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BIOLOGY
Birds Do It
If warblers, robins and other models of monogamy are doing
it, must we then conclude that extra-pair copulation--or
adultery--is natural?
June 02, 2002|MARLENE ZUK | Marlene Zuk is a professor of biology at UC Riverside and the
author of "Sexual Selections: What We Can and Can't Learn About Sex from Animals."
RIVERSIDE — In the animal behavior class I teach, students seem interested in one thing above all
others: infidelity. As one student put it recently: "Last night on TV they said that cheating on your
spouse is natural, because animals do it, so you expect people to do it too. Are we going to talk
about that in class?"
In fact, it is something we talk about in class--although not in the way it's discussed on late-night
television. Since the advent of DNA-based paternity analysis, scientists have begun intensely
studying infidelity in animals, particularly in birds. It is now relatively easy to sample the DNA
from a set of chicks in a nest and compare it with the DNA of the male and female associated with
the young. As it turns out, so-called extra-pair paternity is quite common among birds. The
percentage of offspring sired by males other than the one attending a female and her nest varies
widely from species to species, from 0% in snow geese to a whopping 90% in a species of brilliantly
colored Australian fairy wren.
This discovery was quite a shock to scientists because outwardly it appeared that most birds were
monogamous. Birds have always looked so admirable, so industrious. The way the male and female
rush back and forth to their demanding brood of chicks seems like nature's model of good parenting.
And now we find that they're actually in the same situation as millions of modern-day husbands and
wives, eyeing a child warily and making uneasy jokes about the milkman.
My students are no less shocked than the scientists. In fact, they are horrified. If warblers, robins and
other models of monogamy are doing it, they worry, do we then have to conclude that extra-pair
copulation--or adultery--is natural and expected, part of our evolutionary heritage and therefore
nothing to make a fuss about?
But the truth is, the birds aren't "cheating"; they are just doing what they do. They don't have rules
about the pair bond between a male and female, and it isn't cheating if there are no rules to break. If
we try to use their behavior as a model or justification for our own, we might find ourselves making
decisions about morals on very shaky grounds. Because chimpanzees masturbate publicly, does that
mean humans should?
We can make lists of things humans do that animals don't--pay taxes, watch television, wear socks-or things they do that we don't--lay eggs, change the color of our skin to match our surroundings,
store sperm in the uterus for years. We are understandably fascinated with the attributes we share
with some animals, like mate choice, parenting and craving sugar. But trying to model our behavior
on the traits of other species is pointless. It brings to mind a new version of an old parental
admonition: "If all the lemmings jumped off a bridge, would you jump too?"
In the end, modeling human behavior on animal behavior is absurd because for each species that
exhibits an "admirable" behavior by human standards, you can probably find one that's
"contemptible." And it's equally silly to be disturbed by what the animals do. Yes, we've discovered
some philandering finches, but real, true, till-death-do-us-part monogamy is still seen among many
animals, including snow geese and some sea birds.
Still, humans of all ideological stripes seem to feel compelled to assert that their viewpoints are
supported by nature itself. Traditionalists note that, with few exceptions, males in nature are
physically larger and behaviorally dominant--at least in many vertebrates--and that females tend to
play the dominant role in nurturing infants. Feminists respond by noting nature's exceptions-bonobos have lesbian sex, females in most species are the sex that does the choosing during mating.
And of course feminists can usually end the discussion by bringing up the example of praying
mantids, whose females eat the males after mating.
Ultimately, though, these arguments are pointless. Humans live in societies of their own making,
and we have shaped society's rules not so much to conform to nature but in pursuit of the common
good. Murder is illegal because it destabilizes society. So what if intra-species killing is common in
certain parts of the animal kingdom? I like to think we are perfectly capable of choosing our own
visions of an ideal society without the help of snow geese or wrens.
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