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Females Pick Frog Princes

Finicky females have long mystified both suitors and evolutionary biologists, particularly when the female tends to pick the most flamboyant male--even if he doesn't appear to have any other redeeming qualities. Some biologists have argued that the exaggerated traits are a sign of less obvious "good genes" that will lead to fitter offspring. Now an elegant series of experiments, reported in tomorrow's issue of Science, boosts that theory by showing that male tree frogs with long calls--known to be favored by females--sire higher quality young than do those with short calls.

Terse suitors don't fare well with female gray tree frogs. Females will head for long calls (up to 2 seconds) heard through a loudspeaker, even if the short calls are closer and louder, says behavioral biologist H. Carl Gerhardt from the University of Missouri, Columbia. To explore whether the longer call signaled good genes, Allison Welch, a graduate student, played Cupid. For 2 years she removed the eggs from about 10 female tree frogs, then fertilized half the eggs with sperm from a short-calling male and half with sperm from a long caller. Next, she compared how the offspring fared as tadpoles and frogs, measuring their growth rates under regimes of scarce and plentiful food. Descendants of long callers won out. "Every single significant effect was in favor of the long callers," says Gerhardt.

Other researchers praise the study because it neatly circumvents problems plaguing other "good genes" experiments, such as mothers favoring one egg over another, which alters the offspring's chances. And unlike many studies, Welch's work can rule out the possibility that in this case at least, flamboyant males offer some benefit to their offspring other than good genes, such as food or rich territories, says population geneticist Mark Kirkpatrick of the University of Texas, Austin. Male tree frogs have no contact with their offspring except for fertilization, so their only contribution is genetic.

Welch's team members say it may be a while before they can answer the next question--what makes the long callers and their descendants more fit. However, because longer calls are costly to sustain, the same genes--perhaps for stamina or more efficient metabolism--may underlie both calling and the enhanced survival of the young.