A switch in traditional gender-role behavior gives an advantage to male and female shorebirds called jacanas. Found in tropical regions, jacanas have evolved a social system that confused early ornithologists into mistaking males for females.

In a sexually reproducing species, males and females are defined by the kind of gametes (reproductive cells) they produce. The individuals who produce lots of small, mobile gametes are called males. Those who invest everything in a relatively few large gametes are called females. Because of all the resources a female will put into each egg, it makes sense, in most cases, for her to be choosy about whose genes she allows to combine with it, and to continue to invest in its growth and survival after fertilization. For the male, it usually pays best to compete with other males for access to as many eggs as possible. This tends to give rise to the more traditional male/female sex roles.

But there is nothing inevitable about these familiar sex roles. Evolution favors whatever works, and for the jacana, what works is quite different.

To begin with, female jacanas weigh 60 percent more than males and can lay many successive "clutches" of eggs in a single breeding season. Once their eggs are on the ground, the females are gone, leaving the males to incubate the eggs and take care of the newborn young. Males even have special wing adaptations that allow them to carry two chicks under each wing.

The aggressive females, meanwhile, are busy gathering harems of as many as five males, mating promiscuously and defending territory against other females. Not only that, female jacanas will move in on another female's territory, go to the nest, and kill the chicks. Freed from his parenting duties, the male then becomes the object of the invading female's courting. If successful, the mating is followed a week later by a new clutch of eggs for the male to incubate.

This entire system based on female dominance has evolved, some researchers believe, to compensate for an extraordinarily high rate of egg loss to predators. The world of the jacana has, in effect, turned the female into "an egg-making machine," says Cornell University biologist Stephen Emlen, who has spent years studying the remarkable adaptation. And it is the males who are primarily responsible for the care of the young.

One lesson from the jacana is that the behaviors we automatically label "male" or "female" aren't something inherent in the physiology or looks of the individual. For the jacana, it's just an unusual way of giving their offspring -- and their genes -- a better chance at survival.

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