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March 3, 1998
Birds' Design for Living Offers Clues to Polygamy
By NATALIE ANGIER
oor Monica Lewinsky. She thought Linda Tripp was her best friend, but Ms. Tripp proved treacherous, a wiretapped Anne Baxter to her feckless Bette Davis in the Beltway farce, "All About Eavesdropping."
If Ms. Lewinsky were a European oystercatcher, the scenario would be reversed. She would think she knew who her enemies were, only to watch her worst rival be transformed overnight into her closest pal.
Scientists have found that female oystercatchers, mid-sized wading birds found along the shores of many European countries, engage in a type of cooperative behavior rarely seen in nature, let alone in a bird known for its scabrous temper and a general hostility toward others.
On occasion, two females that have spent weeks or months fighting viciously over a choice piece of territory, and the male settled therein, suddenly seem to decide, as though in a bolt of Amazonian revelation, that there might be value in sorority. They stop fighting and start preening each other. They softly pip-pip-pip, the oystercatcher's "Song of Songs."
They agree to share the territory and the male. They take turns copulating with him, and then they seal the pact by copulating with one another. They nest together. And woe to any predator, or designing oystercatcher, who might venture into the triad's territory.
The three birds will defend their home and nest with synchronized wrath.
The discovery of such an unusual and flamboyant form of female cooperation demonstrates how behaviorally supple even a sexually conventional species can be.
As a rule, oystercatchers are monogamous birds, with one male and one female pairing up to defend a territory and rear their chicks. But the environment in which they breed is so mercilessly competitive that some birds are driven to try radical alternatives like polygyny (the mating of one male with two or more females) and homosexuality -- anything to shake up the status quo and get their claw in the door. The results have oblique implications for human mating strategies.
Scientists from the University of Groningen in the Netherlands had been studying oystercatchers for almost a decade before a team member, Dik Heg, made the first sighting of female-female consortship. When he saw it, his jaw practically dropped to his mud-splattered field boots.
"I thought I was crazy, I thought I was imagining things," he said in an interview. "They were two females who had spent the whole winter fighting, very aggressively, and then one day, there they were, copulating."
Dr. Heg and his colleague Rob van Treuren described their findings in a recent issue of the journal Nature.
"This is one thing you wouldn't have expected in oystercatchers, but there it is," said Dr. Bruno J. Ens of the Institute of Forestry and Nature Research in the Netherlands, who wrote a commentary that accompanied the report. "Of course, you're talking about a bird that can live to be quite old, 40 years or longer, which gives it a bit of time to build up complicated relationships."
In fact, the oystercatcher's impressive longevity is one reason the competition is so brutal.
With a limited amount of desirable breeding grounds and the tendency of established pairs to hang onto the choicest spots year after year, there is always a large pool of rapacious nonbreeders surveying the scene in search of weaklings to depose.
"It's a terrible social situation," Dr. Heg said. "It's a very harsh life."
Once considered a pest species because it dared to feed on a bivalve prized among people, the European oystercatcher, Haematopus ostralegus, now subsists mostly on mussels, cockles and various types of worms. They are handsome birds, slightly smaller than crows but just as squawkish, with raven-dark backs, white bellies, red beaks, red eyes and bright pink legs.
It is every oystercatcher's fondest dream to settle down with a mate on a high-quality breeding ground and start laying eggs.
The birds nest on land but get their food from the water, so the most valuable terrain is on the edge of the shore.
And it is over these waterfront properties that the most ferocious, and creative, clashes occur. When challenged by an outsider, couples defend their territory as a unit. They posture aggressively and flaunt their solidarity, mostly by copulating.
"We think that copulations function mainly as a display against the neighbors, to show that here is a cooperating pair willing to defend its territory," Dr. Ens said.
The most violent clashes take place between a resident female and an intruding female hoping to steal both her territory and her mate. The interloper starts in with a harsh, loud, repetitive scream.
She points her beak downward, mouth agape, the picture of oystercatcher menace.
The resident female retorts in kind, piping and pointing.
They run back and forth in parallel, beaks pointed toward the ground. They peck at each other and aim for the eyes.
Hour after hour, week after week, sometimes month after month, the struggle continues.
The male intermittently rises to his mate's defense, but it is the female that feels most threatened in this case, and thus fights the hardest.
Feathers fly. Blood may flow. A wing may snap. Sometimes the resident female wins, and chases the interloper away. Sometimes the invader succeeds in ousting her rival.
