Looking Out for Number One
Some animals help others of their kind, but their motives may be less altruistic than we think
- Bruce Fellman
- Dec 01, 1991
Here's a quiz: Suppose there's a colony of ten acorn woodpeckers in central California, and a couple of the birds are in a quandary. They've just reached breeding age and nature is "calling" them to go forth and breed. But their family is calling them, too. "Stay home," their kin seem to be saying. "Help out. We need you."
Question: What do they do?
According to Charles Darwin, the central principle of the natural world is "every creature for itself." Evolution favors the selfish. Naturally, one might figure the woodpeckers would heed nature's call and hunt for mates. Surprisingly often, though, that's not what happens.
Over the years, biologists have found more and more animals that behave like Mother Teresa instead of J. R. Ewing. For example, about two-thirds of breeding-age acorn woodpeckers never leave home. Instead, they become nature's "spinster" aunts and "bachelor" uncles. Like people who put off having their own kids, to help rear nieces and nephews, these unmated birds spend their time ensuring that relatives' offspring have the best possible start in life.
Such altruistic behavior, observed in animals as different as bees and bee-eaters, coatimundis and naked mole-rats, has long given researchers fits: It doesn't make scientific sense. As Paul Sherman, an evolutionary ecologist at Cornell University, explains, "To a biologist, an altruist is someone who seems to be breaking the rules of Darwin."
The Father of Evolution based his theory on the idea that individuals alive today are descendants of those that, for any number of reasons, were more successful than their peers at reproducing. The winners in this numbers game left the biggest families. It simply didn't pay for one animal to forsake reproduction to help another raise its young.
Now, however, scientists have determined that Darwin can rest in peace. Evolution is selfish after all. You just have to know where to look. British entomologist William D. Hamilton pointed researchers in the right direction when, in 1964, he declared that selflessness in animals was an illusion. "We like to say that Hamilton took the altruism 'out of altruism," quips Cornell bird biologist Stephen T. Emlen.
Scientists define altruistic behavior as an action that is detrimental to the giver and beneficial to the recipient. "Hamilton's ingenious solution to the altruism paradox was to show that behaviors we thought were costly in fact aren't," says Emlen. "The doers are actually helping themselves, either now or sometime in the future."
This intriguing theory, called "kin selection," prompted scientists to take a closer look at the lives of social animals. A number of independent studies around the world in the 1970s and 1980s--coupled with advances in scientists' ability to determine kinship among creatures--have cast a self-centered light on the subject of altruism.
Here's how Hamilton's theory works--and why you're happier working for your parents than your in-laws. When one acorn woodpecker helps raise another woodpecker's young instead of producing its own, the helper seems to be making a tremendous sacrifice. But according to the kin selection theory, behind that seeming generosity lurks a selfish motive. If the point of reproduction is to pass on genes, then sexual reproduction (the kind most familiar to people) has a fundamental limitation: Each partner can bequeath only half of its genes to its offspring.
But suppose there aren't enough mates or breeding territories to go around. Some birds are left out of the reproduction sweepstakes. In terms of genetic payback, rearing an offspring and rearing a full sibling are equivalent; the same is true for rearing a grandchild, niece or nephew.
Conclusion: In a world where living conditions are harsh, food is scarce, competition is fierce, predators are hungry and mates are few, it may be wiser to stay home and help raise relatives than risk leaving home and trying to find a mate to have children of your own. Sacrifice, as it turns out, isn't so selfless at all.
Genetic self-interest may be the tie that binds animal societies together. Consider the naked mole rat, a subterranean denizen of western Africa. Sherman has long studied the apparently benevolent behavior of this spectacularly ugly creature (imagine a wrinkled bratwurst with tusks). He explains that each mole-rat colony contains a queen, several male consorts that breed with her and an entourage of "sociologically sterile" helpers, which stay out of reproductive matters.
To understand what's in it for the helpers, Sherman and his colleagues at Cornell use DNA fingerprinting to determine how many genes the 75 or so colony members share. The answer is plenty. "Individuals from the same colony are virtually identical; they're almost like clones," says the biologist, who has measured genetic similarities among mole-rats as high as 90 percent. (The similarity between human siblings is 50 percent.)
