How Sex Drives Evolution

It cannot be supposed, for instance, that male birds of paradise or peacocks should take such pains in erecting, spreading, and vibrating their beautiful plumes before the females for no purpose.

—Charles Darwin

There are few animals in nature more resplendent than a male peacock in full display, with his iridescent blue-green tail, studded with eyespots, fanned out in full glory behind a shiny blue body. But the bird seems to violate every aspect of Darwinism, for the traits that make him beautiful are at the same time maladaptive for survival. That long tail produces aerodynamic problems in flight, as anyone knows who has ever seen a peacock struggle to become airborne. This surely makes it hard for the birds to get up to their nighttime roosts in the trees and to escape predators, especially during the monsoons when a wet tail is literally a drag. The sparkling colors, too, attract predators, especially compared to the females, who are short-tailed and camouflaged a drab greenish brown. And a lot of metabolic energy is diverted to the male's striking tail, which must be completely regrown each year.

Not only does the peacock's plumage seem pointless, but it's an impediment. How could it possibly be an adaptation? And if individuals with such plumage left more genes, as one would expect if the raiment evolved by natural selection, why aren't the females equally resplendent? In a letter to the American biologist Asa Gray in i860, Darwin griped about these questions: "I remember well the time when the thought of the eye made me cold all over, but I have got over this stage of complaint and now trifling particulars of structure often make me very uncomfortable. The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!"

Enigmas like the peacock's tail abound. Take the extinct Irish elk (actually a misnomer, for it's neither exclusively Irish nor an elk; it is in fact the largest deer ever described, and lived throughout Eurasia). Males of this species, which disappeared only about 10,000 years ago, were the proud possessors of an enormous pair of antlers, spanning more than 12 feet from tip to tip! Together weighing about 90 pounds, they sat atop a paltry 5-pound skull. Think of the stress that would cause. It's like walking around all day carrying a teenager on your head. And, like the peacock's tail, these antlers were completely regrown from scratch each year.

In addition to gaudy traits, there are strange behaviors seen in only one sex. Male tungara frogs of Central America use their inflatable vocal sacs to sing a long serenade each night. The songs attract the attention of females, but also of bats and bloodsucking flies, which prey on singing males far more often than on the noncalling females. In Australia, male bowerbirds build large and bizarre "bowers" out of sticks that, depending on the species, are shaped like tunnels, mushrooms, or tents. They are festooned with decorations: flowers, snail shells, berries, seed pods, and, where humans are nearby, bottle caps, pieces of glass, and tinfoil. These bowers take hours, sometimes days, to erect (some are nearly 10 feet across and 5 feet tall), and yet they're not used as nests. Why do males go to all this trouble?

We don't have to just speculate, as Darwin did, that these traits actually reduce survival. In recent years scientists have actually shown how costly they can be. The male red-collared widowbird is shiny black, sporting a deep crimson necklace and head patch, and laden with immensely long tail feathers—roughly twice as long as its body. Anybody seeing the male in flight, struggling through the air with its tail flopping behind, has to wonder what that tail is all about. Sarah Pryke and Steffan Andersson of Sweden's Göteborg University captured a group of males in South Africa and trimmed their tails, removing about one inch in one group and four inches in another. Recapturing the males over the breeding season, they found that longer-tailed males lost significantly more weight than shorter-tailed males. Clearly, those extended tails are a considerable handicap.

And so are bright colors, as demonstrated in a clever experiment on the collared lizard. In this footlong lizard that lives in the western United States, the sexes look very different: males sport a turquoise body, yellow head, black neck collars, and black-and-white spots, while the less gaudy females are grayish brown and only lightly spotted. To test the hypothesis that the male's bright color attracts more predators, Jerry Husak and his colleagues at Oklahoma State University put out in the desert clay models painted to look like male and female lizards. The soft clay would preserve the bite marks of any predators mistaking the models for real animals. After only a week, thirty-five of the forty garish male models showed bite marks, mostly by snakes and birds, while none of the forty drab female models were attacked.

Traits that differ between males and females of a species—such as tails, color, and songs—are called sexual dimorphisms, from the Greek for "two forms." (Figure 23 shows a few examples.) Over and over, biologists have found that sexually dimorphic traits in males seem to violate evolutionary theory, for they waste time and energy and reduce survival. Colorful male guppies are eaten more often than are the plainer females. The male black wheatear, a Mediterranean bird, laboriously erects large cairns of stones in various locations, piling up fifty times his own weight in pebbles over a period of two weeks. Male sage grouse t . .


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figure 23. Examples of sexual dimorphisms, showing marked differences in the appearance of males and females. Top: the swordtail (Xiphophorus helleri); middle: King of Saxony Bird of Paradise (Pteridophora alberti), whose males have elaborate head ornaments that are sky blue on one side and brown on the other; bottom: the stag beetle Aegus formosae.

perform elaborate displays, strutting up and down the prairie, flapping their wings, and making loud sounds from two large vocal sacs.32 These shenanigans can use up a tremendous amount of energy for a bird: one day's display burns up the caloric equivalent of a banana split. If selection is responsible for these traits—and it should be, given their complexity— we need to explain how.

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