Why We Take Risks
DISCOVER
Vol. 22 No. 12
Richard Conniff
December 2001
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When it comes to evolution, survival of the fittest is only half the story. The handicap principle holds that humans make showy and sometimes dangerous displays of courage to increase their status and attract mates.
Two thirds of the way into his August 1998 attempt to fly round the world by balloon, Steve Fossett ran into a thunderstorm at 29,000 feet above the Coral Sea and began to plunge uncontrollably as wind and hail whipped his ruptured balloon. At 4,000 feet, he climbed through the hatch atop his capsule and cut away the fuel and oxygen tanks to slow the descent. Then he lay down on a bench to distribute the impending impact across his back. "I'm going to die," he said out loud.
I'd met Fossett the year before, and he was mild and Midwestern, a multimillionaire with no particular need for publicity. Now he was falling out of the sky in a broken balloon. Why? For that matter, why did another American businessman recently pay $20 million to get himself launched into space on a Russian rocket? Why do ordinary people climb Mount Everest?
Risky behavior might seem like just a quirk of the oddly named species Homo sapiens, except that a taste for grandstanding is common in the natural world too. For instance, antelope pursued by hungry cheetahs often leap acrobatically straight into the air, a practice called stotting. Common sense says they should be sprinting straight for the far horizon. Even lowly guppies dance right under a predator's nose before darting away. Why do humans and animals alike do such dumb stuff? Stuff that is unnecessary, flamboyant, and often downright deadly?
In search of answers, I found myself at five o'clock one recent morning rattling across an Israeli desert in a dusty little Peugeot with Amotz Zahavi, the 73-year-old b te noire of the biological world. "This is a minefield," Zahavi said, indicating a fenced-off area just to our left. He veered right, both hands on the wheel, down into a wadi, or dry riverbed. "So we won't go there." We were looking for Arabian babblers, birds he has been studying for 30 years at the Hatzeva Field Station near the Jordanian border. The babblers, when we found the first group a few minutes later, were brownish, forward-leaning birds, about the size of mockingbirds, with long tails and sleek heads. Unflamboyant. Not, at first glance, worth the trip. But Zahavi introduced them as old friends, with names corresponding to the letters on their ankle bands: Pusht (PVST), Taxas (TXXS), Tasha-Sham (TSSM), and so on. The birds also knew Zahavi. They gathered at his feet and cocked one eye up, waiting for him to toss a bread crumb or the occasional mealy worm.
As Zahavi fed them, he rattled off individual stories out of a Middle Eastern melodrama: A brother killed in a trap three months ago, a mother forced to become a refugee, an exiled stepsister, Zatash (ZTAS), who'd returned and pushed her way back into the group. He knew the birds better than most people know their human neighbors, better perhaps than the birds knew themselves. "Go and copulate, lady," he commanded at one point, a little vexed with one coy babbler. No aspect of their lives was too trivial for Zahavi to mull over. "You sit here in the desert and say, 'Why is it like this and not like that?'" he remarked as we huddled under an acacia one evening waiting for a group of babblers to come to roost. "Being alone in the desert, all these things creep in sooner or later."
One of the things that crept in during the early years of his research was the far-ranging and controversial idea for which he is best known. Zahavi's handicap principle attempts to explain why babblers risk their lives by yelling at predators, why peacocks carry splendid but cumbersome tails twice the length of their bodies, and even perhaps why Ted Turner gave $1 billion to the United Nations. Zahavi's handicap principle holds that animals and humans alike prosper not in spite of our riskiest and most extravagant behaviors but because of them. These behaviors are the way we advertise how prosperous, how fit, how fearless we are. And because the world is a jaded, cynical place, we have to incorporate a significant cost, or handicap, in our advertising to make it persuasive. Thus antelopes really are indulging in a dangerous waste of energy when they stot in front of a cheetah. But their willingness to risk it is how they tell the cheetah: "Don't even bother trying."
When Zahavi first proposed this idea in the 1970s, the biological establishment reacted as if something bad had gotten stuck in its throat. In the first edition of his book The Selfish Gene,Oxford evolutionist Richard Dawkins called Zahavi's handicap principle "maddeningly contrary," and he wrote, with a cadence and clarity rarely seen in scientific writing: "I do not believe this theory." Robert Trivers, a Rutgers University evolutionist, used to kid Zahavi that if you took his idea to the logical extreme, you'd end up with a bird where both sexes fly upside down to show each other how good they'd be if they were flying right side up.
