Naturalists have long tried to understand why males in some species of animals use a variety of elaborate sexual displays to attract mates. Biologists at the University of Maryland have discovered that the secret to male variety may actually lie with the ladies.
In a paper published in the April 15 edition of the journal Nature, the Maryland team reports that in their two-year study of the Australian satin bowerbird, they found that females of different ages have different preferences for male mating display.
"There's been a lot of research on the complexity of male mating displays, but diversity of female mate choice hasn't had a lot of attention," says Maryland doctoral student Seth Coleman, lead author of the paper. "Previous work on bowerbirds has shown that multiple male display traits have evolved to provide females with different kinds of information in different stages of the mate choice process. But our study of the bowerbird suggests that complex male displays also may have evolved because young and old females choose mates based on different traits."
"What is new is that we show experimentally that females in a population may choose males for different reason. This forces successful males to have multiple types of display to attract multiple females," says biology professor Gerald Borgia, another of the study's authors.
The satin bowerbird of Australia and New Guinea puts on one of nature's most elaborate sexual displays. To begin, the males build complex display courts, called bowers, and decorate them with feathers, flowers, fruits, and other brightly colored objects, blue being the favorite color for attracting females.
The research team divided the adult males into two groups. Males in one group got to adorn their bowers with researcher-supplied blue decorations, while the other males received nothing. They then observed how the addition of the decorations affected females in their search for a mate.
In the first of three stages of bowerbird courtship, the females cruise the neighborhood when the males are away from their bowers, checking out the bachelor pads and the baubles. They make a second visit when the males are at the bower, to see how well the male performs his raucous mating display dance.
"The males flip out their wings, fluff their feathers out to the side, make loud buzzing vocalization and run from side to side, all very close to the female," says Coleman.
In the third courtship stage, the female goes off to build her nest, then returns to her mate of choice to consummate the relationship. Each female mates with only one male, but the guys with the best bowers and display techniques may mate with as many as 20 or more females in the two-month mating period. The less successful males may not mate at all. The female raises her chicks on her own, not connecting with males till next year, same time, maybe same place.
The Bachelorettes Choose
The study's major finding is that there is a marked difference in the courtship display preferences between young females and the older, more experienced females. Following those initial bower visits, when the males were gone, all females returned to the bowers with the added blue decorations, but the similarity ended there.
The inexperienced females -- one or two years old -- chose to mate only with the guys in the experimental group, those with the most blue decorations. But older females, with a few years of courtship experience under their feathers, ultimately chose the males with the most intense courtship displays, regardless of their yard decor.
"The young, inexperienced females chose based on a trait that can be easily assessed -- does he have a lot of blue?" says Coleman. "The younger females are highly threatened by the intense displays. Even if she is attracted to his bower, she might be scared off by his displays. By emphasizing non-threatening male traits, young females can choose a good male and avoid being startled by the threatening behavioral displays."
The experienced females, on the other hand, evaluated several traits, in seven or eight males, to decide on the lucky man, and the blue trinkets alone didn't cut it. "The number of blue ornaments had no effect on the return rate of the older females. They ultimately chose the males that put on the most intense displays," a result, Coleman says, of the older females' experience in assessing the display traits. "They are really choosy with multiple display traits. The female wants to maximize her own fitness by producing vigorous, healthy offspring. An intense mating display may be a trait that shows vigor."
Coleman thinks the bowerbird discovery may contribute to understanding why males develop more than one romantic technique. "Ultimately, when females differ in their preferences for particular male traits, then a male who has multiple traits to appeal to multiple female preferences will be evolutionarily favored over the one-trick bird."
The findings "can have implications for conservation and species propagation," says Borgia. "Ultimately such highly detailed information on female choice, which is rare, is valuable for distinguishing among different models that attempt to explain male display trait evolution."
Gail Patricelli, a former doctoral student in the Borgia Lab and now at Cornell University, also collaborated on the project. Funding was provided by the National Science Foundation.
For more information on this study, contact:
Seth Coleman, 301 405 6943, firstname.lastname@example.org; Gerald Borgia, 301-405-6943, email@example.com ; or Ellen Ternes, 301-405-4627, firstname.lastname@example.org
More on the Borgia Lab's bowerbird research:
Robotic Female Lures Male Bowerbirds Into Experiment: