticity of penis morphology and delayed reproductive maturation in response to male competition in waterfowl". This could also explain why smaller ducks kept their penises less than half as long as larger ducks - in ruddy duck society, having a penis makes you a target. "In ruddy ducks, the stress from male aggression may be a contributing mechanism for why small males in the groups can't grow a long penis", Brennan said. "They're at the evolutionary end of how far you can push a penis", she said. Beneath the water's lapping meniscus - indeed, beneath the ducks themselves - a ruthless battle for genetic dominance is taking place. The organ itself is unique, with a long spiralling shape that can be about as long as the duck's whole body.

Prum, author of the book The Evolution of Beauty, points out that nearly all other birds lost their penises through evolution after splitting off from other reptile and mammalian groups.

The biggest challenge during the study, says Brennan, wasn't measuring the ducks-it was simply keeping them housed and fed. "It's a cool example of yet another way in which evolution can result in elaborate genitalia", she said.

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