Such genetic closeness comes from generations of inbreeding. "The naked mole-rat is by far the most inbred mammal we know of," notes Sherman. "About 80 percent of the matings are between parents and offspring, or between brothers and sisters." Forced for safety reasons to stay at home, mole-rats end up breeding with their kin. After many generations, colony mates wind up more closely related to each other than they could possibly be to their offspring. By staying home and rearing relatives, the creatures preserve far more of their genes than if they tried to establish their own colonies.
Many of the best studies demonstrating less-than-altruistic altruism have involved birds. "If you're talking about rearing someone else's kids," says Emlen, "that behavior is more common in birds than other kinds of vertebrate animals."
Sexually mature Florida scrub jays, for example, often stay home to help their parents raise young--a seemingly noble gesture. But don't be deceived, says Glen E. Woolfenden, a biologist at the University of South Florida who has studied the species for more than two decades.
"Sure, they'd do better to go out and breed on their own," he says. "But frequently there's no quality place to go and no one to go with. By helping, they're furthering their own genes while they wait for a breeding slot to open up."
Acorn woodpeckers offer another good example of what Woolfenden calls "selfishness through cooperation." The handsome black-and-white birds live in colonies of as many as a dozen birds. Members spend much of their time gathering acorns and stuffing them into thousands of holes pecked in the thick bark of the family "granary." These natural storage bins, often drilled into oak trees, are critical to woodpecker breeding success.
"The granary is a valuable piece of real estate for the birds, and they do poorly if they don't have access to one," says Walt Koenig, an acorn woodpecker specialist with the University of California at Berkeley. "Dispersal is risky business, so no matter how bad it is living in a group, it's probably better than the alternatives."
As long as the woodpecker is staying home, it might as well perpetuate the family genes by helping rear a relative's young. Besides, the neighborhood is a safe "home base" from which young birds can seek out breeding opportunities in other colonies. The quest often leads to what Koenig calls "power struggles," which are usually won by those woodpeckers that arrive with reinforcements--say, a brother or sister or two. "It's possible," says Koenig, "that by helping to raise siblings, these birds may be more likely to have a large support group to fight with."
Another seemingly altruistic species with selfish ulterior motives is the white-fronted bee-eater. This East African bird lives in extended-family clans of 10 to 14 with a membership that typically spans several generations. Within these groups there Wiry be a handful of breeding pairs and lots of potential helpers.
And a good thing, too. Raising youngsters on the cliff faces of riverbanks is hard work, sass Stephen Emlen, who studied bee-eaters in Kenvd. "These birds live in a harsh environment, so, from a breeder's perspective, it's advantageous to get as many helpers as possible. The helpers have an absolutely awesome effect on breeding success."
Nearly half of unpaired bee-eaters Emlen studied, however, did no volunteer work at all, and those that did were extremely picky about whom they served. "In 90 percent of the cases where helpers had a choice," he says, "they chose to help the closest available relative." In doing so, they also were indirectly helping themselves by perpetuating their own genes.
Breeding pairs need all the help they can get, and that can lead to interesting conflicts within the family group. Indeed, life among the bee-eaters has all the intrigue of an episode of "Dallas."
One of the helpers' tasks is to guard the nest to keep other bee-eaters--usually females lacking nests of their own--from sneaking eggs into the clutch, a practice known as "egg parasitism." Successful egg parasitism is tricky, says Emlen. Not only does the intruder have to get past the sentry (often the daughter of the breeding pair), but her timing has to be perfect. A nesting bee-eater will pitch any egg that shows up in her nest before she begins laying her own. Once she has started, though, she will accept interlopers' eggs.
It seems a case of altruism pure and simple, but there's a twist: The egg-bearing intruder is often the security guard herself. "The daughter has the inside track on how to sneak in an egg," says Emlen. "The extra burden on the nesting pair from that one egg is great, but if the parasite is a daughter, they at least get her help." For a price.
Nor is there true benevolence on the male side of the family, as Emlen has discovered. Fathers often chase away or otherwise harass their sons' mates to disrupt their breeding efforts, then conscript the young males as helpers. Among the youngsters' duties: to guard the nest, bring food for their mother and other siblings, clean out the nest area and sound the alarm when danger threatens.
While the stay-at-home males may abandon their own nesting plans to work for the family genes, their mates stay out of the labor pool entirely. "The in-laws don't raise a feather," says Emlen. "There's nothing in it for them."
People help strangers, even die for their country. But there are few heroes in the natural world, where only the closest of kin get the time of day. Among animals, it seems, blood truly is thicker than water.
Connecticut writer Bruce Fellman helped raise two teenage stepchildren.