Sometimes, through a mysterious, mutual signal, the two of them call it a draw.
They stop arguing and start to act like a pair of lovebirds. "They follow the dictum, 'If you can't beat 'em, join 'em,' " Dr. Ens said.
It is a rare outcome to conflict, involving fewer than 2 percent of all female oystercatchers, but it is nonetheless performed with great zeal. The new friends use the same preening behaviors that males and females rely on to reinforce their bond, and to brandish that bond before their neighbors.
And so they copulate, hour after hour, day after day, taking turns playing the male on top and the female on the bottom, as well as taking turns with the resident male.
Eventually, the cooperating females lay fat clutches of eggs in one big nest.
The strategy should work.
As a threesome, the birds present a more formidable front to the world than a couple ever can.
But somehow, the naturally monogamous oystercatchers do not quite have the mechanics of polygyny down pat. The double dose of eggs turns out to be too big for any single bird to brood, and the birds fail to brood communally. Instead, the three of them alternate their stint on the nest, the way each parent in a monogamous couple will, with the result that few if any of the eggs are kept incubated well enough to hatch.
"It's kind of sad," Dr. Heg said.
If many of their eggs fall flat, why in the name of Alfred Hitchcock would the birds bother cooperating? The Dutch scientists have determined that polygyny for the oystercatcher is a short-term strategy with long-term benefits. Females generally cooperate for just one season.
And the next year, the interloping female has a much better shot at getting her own breeding ground nearby than she would if she were to come in cold as an outsider.
On average, a nonbreeding female floater has has only a 9 percent chance of breeding the next year.
By contrast, a female who has been in a cooperative troika has a 67 to 73 percent chance of breeding monogamously in the next season.
As for the resident female, she gains little from the capitulation beyond peace of mind, an end to a skirmish that she could not win, and perhaps an innate understanding that the polygynous compact will soon expire.
Scientists also propose that the cooperative behavior might be an evolutionary work-in-progress, and that eventually the birds will figure out how to brood their eggs cooperatively, yielding a large flock of offspring, a genetic bounty for all.
"Oystercatchers can learn a lot of things in life," Dr. Ens said. "They're more intelligent than we think."
Oystercatchers may be far removed from Homo sapiens taxonomically, but to some evolutionary biologists, human family arrangements have more in common with some bird families than with the lives of other primates. Males and females couple up, and fathers contribute to the welfare of their young.
But because a man theoretically can sire far more children than any one wife can bear, a number of theorists have insisted that men are polygynists at heart, rather than the loyal menschen of sentimental comedy. They insist that most cultures, historically and prehistorically, have been polygynous, and that men will always acquire multiple wives when they have the economic and political wherewithal to do so.
Recently, a few evolutionary psychologists and their popularizers have suggested that polygyny not only benefits men, but also women. They propose that a woman may do better, reproductively speaking, to be one of many wives of a wealthy man than the exclusive partner of a financial loser.
At least some Mormon women appear to agree.
Elizabeth Joseph, a Utah lawyer and journalist who is one of seven wives of a Mormon man, recently extolled the benefits of polygamy at a meeting of the National Organization for Women.
"It's helpful to think of polygamy in terms of a free-market approach to marriage," she said.
"Why shouldn't you or your daughters have the opportunity to marry the best man available, regardless of his marital status?"
Ms. Joseph presented polygamy as "an empowering life style" for women.
"The women in my family are friends," she said.
"You don't share two decades of experience, and a man, without those friendships becoming very special."
Yet some theorists say that as the oystercatcher study shows, the ostensible benefits of polygyny should not be counted before the evidence has hatched. In some African societies where polygyny and monogamy coexist, the first wife in a plural marriage does, indeed, tend to thrive, and may have as many children as the wife in a monogamous marriage or more.
But the subsequent wives of a polygamist often fare worse than their monogamous counterparts, bearing fewer children and receiving comparatively little of their husband's wealth.
"A lot of times, females enter into a polygamous arrangement because it's the best of a bad situation," said Steve Josephson, who has studied polygamy in different human societies and is completing his doctorate at the University of Utah.
"If it's polygamy or nothing, it's obvious a woman had better opt for polygamy. If it's polygamy or monogamy, she'd better examine her options very carefully."
Birds can change their minds, Josephson said, and abandon polygyny for monogamy as soon as the opportunity presents itself. For a woman, however, packing up and leaving may not be an option. "If she chose polygyny and she chose wrong, that could be the end of her," he said.
Polyandry, anyone?
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