It didn't help that Zahavi was himself maddeningly contrary. The standard scientific practice of testing ideas with mathematical models was alien to him. He hatched his ideas based on observation and intuition alone, and he tended to question the intelligence of those who failed to embrace his conclusions, including prominent scientists who had risen by more traditional routes. When I reminded him of Trivers's joke one day, Zahavi said, "But they do fly upside down," and proceeded to rattle off bird species in which somersaults and other inversions are part of the mating display.
As we moved from one babbler group to the next, Zahavi was clearly happiest when he was being provocative. He argued, for instance, that a baby screaming for attention at the nest is actually blackmailing its parents, in effect saying, "Fox, fox, come and eat me. My parents don't care." Children on the sidewalk screaming at an unresponsive parent do the same thing, he added, and this is why, when they run away, they often run toward danger rather than away from it. Suicide is also an instance of the handicap principle, he said, a bid for help that we take seriously in direct proportion to the individual's actual risk of death. Indeed, Zahavi's own conversation seemed like a prime instance of the handicap principle, a contra dance at the edge of the unlikely. He summed up his big idea in a phrase: "Something can be good because it's bad."
In spite of Zahavi himself, evidence for the handicap principle began to accumulate. One study showed that African wild dogs and hyenas in fact avoid animals that stot, apparently because nonstotters are easier to catch. More important, an Oxford biologist named Alan Grafen demonstrated with a mathematical model that the handicap principle made sense in evolutionary terms. In the second edition of his book, Dawkins grumbled about the possibility that "theories of almost limitless craziness can no longer be ruled out on commonsense grounds." But he added, "if Grafen is right—and I think he is—it might even necessitate a radical change in our entire outlook on the evolution of behavior."
In fact, Zahavi's principle addressed one of the central problems in evolutionary thinking. Charles Darwin is of course best known for his theory of evolution by natural selection. But in his 1872 book, The Descent of Man, he developed an equally important idea, largely ignored until the mid-20th century. Evolution by sexual selection holds that genetic change is also influenced by the ability to attract and win access to members of the opposite sex. The trouble with these two ideas is that they often seem to contradict each other.
Natural selection essentially means that nature weeds out unfavorable traits by killing individuals who display them. Thus an arctic fox with a red bull's-eye on his side would quickly end up in a polar bear's belly. Individuals with more favorable traits, such as plain white fur for camouflage in snow, tend to survive and reproduce.
But in most species, females control mating, and they often have an irresistible attraction to the male with the equivalent of the red bull's-eye on his side. That is, they seem to select males for traits that make them less likely to survive. In peacocks, for instance, a bigger tail requires the male to waste huge amounts of energy. It also inhibits his ability to fly and makes him more vulnerable to predators. The female's own drab coloration attests to her abiding faith in the value of camouflage. But she will nonetheless choose the male with the bigger, showier tail nearly every time. The natural world is full of females falling hard for stupid male display behavior, including bright feathers, big antlers, and bombastic courtship rituals. Then again, male sexual selection sometimes forces irrational displays on females too. Most men prefer women with large, cumbersome breasts, for instance, though small breasts are just as good for nursing babies. It is, says Zahavi, a handicap, a cost ancestral women undertook to display greater nutritional fitness in the form of visible body fat.
Given the ruthless efficiency of the natural world, how could such costly traits have evolved in the first place? The classic explanation enshrined in biology textbooks is the "runaway process," a variant on sexual-selection theory devised by the British mathematician R.A. Fisher. Let's say ancestral peacocks started out dull-eyed and unbushy-tailed. Then, through some minor genetic shift, a few females developed a random hankering for males with slightly longer tails. If this flourish happened to occur in bigger, better males, then females who chose them would probably rear more offspring. Longer tails would proliferate in males, and the preference for them in females. So far so good. But the conventional understanding of Fisher is that females soon come to focus single-mindedly on this trait, regardless of a male's overall quality. And thus begins the cycle of runaway one-upmanship: Over thousands of generations, the peacock's tail continually lengthens and spreads out until no self-respecting male can get a date without a huge fan rattling on his rear end. Natural selection steps back in to stop the runaway process at the point where the quest for better sexual ornamentation is attracting enough predators to kill the hapless males.
The trouble with this idea, a student of Zahavi's argued one day, is that sexual selection isn't that whimsical. In the real world, females rigorously test males to see if they will make good mates (and males of course test females, too, though perhaps less rigorously). As Zahavi thought about it, he came to think females would choose an exaggerated trait only as long as it reliably displays that the male would be a better mate. So what good does a huge tail do for a peacock? And why would Irish elk have developed antlers with a 12-foot span?
