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The feathers that pestered Darwin

Creature Courtship
by Peter Tyson

"The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!"

—Charles Darwin, in a letter to botanist Asa Gray, April 3, 1860

For most people, the glorious train of the peacock is a joy to behold. But for Darwin in the years immediately following the 1859 publication of Origin of Species, in which he laid out his theory of evolution by natural selection, the peacock's resplendent train amounted to an eyesore.

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?

Darwin had to devise an entirely new theory to explain the peacock's ostentatious feathers.

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.

Necks scarred from previous battles, bull elephant seals fight it out on an Antarctic beach.

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.

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.

A pair of rutting bull elk lock horns.

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.

Continue: The meek shall inherit

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