When it comes to sex, some female ducks have taken "no" to a new level. They have evolved vaginas with clockwise spirals that keep out the oppositely spiraled penises of undesirable males, scientists have discovered.

When the female wishes to say "yes," she has some tricks that make it easier for her preferred mate to slip his corkscrew-shaped penis easily inside and achieve fertilization.

"In species where forced copulation is common, males have evolved longer penises, but females have coevolved convoluted vaginas with dead-end cul-de-sacs and spirals in the opposite direction of the male penis," said lead researcher Patricia Brennan of Yale University. "This coevolution results from conflict between the sexes over who is going to control fertilization."

The upshot: In the constant battle between the sexes over who's in charge, females win among these Muscovy ducks (Cairina moschata). The findings are detailed in the Dec. 23 issue of the journal Proceedings of the Royal Society B.

Odd sex organs

Back in 2007, Brennan's team described the strange morphology of this duck's sexual organs. Most male birds don't sport phalluses, and instead have sex by bringing together their so-called cloaca (opening to such regions as the reproductive tract) in order for the male to transfer sperm to the female. Not only does C. moschata have a penis, but it's a relatively large, flexible penis (even when erect) extending up to nearly 8 inches (20 cm) inverted inside the body.

(For comparison, the average human erect penis extends just 6 inches, or 15 cm.)

When mating, the male everts its penis to extend the length of the female's vagina.

Being so well-endowed is supposed to give the male a reproductive advantage in forced mating when a male essentially jumps the female with no consent. But Brennan hypothesized that females, with their complex genitalia, could turn the tables and make copulation difficult for such macho males. 

First the team used high-speed video to see how the odd organs get the job done, finding the eversion process was explosive — taking about a half-second. The results came from ducks at a commercial duck farm in which the animals were trained to provide semen for artificial insemination.

Then they examined how duck penis eversion worked in a set of glass tubes with different shapes — a straight tube, one with counter-clockwise spirals (to mimic the male penis), and another with clockwise spirals (matching that of females).

The glass tubes with clockwise spirals could completely stop the penis from extending all the way in. That wasn't the case for the straight tubes or counter-clockwise ones, which didn't slow down eversion.

The results suggest the female vaginal shape can impede copulation with males.

"Female ducks are subjected to forced copulations by unwanted males and usually they cannot escape," Brennan told LiveScience. "The genital morphology allows them to regain control of reproduction by making it difficult for these unwanted males to achieve fertilization."

Females in charge

So how do these birds procreate?

For those males that do a pre-copulatory ritual and get the go-ahead from the potential mate, that female will assume a receptive posture, the researchers found. In this posture, the opening to her vagina gets lifted and begins to pulsate as if laying an egg.

"The female oviduct is very elastic," Brennan said, "and we think that a combination of this pulsating and receptive posture help to relax the oviduct during copulations with the female's own mate, therefore allowing him to fully evert his penis."