From the WSJ:
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
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