In ducks, the war between the sexes appears to have spiraled out of control. New research reveals that as duck penises evolved to be larger and fancier, twisting in a counter-clockwise fashion, females evolved spiraling vaginas--but that twist in the opposite direction. This apparent barrier might allow females to keep out unwanted males, say scientists.
Competition among males shapes their mating equipment. For example, swans that mate for life have relatively small penises. However, ducks that prefer to mate with multiple partners have penises on the large end of the spectrum, presumably as a result of males competing with each other for females. But many researchers think that males and females also compete to minimize the costs of reproduction while maximizing their benefits. Such sexual selection results in an "arms race" between the sexes: Males evolve better ways to inseminate females, while the females evolve improved ways to maintain control of who fathers their brood. Evolutionary ornithologists Patricia Brennan and Richard Prum at Yale University wondered what consequences penis size and shape had on the females' reproductive organs.
To find out, Brennan, Prum, and colleagues dissected reproductive organs from breeding pairs in 16 duck species. The researchers selected breeding birds because the reproductive organs in waterfowl wither after each mating season. They found that the species with the largest penises--which corkscrew counterclockwise--also had fancy vaginas. The muscular tube spiraled in the opposite direction of the male penis and sported sacs that branched off near the entrance and dead-ended quickly. Because the species with the most elaborate penises and vaginas are least likely to be monogamous, Prum postulates that the clockwise spirals and dead-end sacs have evolved in females to counter unwanted advances by males. He says trying to determine how the males and females fit together might be problematic. "We haven't tested how these things work. I mean, how are we going to do that?"
Evolutionary biologist Joan Roughgarden of Stanford University in Palo Alto, California, thinks the researchers have hastily concluded that the data support sexual conflict. "Nothing's been demonstrated. They haven't made a good faith effort to imagine other possibilities," says Roughgarden. She says that the researchers are ignoring potential cooperation between the sexes. For example, she suggests that the unique genitals might promote communication and signaling between the breeding couple. What's obvious, though, is that the research puts a new twist on our picture of duck love.