Females Don't Always Go for Hottest Mate
At first glance, the "sexy son hypothesis" makes perfect sense. According to this pillar of evolutionary biology, a female who chooses a high-quality male will have sons who inherit dad's allure. They, too, will therefore have their pick of females, allowing mom to hit the jackpot: grandmotherhood. But when scientists followed male flycatchers whose dads were real catches (as judged by a forehead patch that is this bird's equivalent of perfect abs), they found no such thing. The sons "did not inherit their father's ... mating status," the Swedish researchers wrote in the February issue of American Naturalist. As a result, mom got fewer grandkids than did females who settled for less-attractive males. The studs were so busy mating they had no time to raise offspring, causing their health and fecundity to suffer. Homelier birds were better dads, raising sons who had more mating success. Poor Darwin. After he developed his theory of how organisms change through variation and natural selection, his thoughts turned to sex. Because females have few eggs (compared with males' limitless sperm), their best strategy is to select the highest-quality males for mates, he wrote in 1871. That way, their progeny also would have superior traits. The offspring would survive and reproduce better, making mom's fondest wish -- to become a grandmother -- come true. (In evolution, success means reproduction, not only for you but for your descendants unto the nth generation, too.) The theory of sexual selection -- that females choose males with the best genes, causing those genes to become more prevalent in succeeding generations -- is invoked to explain why peacocks have rococo tails and bucks have huge antlers. Neither trait has real survival value, but females choose males that have them, exerting selective pressure for ever-showier versions. Or so textbooks say. Just as Darwin's theory of natural selection is under attack by America's religious right, his less-known theory of sexual selection is catching flak from some biologists. "In a number of species, reproductive behavior does not conform to Darwin's theory of sexual selection," says biologist Joan Roughgarden of Stanford University. "The idea that females choose the genetically best males is wrong. Instead of choosing mates who will increase the genetic quality of their offspring, females make choices that will increase their number of offspring." As in the flycatcher study, mating with "sexy" males isn't necessarily the way to a plethora of descendants. True, in species where males contribute nothing but genes to offspring, this strategy may work. But biologists are finding more and more examples where females benefit from a different strategy. Female crickets mate with just about any male that asks, for instance. Through promiscuity, not choosing the "best" male, they increase the genetic diversity of their offspring, improving the chances that some will survive no matter what pathogens and enemies the kids encounter. Other females are not as enamored of sexy traits as theory claims. While big-antlered red deer are busy fighting each other to show a female who has the best rack, the doe sneaks off to mate with less well-endowed stags. Female red-winged blackbirds are not easily impressed, either. Having the most macho plumage has no detectable effect on how many offspring a male sires, David Westneat of the University of Kentucky reported in American Naturalist this week. Nor is flaunting their charms and competing against other males necessarily the best reproductive strategy, as Darwin thought. In some species, cooperation can bring greater success. Bluegill sunfish, for instance, form trios of one small female, one large territory-holding male and one small male that infiltrate that territory when the female releases her eggs. That lets the little scrawny guy, despite the lack of female-attracting heft, become a dad. Such strategies, Prof. Roughgarden says, show that "each kind of male has its own way of going about its life. Each works out fine." As she and colleagues wrote in February in Science, "animals cooperate to rear the largest number of offspring possible." Another problem with sexual selection is that it fails to explain the persistence of, shall we say, homely males. If females choose the male with the best traits, as claimed, then after enough generations every peacock should have a tail to die for. But they do not. Every flock has studs and duds. "Shouldn't all the tails be great?" asks Prof. Roughgarden. Other scientists are not ready to jettison sexual selection, calling it (as biologist Jerry Coyne did in a review) "powerful and largely correct." But some aren't so sure. Primatologist Sarah Blaffer Hrdy (pronounced "herdy") calls it "ill-advised" to "give precedence to [females'] quests for supposedly the 'best' genes" when they choose a mate. Mating can indeed be a competitive sport (see: spring break). But many traits that attract females have nothing to do with good genes. For mysterious reasons, females just developed an attraction for them. Men on a quest for perfect abs can take that as fair warning.
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