Ballistic penises and corkscrew vaginas — the sexual battles of ducks By Ed Yong December 22, 7: Normally, the duck keeps its penis inside-out within a sac in its body. The whole process takes just a third of a second and Brennan captures it all on high-speed camera.
Duck penises are a wonderful example of the strange things that happen when sexual conflict shapes the evolution of animal bodies.
Many ducks form bonds between males and females that last for a whole mating season. But rival males often violently force themselves onto females. To gain the edge in these conflicts, drakes have evolved large corkscrew phalluses, lined with ridges and backward-pointing spines, which allow them to deposit their sperm further into a female than their rivals.
But female ducks have developed countermeasures. Their vaginas are equally long and twisting, lined with dead-end pockets and spirals that curve in the opposite direction. Two years ago, Brennan showed that duck species whose males have the longest penises tend to have females with the most elaborate vaginas. Now, she has found further evidence that these complex genitals are the result of a long-lasting war of the sexes.
Duck penises and those other birds work in a different way to human ones. For a start, they are flexible when erect. That presents a problem. You need to give it something to have sex with.
Brennan used four different shapes: Brennan took these tubes to a commercial duck farm in California, where Muscovy drakes provide semen for artificial insemination.
She watched as the handlers placed a female in the same cage as a randy drake, and waited until the male mounted her. Just as he was getting ready to mate, Brennan intercepted him with one of the four oiled tubes, touching it to a pair of sensitive spots that immediately trigger the explosive erection and ejaculation.
Her videos revealed the process of duck erection in painstaking detail, including how long it takes to evert the different parts of the penis, how fast the ejaculate is and how long it takes to retract the appendage around 2 minutes. They showed that the drakes easily launched their penises into the straight tubes and the ones that matched the direction of their spiral.
The tubes that spiralled in the wrong direction or bent at a sharp angle proved to be more challenging, and mostly stopped the males from achieving a full erection. Not all males are hit equally hard by these defences. Those that the female actually wants to mate with have an easier time.
She repeatedly contracts the walls of her genital tract, relaxing them for long enough for favoured suitors to achieve full penetration. Males who try to force themselves upon her receive no such help and have to cope with vigorous struggling.
The female may not be able to resist such advances, but her convoluted vagina gives her ultimate control over where the sperm of her current partner ends up. Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia Proceedings of the Royal Society B: