For the father of evolution, baroque ornamentations like that train and other seeming extravagances—such as the gaudy tailfin colors of the male guppy, the saber rattling of the rutting bull elk, and the elaborate bowers of the bowerbird—seemed to fly in the face of natural selection. How could such ostentation, so costly to the creature in question in terms of expended time and energy, benefit the animal and its offspring in the survival of the fittest?
The other selection
In the end, Darwin came up with an entirely new theory to explain the extraordinary lengths many animals will go to in order to woo a potential mate. He called it sexual selection. Simply put, sexual selection is the evolutionary process that favors adaptations that increase an animal's chances of mating. Darwin identified two kinds. In the first, males compete fiercely with each other for access to females. This kind favors the evolution of secondary sexual characters, such as large size and armaments like horns, that enhance a male's ability to fight. In the second, males compete to win over a female. This variety favors the evolution of vivid color patterns, intricate courtship displays, and specialized structures such as plumes and frills, which heighten a male's attractiveness to the opposite sex.
As we'll see, sexual selection is much more intense among males than among females. The reason can be summed up in the phrase "sperm is cheap." Since males bear an inexhaustible supply of sperm, it's in their interest to copulate with as many females as possible, thereby engendering the most offspring. A female, by contrast, has a limited cache of eggs. So as not to waste them on less-than-ideal individuals, it pays for her to be discriminating.
That, in short, is why the feather that made Darwin sick belongs to the peacock and not the peahen.
So what are some of the enhancements males invest in to court a potential mate? As the following sampling will suggest, the range is limited only by the imagination.
Clash of the titans
For males of many species, courtship begins with a thud. Crashing head-on into one's rival—with painful and sometimes even lethal results—is the order of the rutting day for many mammals bent on securing sexual favors from nearby females.
One of the most fearsome of such battles occurs every spring among bull elephant seals. The Sumo wrestlers of the animal world, male elephant seals are quivering masses of blubber weighing up to 6,600 pounds. When they go at each other, a pair of bulls rear up, roar loud enough to make the earth shake, and collide with a thunderous crash. The whites of their bulbous eyes showing, they gnash at each other's fleshy necks with blunt teeth that leave grievous-looking wounds and nearby beach water stained crimson with blood. It's a dangerous game, but the payoff is enormous. On islands off the Pacific coast of California, for instance, beachmasters may control harems of 100-plus cows.
The male clown fish has evolved an even more ingenious means of getting the girl: He becomes one.
Males of other species prefer a show of force to force itself. The North American elk is a good example. Bulls are big-shouldered beasts with majestic sets of many-tined antlers. Come mating season, they start "bugling" for all they're worth. This call starts as a bellow, quickly becomes a shrill whistle or scream, and winds up with a series of grunts. The bull that calls the longest and most frequently often succeeds in scaring off potential rivals with his call alone. If challenged, however, he and his adversary walk in parallel to show each other their "manly" profiles. If this doesn't convince one or the other to back off, the pair resort to elephant-bull tactics and ram each other.
In rare instances, such jousts can cause serious injury or death. But typically neither is hurt, for the elk's magnificent antlers, which originally did evolve for fighting and defense, are now largely ornamental. An embellishment to please the softer sex, the antlers are cast aside at the end of the rutting season like ornaments after a holiday.
The meek shall inherit
While sexual selection has favored the biggest individuals in species like elephant seals and elk, sometimes tremendous size can work against its owner. Bull elephant seals, for instance, can reach five times the weight of their mates, and among the northern subspecies, roughly one in 1,000 cows dies of suffocation while copulating with such behemoths. Though this death rate is low, Mother Nature appears to have taken notice and placed a possible check on the evolutionary trend toward Brobdingnagian. While beachmasters are battling it out, smaller males are sometimes able to snatch sex from cows apparently willing to forgo the biggest for the craftiest.
Biologists have recently identified a similar strategy of trumping the Schwarzeneggers of one's kind in a species a fish found in lakes along Africa's Great Rift Valley. The fish in question is the shell-brooding cichlid Lamprologus callipterus. Proportionally, this species boasts the largest males in the animal kingdom; mature males are up to 30 times the size of their mates. These comparatively massive males fill their lake-bottom territories with empty aquatic snail shells. Interested females enter the shells and spawn, while the giants hover outside, spewing sperm all over the place.
These males might seem to have it made, yet L. callipterus features another kind of male, one that goes in for brains over brawn. Looking for all the world like a nonbreeding female, this David swims unmolested into an occupied shell and, unbeknownst to the Goliath hanging around outside, fertilizes the female, ensuring a future for his genes.
The male clown fish has evolved an even more ingenious means of getting the girl: He becomes one. Among clown fish, which form monogamous pairs, the female is heftier than the male, but if she dies or disappears, the male puts on weight, changes sex, and bonds with a new male partner.
While gender-jumping is common among fish (though it usually happens in the opposite direction, from female to male), most males in the animal world don't have such a convenient option. And judging from appearances, many males would just as soon not have to grapple with others of their gender. These males eschew Darwin's first kind of sexual selection for his second. On the surface, bright colors, fancy appendages, and flashy displays may seem a kindler, gentler form of competition than that evinced among those built for armed combat. But the drive for success is no less steely.
The long-tailed manakin presents a rigorous test of Darwin's theory of sexual selection.
Males in many species attract members of the opposite sex with dazzlingly colored feathers, skin, or other body parts, which put comparatively drab females of the same species to shame. Such color dimorphism is particularly prevalent among birds. Many species show it, from such commonly seen birds as the cardinal to the elusive quetzal, the most strikingly hued bird in the Americas. The males of this species have flaming red crests and emerald tail feathers almost three feet long, neither of which the females possess. That's because such structures—arguably exemplified by the sexual billboard that is the peacock's train in full display—are another attribute males have evolved over the eons to excel in the mating game.
Even the most foppish males seem to realize, however, that beauty is only skin deep, so they combine appearances with action. The cardinal lures the female with a series of calls and gifts of tasty sunflower seeds. The quetzal courts with a painstaking song-and-dance routine. Even the lordly peacock shakes its brilliant billboard for added effect.
Some of these ritualized courtship displays can be quite involved. On the western prairies of North America, for instance, male sage grouse congregate on vast display grounds, where they strut their stuff before females. Mate markets of this sort are common among birds; ornithologists know them as "leks," which comes from a Swedish word for play.
Each sage grouse male is a wonder of pumped-up masculinity. Standing tall on his patch of preferably raised ground, he holds his wings at his sides like medieval shields, puffs out his robust chest of downy white feathers, throws up a daunting array of pointed tail feathers, and begins to stomp vigorously on the ground. (The bird's singular two-step inspired the foot-stomping ceremonial dances of the Sioux Indians.)
Getting worked up, the grouse then takes a few steps, draws back his head, and rapidly deflates a pair of scrotal-like air sacs jutting from his chest. The resulting "plop" can be heard several hundred yards away. At the same time, the bird briskly rubs his breast feathers against the wings at his side, generating a swishing sound. When this sequence is finished, he stands still as a statue for a few moments, then repeats the performance, eager to have nearby hens take notice.
As with elephant seals, the payoff is tremendous for the cock that impresses the most hens. On one such display ground, for example, a single male sage grouse enjoyed fully three-fourths of 174 couplings.
While the sage grouse lek has males fervently competing with one another, the lek of the long-tailed manakin, a diminutive bird found in the rainforests of Costa Rica, is altogether different. In this lek, two male manakins cooperate with each other so that the dominant one of the pair can mate with a smitten female.
The display begins with the two males patiently perched on a horizontal branch near the ground, calling for females with a distinct toe-le-doe whistle. When a female lands on the branch, indicating she's ready to be courted, the two males, which are black with turquoise backs, crimson crowns, and twin tail feathers as long as their bodies, launch into a prolonged acrobatic display, whistling their flute-like call all the while. They step daintily and hop energetically; they somersault and leap-frog; they take turns hovering in the air.
As the tempo picks up, the males emit a buzzing sound, and the female becomes more and more excited. At some critical point in the crescendo, the leading male utters a shrill cry; this is the lesser male's cue to make himself scarce. Alone at last, the vanquisher performs a brief dance before his prize and then quickly mounts her.
The long-tailed manakin presents a rigorous test of Darwin's theory of sexual selection. Not only have males evolved a handsome dress code, including radiant colors and lengthy tail, but they also dance to beat the band. They'll go on sometimes for 20 minutes, proving just how fit they are. On top of that, if the pas de deux is successful, only the lead male gets the spoils. How can the subordinate male's costly extras and exhausting performance possibly help him in the survival of the most reproductively fit if he knows even before he begins a dance that he has no chance of mating? Is it all for naught?
Not at all. He learns the ropes, so that when the dominant male dies or becomes less sprightly with age, his apprentice can take over the lek or perhaps form his own. Nature has it all figured out.