 |
 Illustration by Helen Fitzgerald
Who loves ya, baby?
Having more offspring than your competitors is the key to
evolutionary success, but it’s not always easy to tell which adults
produce which young. Although close observation can reveal who
spends time with whom, recent advances in DNA profiling show that
time together doesn’t always mean what we think it means. Take, for
instance, a pair of songbirds working hard to feed their clamoring
youngsters.
“The assumption up until a decade or so ago was that when you
see a male and a female at a nest together raising young, that all
the young in that nest belong to that male and female,” says Mike
Webster. “And then somebody decided to test that genetically—and
surprise, lo and behold, not all the young in that nest belong to
those parents!”
It turns out that birds, long held up as models of dutiful
monogamy, are in fact randy little rascals, and many bird societies
are cauldrons of adulterous hanky-panky. Most kinds of birds engage
in at least an occasional extra-pair mating. Some mate with others
more often than with their partner. In the Australian fairy wrens
Webster studies, nearly half—half—of all chicks are sired by
a male other than their mother’s social mate.
Surprising as it was, that discovery helped explain something
that has puzzled evolutionary biologists ever since Darwin—the
bright plumage, dazzling songs, and flashy courtship displays so
common among male birds. The puzzle was how birds could evolve
traits that would make them more conspicuous to predators.
Webster says Darwin came up with the idea of “sexual selection,”
a form of natural selection in which competition for mates drives
the evolution of key features. If bright feathers and loud songs
enable male birds to have enough offspring to make up for the
greater risk of being eaten by a predator, those traits will
evolve.
There was a problem with that idea, though. Most songbird males
don’t seem to compete much for access to partners. In Webster’s
wrens, some of the males have bright red or orange plumage; others
are drab brown. Both kinds get a social mate without much fuss. So
where is the sexual selection?
The discovery of extra-pair matings may have solved that puzzle.
The males do compete—after they’ve found a social partner. And it
is quite a competition, complete with offerings of flowers to the
object of their desire.
“It’s a really beautiful display,” says Webster. “The males
present flower petals to the females. They’ll pick a flower petal,
and they’ll fly in and present it to the female. They fluff up
their feathers, and they dance around. It’s just spectacular.”
Webster wants to understand the competition from the perspective
of both the females and the males. Using observations of the adults
and paternity tests on every chick in a population, he’s found that
female fairy-wrens don’t mate with just any extra-pair male that
comes along. Rather, they show a strong preference for more
brightly colored males. Webster thinks a female accepts her first
suitor as her social mate, regardless of what he looks like, just
to get started on a family. Later she copulates with higher-quality
males if her partner is not such hot stuff. Webster doesn’t know
yet why females prefer the colorful males. Bright color may
indicate better food-finding skills or better resistance to
disease; or the preference may have arisen by chance.
For male wrens, stepping out seems more straightforward. Males
who mate with more females sire more offspring and pass more of
their genes on to the next generation. During one season, one male
Webster’s group studied sired 15 chicks, 10 with females other than
his social partner.
Extra-pair mating may also be a form of insurance for the
males.
“If a male has all his eggs in one basket, so to speak, or all
his young in just his social mate’s nest, and a predator gets that
nest, he has zero reproductive success for that year,” Webster
says. “From that perspective, I don’t think they care which female
they mate with. They just want to get their young out in several
nests.” And if flashy colors help them do that, there will be
strong selective pressure to go for the glitz.
Page
1
2
3
4
5
Continued
|
|
In which we offer a variety of side trips into the realms of
beetles, evolutionary developmental biology, systematics (how all
living creatures are related to one another), species formation,
sexual selection, and other fascinating places. Feel free to roam
at will.
| |
.jpg)
