For all the time we spend watching birds during the breeding season, rarely do we get to see birds actually breeding. That’s because avian copulation usually lasts only seconds. Birds mate by joining at the cloaca – an opening beneath the base of the tail. We call this union – the male perched and wobbly on the female’s back – the “cloacal kiss.”
Among ducks, however, there is no kiss. Not even dinner and a movie. (Speaking of which, this column is about to become rated PG – politely graphic.) Male ducks seize upon females in what biologists euphemistically call forced copulation.
As it turns out, male ducks are among the few birds with a penis. But this is no ordinary organ. It’s shaped like a corkscrew and resides most of the time inside his cloaca. As a male overtakes a female, he extends his penis into her reproductive tract. This extension happens quickly, in less than a half second in some ducks, a move biologists call explosive eversion. (I warned you.)
Having a corkscrewed penis would seem to be a favorable trait in an aggressive mating system. Favorable to him, at least. Forced copulation presents risks to females, including injury, death, vulnerability to predators, and lost opportunities to mate with more suitable males. It’s an example of how males and females rarely share equally the burdens of reproduction.
But in their evolution, female ducks have developed a cunning defense.
It turns out that the female’s reproductive tract spirals in the opposite direction of the male’s penis and is outfitted with various cul-de-sacs along its route. Her countervailing tract and its detours present a literal and genetic dead end for an undesirable male. When the female is receptive to a particular male, however, and her body lies prone and relaxed, her tail high and her cloaca exposed, the contours of her tract present no such barrier to fertilization.
Courtship and breeding among wildlife most often feature males competing with other males, employing ornate displays (peacocks) or aggressive rivalry (rams) to attract the attention of a female. That’s classic Darwinian sexual selection. But among ducks (and some other animals) males and females also engage in a kind of sexual competition – an evolutionary arms race over who controls fertilization.
The shape of her reproductive tract doesn’t protect the female from forced copulation and its risks. But it does allow her to control which male helps produce her young. At least that’s the working hypothesis of researchers at Yale University. I won’t go too explicitly into the details of their experiment, which involves birds at a commercial duck farm and custom-made glass tubes of various shapes mimicking a female’s reproductive tract. Maybe you get the idea.
Suffice to say that some of this research is, well, twisted.