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ARLINGTON, Va.—When looking for sex partners, younger females prefer males who decorate their place with a little extra blue, be it plastic or feathers. They also prefer males who tone down the intensity of their courtship behavior. At least, that's how it looks for satin bowerbirds, according to research findings published this week in the journal Nature.
The study - conducted in New South Wales, Australia, in 1999-2000 - found that not all females find the same traits attractive in mates. As they choose a mate, females make a series of complex decisions related to male courtship behavior and to the colored decorations males collect and place around the bower, a stick structure that protects females during courtship display. Older females focus more on the male's intense courtship display while younger females are attracted by the blue bower decorations.
The research, funded by the National Science Foundation's Animal Behavior Program, was conducted by biologists Gerald Borgia, Seth Coleman and Gail Patricelli of the University of Maryland. NSF is an independent U.S. federal agency that supports fundamental research and education across all fields of science and engineering.
According to Borgia, learning more about such sophisticated courtship behavior can increase our understanding of mate choice, a fundamental process that affects the genetics of most animal species. Thus, clues to courtship can also aid wildlife conservation and species propagation.
"It's very important," he said, "because it helps us develop a general model of mate choice. We show experimentally that females in a population may choose males for different reasons. This, in turn, explains why males have complex displays - that is to accommodate the different mating preferences of females they engage in courtship."
A male bowerbird's intense performance of "Pick me! Pick me!" involves loud buzzing calls, raising its feathers on end, and running vigorously back and forth, with its wings extended, across its platform stage - the bower. Males build their bowers primarily from sticks, creating a U-shaped chute that they adorn with blue decorations, such as feathers, blue clothespins, and scraps of plastic.
To see if bluer-is-better interior decorating affects females' preferences, the researchers supplemented some bowers with blue plastic tiles and strands. Because male bowerbirds commonly steal from each other, researchers anchored these extra accoutrements with screws and glue.
According to Jane Brockmann of NSF's animal behavior program, "This is an excellent system for studying mate choice. Dr. Borgia watches the bowers by using video cameras that are tripped when a female enters a male's bower, so he has a very accurate record of whether a female just visits a male or mates with him. He has also been studying the same large group of marked birds for years, so he knows when a female's choice changes with age and experience."
Very few studies have this sort of long-term data, and most studies of mate choice treat females as if their decisions do not vary or change, said Brockmann.
"Males of most species show a lot of variation in their behavior," she said. "It is unreasonable to think that females would not similarly show variation. Yet we know very little about variation in how females choose mates. It does matter."
Particularly to males.
Michael Greenfield, who co-directs NSF's animal behavior program with Brockmann, said, "As males are under some pressure to be attractive to and mate with as many females as possible, their display is a means of covering all possibilities in luring potential mates. We still don't really know why female preference varies with age, but one possibility inherent in this study is that learning - which is generally disregarded in studies of sexual selection - plays a role."
As scientists learn more about how females choose mates, biologists will be better able to test various models of how courtship displays evolve among males, Borgia said.
In a previous study, his group found that female satin bowerbirds will return the next year to attractive mates, but "females which fail to encounter very attractive males typically reject their previous mates and search for more attractive males in the following year."
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