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A Head Start for Hunks

Male animals have evolved everything from showy feathers to flashy fins--all in the hope, scientists suspect, of strutting their genetic stuff to potential mates. But do females just sit back, pick a favorite, and hope for the best? Not zebra finches. After a liaison with an attractive male, the female gives her offspring a hormonal head start even before it hatches.

To see if a female's ardor has an effect on egg content, evolutionary ecologist Diego Gil, now of the Université de Paris X in Nanterre, France, and his colleagues took advantage of a peculiar taste of zebra finch females. The birds seem to find males wearing red leg bands particularly attractive, while tending to ignore males wearing green leg bands. No one is sure why certain colors are a turn-on, but because females pursue males with especially red beaks, it's possible that leg bands trigger the same reaction. Whatever the explanation, this fetish allowed researchers to vary a male's attractiveness and thus distinguish the effects of his sex appeal from those of his genes.

The team randomly gave males either a red or green leg band, then allowed each of six females to mate with a green-banded male. After collecting the eggs, the scientists mated the same females with red-banded males. Another six females mated first with a red-banded male, then received a green-banded suitor. Eggs fathered by red-banded males contained higher levels of the male sex hormone testosterone than did eggs fathered by green-banded ones, the team reports in tomorrow's issue of Science.

Other work in canaries has suggested that developing chicks that receive more testosterone from the mother beg more vigorously for food and grow faster than other chicks--whether they be male or female. Therefore, the researchers conclude, it is not clear whether the father's "good genes" or the mother's help should get the credit for any extra success enjoyed by offspring of an especially attractive father.

Why the offspring of attractive males should be accorded such favor remains a mystery. But the finding raises doubts about other experiments meant to show that attractive males really do pass good genes to their offspring, says evolutionary ecologist Doug Mock of the University of Oklahoma, Norman. The good genes theory "is a very sexy idea, but people will have to be careful" in testing it, he says.