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wearing a red leg ring over males without one. The females show little interest in males with green leg bands. This discovery saved the researchers from having to figure out which finches are the best-looking. It turns out to be a question of choosing the proper accessories.
    It’s impossible to know for certain why the females prefer males adorned with red rings, but female zebra finches find males with big red cheek patches extremely attractive, Curley said, and the red rings could somehow be mimicking the cheek patches. Even without a firm explanation, this was a phenomenon the researchers could use to their advantage. They put red leg bands on half a group of male finches and green leg bands on the other half. Then they compared the offspring of the attractive males to those of the homely green-banded males.
    The offspring of the attractive red-banded males were found to have distinct advantages. They begged for food more often than the others and were rewarded: mothers gave them more food. Females laid eggs containing more growth hormones when the eggs had been fertilized by the attractive males. You might guess that the attractive fathers simply had better genes, but that wasn’t the case. Somehow, making the male finches more attractive encouraged mothers to devote more resources to the offspring. The attractive males didn’t have better genes than their green-ringed competitors, although the females might have been tricked into thinking that they did.
    Curley called the findings so unexpected as to seem ridiculous. How could a colored leg band have such an important effect on mothers’ behavior? He decided to see whether he could replicate the experiment with his mice, comparing males raised in isolation to “enriched males” raised in a more natural environment. Then he mated each one with a female. The females who mated with the enriched males devoted more resources to their offspring and engaged in more thorough maternal behavior. It was similar to what was going on with the finches—females invested more in their offspring when they had a more desirable mate.
    Encouraged, Curley did another test, this one with stressed and normal males. Females who mated with normal males nursed and licked their offspring more often, and their pups exhibited less anxiety than the offspring of the stressed males. It was yet another demonstration of the same effect: making the males more desirable turned the females into better mothers. And that was good for their pups.
    Continuing along these lines, Curley looked at whether a male’s anxiety could affect his pups in the same way that stress did. To produce high-anxiety males, he took males out of their cages and dropped them into unfamiliar enclosures. Those who were the least willing to explore their new surroundings were the mice with the highest anxiety. He bred these males with females and found that the daughters of the high-anxiety fathers exhibited similar symptoms. The pups were raised solely by their mothers. The researchers concluded that marks on the fathers’ sperm were being passed on to affect daughters’ behavior, independent of any change in mothers’ behavior. (These marks are referred to as epigenetic changes, because they change the operation of genes—whether they are turned on or off—without actually changing the DNA.) And the sons did not inherit their fathers’ anxiety. This, too, parallels other findings. The nutritional status of the Överkalix grandfathers affected only their sons, not their daughters. It’s clear that some of these effects apply only to sons and others only to daughters. The inability to explain this is a sign of how much more researchers need to find out about these odd generational effects.
    Curley and his colleagues are now exploring a gene called Peg3 that likewise has different effects on sons and daughters. The name stands for “paternally expressed genes”: in this family of genes only the father’s copy is