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Running to the Extremes
25 October 2000 7:00 pm
When a lazuli bunting babe sets out to make bunting babies, she doesn't mess with Mr. In-Between. Instead, she goes for the extremes--choosing either one of the snazziest-looking potential mates or one of the drabbest. That's because the best territories go to the blandest or the brightest male birds, according to a new study. The unusual situation--the two extremes being favored over the middle--is a rare case of sexually selected male-male cooperation, the researchers report in the 26 October issue of Nature.
In the bird world, it's often the guys who are the head-turners. Darwin argued, and many researchers believe, that sexual selection--competition for mates--is the driving force for the evolution of such foppish ornamentation as a male peacock's tail and the stunning turquoise, white, and chestnut coloration of an adult male lazuli bunting. But Bruce Lyon of the University of California, Santa Cruz, and Erick Greene of the University of Montana in Missoula wondered why, then, yearling male lazuli buntings were so variable in color, with some birds almost as dull as the all-brown females and others as splendid as the full-grown males. Using bird-bands, photographs, and DNA fingerprinting, the researchers closely tracked the goings-on in a population of buntings living in a shrubby prairie habitat outside Missoula, Montana.
They found that the gaudy males, whether adults or juveniles, managed to win the competition for scarce good territories. Juvenile males with intermediate coloration apparently weren't aggressive or strong enough to win; but the studly guys tolerated the dullest males, letting them settle next door and sometimes even hanging out with them. The genetic fingerprinting revealed that both parties benefit. The more dull-looking neighbors a hunky bunting has, the less chance he stands to be cuckolded, for reasons that aren't entirely clear yet. The dull guys, on the other hand, get to sire significantly more young than do the intermediate-colored males--even though their partners do sometimes cheat on them.
"We were surprised by our findings," says Lyon. Both ends beating the middle--a pattern biologists call disruptive selection--is highly unusual, he says. Marlene Zuk, a behavioral ecologist at the University of California, Riverside, agrees. "There are very, very, very few examples of disruptive selection," she says. "It's a really tight study," Zuk adds. "I was impressed."