Sexual conflict takes shape in ducks

Anatomy differences, other clues suggest coevolution of genital traits

The first intimate duck videos show that it’s mate and checkmate in the battle of the sexes.

High-speed cameras document that a male Muscovy duck can fully extend his 20-centimeter penis in a third of second, says evolutionary biologist Patricia Brennan of Yale University. (Watch the researchers’ video.) That may be about all the time he has with a resisting female trying to escape him. Male Muscovy ducks rank among the waterfowl that often fail to take no for an answer.

Also on camera, duck handlers coaxed males to deploy into glass tubes instead of into females. The tubes featured bends and wrong-way spirals that kept the males from extending very far, Brennan and her colleagues report online December 22 in Proceedings of the Royal Society B.

Real female ducks have such shapes, even spirals that squiggle in the direction opposite to male anatomy. And Brennan has proposed that a female duck refusing to relax the tension in all her complicated bits presents a challenge to an unwelcome male. For a mate she approves, though, a female Muscovy can ease the constrictions.

Even within the same species, what serves females well as far as number and frequency of mates may not be the best for males. In the case of Muscovy ducks, the males, like competitors in an arms race, have developed explosive extension, but females may be countering with resistant geometry.

The new videos support this idea of female resistance, Brennan says. The obstructive powers of internal tracts fit with the scenario that corkscrewy genitals in certain waterfowl evolved through sexual conflict, Brennan and her colleagues report.

This evidence for competing genitals could be a first for vertebrates. As far as he knows, says evolutionary biologist Locke Rowe of the University of Toronto, elaborate reproductive organs in waterfowl are the only evidence so far of sexual conflict driving coevolution of traits in vertebrates.

Biologists pay so much attention to reproductive organs not out of personal weirdness, Rowe says, but because “they’re some of the most diverse traits known.” Myriad forms suggest the organs evolve fast under the pressures of sexual selection. For example, baroque reproductive tracts might benefit female ducks by allowing them to choose only high-quality or high-compatibility sires for their young.

Females obviously have some way of resisting unwanted males, Brennan says. Studies of mallards, for example, show that, despite plenty of trying, unwelcome males end up siring only a few percent of a female’s offspring.

Mallards and some other waterfowl are among the few birds that have any insertable sperm-delivery organ at all. Biologists have marveled at the spiraling extravagance of some waterfowl penises. But the female side of the picture didn’t emerge until 2007, when Brennan uncovered comparable female elaboration. She also observed that species with more contentious sex lives, such as the mallards, tend to have more elaborate genitals in both sexes. In contrast, male Canada geese don’t typically force copulations and have pretty uncomplicated organs.

To see how waterfowl use their organs, Brennan and her colleagues turned to a commercial poultry operation in California, where male Muscovy ducks have been trained to deposit sperm into tubes.

Brennan had used four sets of tubes representing what she thought would be easy or difficult geometries. The first time she tried to film males in action, she found that her camera wasn’t fast enough.

With a camera that recorded 250 frames per second, Brennan got a first-of-its-kind view of male anatomy in action. When not in use, the male penis lies inside out. Lymph, not blood, powers the expansion.

Brennan filmed 56 occasions of male duck extension, either just in air or into one of the tubes. Males easily extended full length into both a straight tube or into one that curled in the same direction as the duck did. However, tubes that spiraled in the opposite direction or had a bend of 135 degrees stopped the male short of full length, Brennan says.

Males do ejaculate even without full extension, she discovered. What the bends and twists do then is prevent the sperm from getting very far into the reproductive tract, Brennan says. Thus a male forcing himself on a female would be less likely to sire young.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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