Normally, the duck keeps its penis inside-out within a sac in its body. When the time for mating arrives, the penis explodes outwards to a fully-erect 20cm, around a quarter of the animal’s total body length. The whole process takes just a third of a second and Brennan captures it all on high-speed camera. This isn’t just bizarre voyeurism. Duck penises are a wonderful example of the strange things that happen when sexual conflict shapes the evolution of animal bodies.
Childhood… violated… Innocence… lost… SCIENCE!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. These extreme penises are even more unusual when you consider that 97% of bird species lack any penises whatsoever.
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. They are organic chastity belts, evolved to limit the effectiveness of the males’ lengthy genitals. 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.