My latest Mind and Matter column in the Wall
Street Journal:
What limits the size of a peacock's tail, the weight of a deer's
antlers or the virtuosity of a songbird's song? Driven inexorably
by the competition to attract mates, these features of animals
ought to get ever more elaborate. There was even once a theory-now
discredited-that the famously gigantic antlers of the Irish elk
became so unwieldy that they caused its extinction. Yet sexual
ornaments do not get ever bigger.
Now comes evidence from studies of peacocks and frogs that what
sets the limit is the ability of females to perceive any difference
between one level of elaboration and the next. Cutting the tails of peacocks short prevents
them from attracting peahens, but having a slightly larger tail
than a rival does not make much difference, because the peahen
cannot tell the difference. Likewise, female tungara frogs are
attracted to males with complex songs but cannot tell the
difference between elaborate songs and slightly more elaborate
ones.
Let me back up. The philosopher Herbert Spencer turned Charles
Darwin's theory of natural selection into a sound bite: "survival
of the fittest." But in his later book, "The Descent of Man,"
Darwin had a second theory: sexual selection. The sound-bite
version of sexual selection is "reproduction of the sexiest."
In other words, some features of animals don't help the
individual to survive-and may even hinder its survival-but they do
help it to reproduce, either by enabling it to compete against
rivals or by attracting members of the opposite sex. Darwin's idea
of sexual selection by female choice was all but abandoned by
biologists in the ensuing century. It seems that the idea of active
female sexual discrimination made male biologists
uncomfortable.
"Darwin posited the idea of sexual
selection, the sound-bite version of which is 'reproduction of the
sexiest.'"
By the 1970s, however, a series of experiments had conclusively
shown that Darwin was right and the ornaments, colors and songs of
(mostly) males had been selected by the persistent and
idiosyncratic preferences of (mostly) females. Once such a habit of
sexual selection gets going, a female that bucks the trend risks
having sons that cannot attract other mates.
In short, sexual selection seems to be a runaway process, like a
nuclear chain reaction: The more it occurs, the more it will occur.
Certainly it can produce bizarre results, burdening males with huge
plumes, exhausting dances or arduous songs. But it does not seem to
accelerate like a chain reaction. Peacocks' tails are not getting
bigger and bigger in each generation. They got big and then
stopped.
The explanation seems to lie in the neuroscience of perception.
As Mike Ryan of the University of Texas at Austin puts it, just as you can easily tell the
difference between a pile of six oranges and a pile of five, but
you cannot easily distinguish 100 oranges from 101 oranges, so a
female tungara frog in Panama can tell the difference between a
simple male song and an elaborate one, but not between an elaborate
and a very elaborate one. The song has evolved to the limit of
female discrimination.
An alternative explanation for the lack of runaway sexual
selection is that predators knock off the most elaborately burdened
males, counterbalancing the females' pressure. But Dr. Ryan's
experiments with tungara frogs prove that this is not the case. The
frog's main predator is a bat, which homes in on the frog's song,
and the bat is like the female frog: It shows no preference for
males whose songs are not just elaborate but very elaborate.
A recent paper by Roz Dakin and her colleagues at Queen's
University in Ontario comes to a similar conclusion about the
plumage of peacocks. The difference between the best and the good
peacock tail is too subtle for peahens to perceive. This news
should encourage the great multitude of peacocks whose plumage is
merely average.
