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Photo: A male peacock displays elaborate plumage
A male peacock's unwieldy outfit suggests that he's robust— or at least it may just look sexy. Regardless, females prefer elaborate, heavily spotted trains.
Photograph by Ingo Arndt
Written by Virginia Morell
Republished from the pages of National Geographic magazine
It's a blustery spring day in the Australian outback, the kind that makes you think rain must be on the way, although there hasn't been a drop in months, and the ground is brown and parched. In some animals, frogs for instance, a dry spring can slow down or stop altogether the normal, romantic inclinations that come this time of year. But the lack of rain hasn't deterred the male spotted bowerbirds.
Under old peppertrees, thornbushes, and stands of oleanders, they've built elaborate U-shaped arenas of dried grasses, 12 to 14 inches (30.5 to 35.6 centimeters) high and 12 to 20 inches (30.5 to 50.8 centimeters) long. They've decorated them with piles of sun-bleached sheep vertebrae, shiny aluminum foil, pop-tops from beer cans, shards of broken windshield glass, and little strips of red and blue plastic. The fanciest bowers feature special, seductive tidbits: a silver fork, the shoe token from a Monopoly game, old gun shell casings, red, blue, and purple glass of the deepest hues. The birds have arranged their treasures with an eye to the light—how does that bone pile look when the morning sun hits it?—and to their symmetry: silver metal hoops of unknown origin, for example, placed at equal distances from opposite ends of the bower.
Now a male can do little more than watch and wait. If he's built a good bower, then he'll succeed in life's ultimate contest and win the top prize: a female who chooses him as a mate.
"That's really what it comes down to," says Gerry Borgia, an evolutionary biologist who has studied the mating behaviors of bowerbirds for 23 years. "So you wonder sometimes when you see poorly built bowers," he says, pointing to one in disarray. "You want to say to the guy: 'Hey! This is about your reproductive success! Get moving! Straighten those straws! Find some more bones! Why be a C student?'"
Borgia, a hefty, middle-aged man with a broad, gap-toothed smile, bends over a video camera he's placed a short distance from the bower and changes the tape. He's stationed similar cameras with microphones at 22 bowers scattered across the sheep and cattle stations near the sleepy town of Nyngan. The cameras are equipped with motion-detection sensors and record whatever the male birds—or their female visitors—do within the bowers. Later, in his University of Maryland lab, Borgia's students will review the tapes, picking out the ones that show what male bowerbirds might dream about: a female entering the straw bower, watching the male perform and sing for her, and, if she is well pleased, accepting him as a mate. Borgia isn't sentimental about this latter event, referring to it simply as a "cop"—short for copulation.
"You watch enough of these cops, and you begin to get a feel for why the female chooses one male and not another," he explains. "It's my guess that this guy isn't going to do well. I mean, that's pathetic," he says, waving his hand at the bird's puny pile of vertebrae. "And the thing is, he took over this site from an older male who died, but who had a great bower with lots of bones. And they're still here! This new guy just hasn't made the effort to move them to his bower."
Borgia shakes his head like a teacher who can't figure out why some kid who has everything handed to him on a platter would still choose to fail. "He's probably not getting any cops this year," the professor says, assigning the bird to possible evolutionary oblivion.
As if in protest, the male, a blue-jay-size bird colored beige and brown, squawks at us from a nearby eucalyptus tree. He rasps out a long series of skraas, then changes to the snarling, spitting sound of a cornered cat, and ends with a laughing call that sounds like a kookaburra.
"He's trying to scare us," Borgia says. "They're great imitators—cats, hawks, kids crying. They use that skraa call in their courtship displays too, so there must be something about it that allows females to find genetically superior males. But he's got a lot to learn about how to build a bower, one that will attract the females and get him some action."
The most successful males are about ten years old and have spent some five lonely bachelor years perfecting their skills. In this species (as with most birds), a male can't force a female to mate. Like a solo rock star, he must devise a bower, song, and dance that wows the gals. Among bowerbirds and most other animals as well, it's the females that do the choosing.
From fruit flies to elephants, females pick the male (or males) with which they want to mate. The males, in turn, compete with each other to get a female's attention, each vying to show her that he will be the best sperm donor for her babies. That is why, evolutionary biologists say, males are most often the ornamented sex. It is why the male peacock unfurls his dazzling train, why male guppies are adorned with bright orange and blue spots, why male frogs call and male canaries sing. It is even why the genitalia of many males, particularly insects, are as fancy as an Aborigine's embellished didgeridoo, with accoutrements far beyond what's required to get the job done.
"Basically, the male wants access to the female's eggs," explains William Eberhard, an evolutionary biologist at the University of Costa Rica. "And he'll do whatever it takes to please her. But it's her game; she sets the rules. And she makes the choice."
Charles Darwin was the first scientist to devise a theory of sexual selection and to recognize that females frequently select mates. He began to develop the notion while writing On the Origin of Species, in which he argued that the related theory of natural selection is the primary force in the evolution of all organisms.
Natural selection goes far in explaining why one individual animal survives to pass on its genes to the next generation, while another dies leaving no descendants. It is why female birds are often drably colored (to hide from predators when incubating their eggs), and why gazelles are built for speed (to outrun their enemies). But natural selection does not explain features that would seem to hinder an animal's survival, such as the male peacock's extravagant plumage or a male elk's heavy and unwieldy antlers. How did such unlikely traits—ones that seem to run counter to every Darwinian rule for staying
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