"Their retreat is so hasty—often leaping up and out of the bower—that they can't possibly have any time to assess the male," Coleman said. (Sensitive males actually modulate this display to match the body language of the female. If she appears frightened, he tones it down. If she is secure, then he cranks up the level for his most intense performance.)
Finding a Quality Guy
The researchers speculated that the younger females use different indicators to size up a male—perhaps the bower with its blue ornaments. Blue objects are rare in the bowerbird environment, and a male whose bower is adorned with many is deemed superior, because it proves he is able to acquire and retain these items in the face of competition from other males.
To test their hunch, the scientists augmented a bowerbird group's environment with blue tiles and plastic strands and left another group untouched. Males from the blue-item-rich group quickly incorporated the blue decorations into their bowers.
Using video cameras, the researchers observed females at each of the three stages of courtship.
The first step in the mate search involves visiting a bower while the male is absent. Males with poorly constructed or asymmetric bowers or a haphazard arrangement of blue items are generally less successful at this stage.
Females impressed by the bower generally return for stage two—the buzz-wing-flip. After the performance the females leave for about a week to build a nest. They return for stage three, where they review encore performances from only the most promising candidates. Once they've narrowed their search to one, they mate.
Coleman discovered that in the absence of a male, all females, regardless of age, were more impressed by the augmented bowers and returned to these sites to view the male.
Decorating vs. Singing and Dancing
For the younger, less experienced females, this was the end of the story—they chose the males with the most blue ornaments because they couldn't tolerate the courtship. The three-plus-year females (with two or more years of mating experience), on the other hand, went with the physical displays, regardless of whether the bower had more curios.
The physical display is believed to be a more honest indicator of male quality. Blue ornaments can be stolen from another bower, but a buzz-wing-flip cannot be faked. "Only males in top physical condition are able to produce vigorous performances—something that sick or weak males can't do," Coleman said. Vigorous displays suggest a strong male with desirable genetic qualities.
"Not much attention has been given to female preferences and how this influences sexual selection, in part because the work is time consuming and laborious to follow individual females back and forth and record their preferences," said John Endler, an evolutionary biologist at the University of California, Santa Barbara.
"What Seth's group has achieved is outstanding—he has shown that females are assessing multiple traits. This is almost certainly true in other species, but no one bothered to ask."
Fortunately for the young female bowerbirds, good decorating skills correlate with high energy—so both young and old females tend to mate with the same group of top males. For bowerbirds, males that can sing, dance, and decorate get the females. That's a combo that could work for humans as well.
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