Zahavi came up with a theoretical answer, then confirmed it one day while watching his babblers. He was trying to figure out why the babblers kept shouting at a hawk, instead of just quietly hiding in a bush. "And then I realized, they are talking to the predator." They were taking on a handicap by revealing their presence to the hawk—and they were advertising that a surprise ambush wasn't going to work. It convinced Zahavi that flamboyant handicaps—the babblers' shouting, the peacock's tail, the elk's antlers—actually serve a useful purpose: The same displays that attract females also discourage predators and rival males.
This was the basic idea Grafen refined into the language of theoretical mathematics. He started with the assumption that females could use handicaps as a gauge for selecting the fittest males. For example, the arctic fox with the red bull's-eye wouldn't survive a single winter unless he also happened to be incredibly quick or cunning. Then Grafen calculated the advantage to the daughters in inheriting greater fitness. Finally, he added in the advantage to the sons who get saddled with Dad's dumb displays but thus attract more females and scare off more rivals. Grafen fine-tuned his model so that sons actually express Dad's handicap displays only in proportion to their own physical abilities. Thus Dad may stot like a champion, but if Number Two Son lacks zip, he soon drops out of the evolutionary equation in the form of fast food. When Grafen put all these calculations together, his model showed that, over time, the handicap principle should result in more offspring.
Zahavi did not wait for this moment of vindication. He'd been sure of his idea from the start, and he had been busily extending the principle into every known corner of the universe. In particular, Zahavi argued that it explains the evolutionary puzzle of altruism. In the narrowest Darwinian terms, acts of charity make no sense. Giving away food or other resources represents an apparent reduction in Darwinian fitness, a loss in the donor's own ability to survive and reproduce. So altruism should long ago have disappeared from the gene pool. But seemingly selfless acts are commonplace in the human and natural worlds. Other biologists have tried to explain this awkward fact through mechanisms like kinship selection (the idea that it pays to get your nephew Louis a job because he's carrying a quarter of your genes into the next generation) and reciprocal altruism ("You scratch my back, I'll scratch yours"). But Zahavi looked at the question from a radically different angle.
In the desert one morning, Zahavi stood over his birds, fondly rattling off a nonstop narration of their every chirp and shimmy. He is a grandfatherly figure, with steel-gray hair brushed up willy-nilly, bushy eyebrows, pouched eyes, and a paunch stretching his blue T-shirt out over the top of his gray shorts. He often imitates his birds, drawing himself up, hands pinned to his chest like wings, and saying, "WEET-WEET-WEET," or slumping his shoulders to mutter a timorous "weet." The effect is comical. But Zahavi believes in truth in advertising. That is, a handicap display must correlate with real quality, and a bird can only give a threatening shout if its body is upright. So imitation is a means of understanding: "If you are good at it, you can decipher from the sound what is happening in the body of the signaler." And Zahavi is good. "He's one of those naturalists," says Robert Trivers, his friend and sometime adversary, "who, if he were made a babbler tomorrow, would know most of the right moves to make."
Zahavi tossed a bread crumb, and the recipient, Tasha-Sham, did not gobble it down. Instead, he flitted up onto a tree where the subordinate male, Pusht, had been doing sentinel duty. Pusht saw what was coming and slipped away. But Tasha-Sham followed him to the ground. Then he held up the prize morsel until Pusht dutifully begged like a nestling, mouth open, wings quivering with sham enthusiasm. Zahavi interpreted: "The dominant says, 'You'll take what I'm going to give you.'"
Despite the babblers' drab appearance, an almost obsessive concern with status seemed to color every aspect of their lives, and altruism was one of their favorite forms of showing it. Tasha-Sham announced his act of charity by lifting his beak and giving a special trill, like a socialite posing for an event photographer at the Red Cross Ball. Watching this sort of behavior, Zahavi concluded that there are no truly selfless acts. "Altruism is advertising," he said. It is a handicap display, an "unseen peacock's tail," a bid for prestige and status. Giving away resources may actually increase Darwinian fitness, Zahavi reasoned, because big donors typically attract more potential mates and discourage more potential adversaries. "I have turned the tables and said altruism isn't loss of fitness," he said, triumphantly, "it is a gain in social prestige."
Zahavi's triumph was, in fact, nearly total. His handicap principle has become so well accepted in the biological world that it risks being, in Alan Grafen's words, "too much of an orthodoxy." But Zahavi characteristically took his argument a step further, and trashed all the alternative theories. Ideas like kinship selection and reciprocal altruism simply miss the point, Zahavi said. And Darwin was narrow-minded to call it evolution by sexual selection, because handicap displays, or signals, aren't just about winning mates; they're also about scaring off rivals. He should have called it evolution by signal selection.