Sure those fancy feathers drive her wild, but are they worth the trouble?
Peacocks' tails have long baffled evolutionary biology. Clearly peahens love them - the bigger and brighter, the better. But it's hard to figure what possible advantage these eye-catching but burdensome appendages offer either cocks or hens in the grim business of survival.
A study now shows that there is nothing capricious about a finicky peahen's habit of sizing up a suitor by his tail. In a recent experiment, zoologist Marion Petrie of Oxford University placed a different peacock in each of eight pens containing a small number of randomly chosen peahens. She then charted the progress of the 350 chicks that hatched. Bizarre as the result may sound, the chicks that grew the fastest and survived the longest, when released into a British park, were precisely those whose fathers' trains had the largest eyespots.
Petrie expected as much. "We think an elaborate train acts as a sort of handicap," she says. "Only the best-quality males can afford to carry it around, so females are using the train as a cue for male quality."
In other words, if a peacock can not only strut his stuff but also find food and evade predators while dragging around a bigger and more conspicuous tail than his rivals do, he must indeed be hot stuff. By fanning out his tail at opportune moments, he's brandishing a Day-Glo, billboard-sized self-advertisement: LOOK WHAT I CAN DO! THIS TURN YOU ON? And since any animal's health and fitness depend partly on the genes it inherits, a fitter-than-average peacock is likely to produce fitter-than-average chicks.
Naturalists have long agreed that among animals that use unusual ornaments or behaviors to win mates, the individuals with the most attention-getting displays tend to be the most successful. What scientists are now beginning to resolve is why this should be so. In the past 15 years a flurry of research has suggested that beneath the apparent silliness of so many of the animal kingdom's courtship displays - the greater bird of paradise's imitation of a feather duster, the synchronized swimming of great crested grebes, the dance of the fruit fly - hidden signals are being sent and received. As with the peacock, these messages are about who'll make a good parent.
Survival of the Cutest
In Origin of Species, Charles Darwin conceded that natural selection alone - survival of the fittest, in simple terms - couldn't account for flamboyant ornaments, which are found mostly in birds and fish. He suggested a second, parallel mechanism, which he called sexual selection. This includes selection for weapons like horns that rival males use to batter one another during the breeding season in a process that determines dominance and thereby access to females. Darwin also described a form of sexual selection that was "of a more peaceful character": female choice. If, over thousands of generations of birds, females chose "the most melodious or beautiful males, according to their standard of beauty," the males of the species would find themselves slowly evolving to meet that standard. The ugly and tone-deaf would go childless, and their frillier rivals would multiply. As to how and why a hen, let alone an insect, would hew to a "standard of beauty," Darwin didn't venture to speculate.
The contemporary researchers have begun filling in the answers. Evolution works in small steps, of course, and what was merely practical yesterday can become beautiful (to the wooed female) today, the scientists say. The elk first grew antlers to fight and defend itself, while the mane of the lion and the ruffled cape of the hamadryas baboon protected their necks from lethal bites. Gradually, females of each species evolved an innate preference for these now-ornamental armaments. To the females, big antlers and heavy manes now mean "marriage material."
In other cases, perhaps most cases, the researchers conclude, the reverse may have happened. …