Feb. 7, 2005 CHAPEL HILL – Attracting a mate can be a costly endeavor, according to a University of North Carolina at Chapel Hill scientist, but new experiments he helped lead show that some male animals economize on courting when the chance of success seems low.
Dr. Keith W. Sockman, assistant professor of biology in UNC's College of Arts and Sciences, studies mating behavior in songbirds and the songs that play such a central role in their reproduction.
"From people to praying mantises, individuals invest everything from their homes to their heads to attract a member of the opposite sex," Sockman said. "When male songbirds sing to attract a mate, they expend energy during times they could otherwise be foraging for seeds and grubs.
"They may also increase their exposure to predators," the biologist said. "This led us to predict that when females are in short supply or infertile, unmated males should reduce these 'costs' by singing less."
In a paper published in the new issue of the journal Biology Letters, Sockman and colleagues Dr. Thomas P. Hahn and Kendra B. Sewall of the University of California at Davis and Dr. Gregory F. Ball of Johns Hopkins University in Baltimore, report that male songbirds are frugal in their efforts to attract a mate.
Studying Cassin's finches, which breed in the Sierra Nevada Mountains of California, the scientists found that males sang the most in response to the loss of a prospective mate but sang very little either in the presence of a mate or when females were unlikely to be nearby.
The researchers concluded that male Cassin's finches adjust their mate-attraction efforts according to their need for a mate and the likelihood of attracting one.
"But the story gets better," Sockman said. "After the breeding period was over, we determined the relative fertility of females by measuring when each began feather molt."
Because molt begins when fertility ends, the scientists could infer when each female had been at the peak of her fertility during the experiment when males were exposed to them.
"To our great surprise, we discovered that males somehow ascertain female fertility and sing the most when trying to attract the most fertile females," Sockman said. "Thus, males temper their efforts in attracting a mate depending on the likelihood of a payoff."
Dr. Mark E. Hauber is a behavioral ecologist at the University of Auckland in New Zealand and an expert in bird behavior.
"Since Darwin's writings, we have assumed that females are the choosy sex, but apparently males are being choosy in their own way," Hauber said. "They simply don't put in the effort for less fertile females.
"Economic decisions of this sort are likely adaptive because males would not incur large costs on investments yielding low returns," he said. "Now the question is how, exactly, a male bird determines the fertility status of potential partners."
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