A Kingdom of Characters

Hilltop Male, a coyote who made his home in a suburban Chicago park, knew how to get around the 'hood. Every night he and his mate set out to hunt, traveling together until they reached a two-lane road. He looked left and right, waited for cars to pass, then darted across to all the tempting fare around the area homes. His mate would watch him leave before turning back to the park. Not once in the year that a research team observed the pair did she ever cross that road.

Meanwhile, Hilltop Male was having a fine time. "He definitely wreaked havoc; he took peoples' cats," says Stanley Gehrt, a wildlife biologist at The Ohio State University in Columbus who's been studying Chicago's coyote population for 13 years. Hilltop Male even devoured a local hospital's prized swans, brazenly feasting on one in plain view of passing patrons. Ultimately, however, his adventurous nature proved fatal; he was struck by a car and killed.

"He was bold, but not every coyote is," Gehrt explains, noting that neither Hilltop Female nor her new mate, Hilltop Male2, has ever ventured across the road separating the park from the suburban houses. "There is tremendous variation in coyotes' behavior," he says, "and the only way to explain it is by understanding their personalities."

For most of the 20th century, the idea that nonhuman animals had personalities was considered anthropomorphic. When primatologist Jane Goodall suggested in the early 1960s that chimpanzees were temperamentally different from one another, she was criticized by animal behaviorists and comparative psychologists. But these days, animal personality research is booming. "Personality is a phenomenon in nonhuman animals," asserts Alexander Weiss, a comparative psychologist at the University of Edinburgh in Scotland who was part of a team that showed that chimpanzees have dispositions remarkably similar to those of humans. "Anyone who has been to the zoo or owns a pet can see it."

A bird in a flock, a fish in a shoal, or orangutans separated by miles of forest—each individual animal is now accepted by researchers as having persistent quirks, tics likes, and dislikes that set it apart from others of its kind.

There are agreeable Scottish wildcats and others that are ornery and aggressively dominant, as well as elephant matriarchs whose confidence, constancy and openness reflect their age and role. Male crickets can be bold, delivering mating calls often and loudly, or shy, singing softly now and then; while some fruit flies are explorers, venturing far to find a new home, others are sitters, barely moving from where they hatched.

"In almost every species we've looked at, we find that individuals are consistently different in their behaviors over time and across situations," says Andrew Sih, a behavioral ecologist at the University of California, Davis. Just as introverted children typically remain so as adults, animals also stick to their personality types—as did Hilltop Male. (Yet, he notes, "they also have some unpredictable element. That's the essence of personality.") The universality of this phenomenon has triggered a host of questions about the origin and purpose of personality; in addition to cataloging the vast array of types within and across species, researchers today are focused on why such differences emerged in the first place.

The Many Masks Of Man And Beast

The study of animal personality is a young field, barely two decades old. It didn't really get underway until the late 1990s, although several early prominent scientists, including Ivan Pavlov and Robert Yerkes, envisioned a holistic science of personality that embraced all animals.

Scientists who watch animals, though, were paying attention to their quirky ways and, when they could, publishing observations. At the Seattle Aquarium in the late 1980s, for instance, biologist Roland Anderson noted that volunteers gave names to giant Pacific octopuses that reflected their individual habits. There was Lucretia McEvil, who destroyed items in her tank; Emily Dickinson, who tried desperately to stay hidden from view; and Leisure Suit Larry, who eagerly latched his arms onto anyone unlucky enough to be nearby when his tank was open.

Could octopuses have personalities? Anderson teamed up with Jennifer Mather, a psychologist at Canada's University of Lethbridge, to find out. They adapted a test for measuring temperamental traits, such as shyness and boldness, in humans and applied it to 44 red octopuses that Anderson caught and held briefly at the aquarium. Researchers assessing the personality of people ask a standard set of questions; this, of course, can't be done with other animals, so the two scientists came up with 19 different behaviors to assess. One involved simply watching how the octopuses responded when a person opened the tank lid: Shy octopuses would hide; bold ones wouldn't budge. In another test, Anderson touched each octopus with a soft brush. The easygoing ones didn't mind, but those with a reactive personality jetted away, leaving behind a cloud of ink as they fled. Most impressively, the octopuses were consistent in their responses and could be slotted into one of three personality types—shy, passive, or aggressive. Mather and Anderson eventually reported this landmark study in the Journal of Comparative Psychology. It was the first to identify and measure personality traits in an invertebrate.

If octopuses have personalities, why not other species? Indeed, in 1999, Sam Gosling, then a graduate student in psychology at the University of California, Berkeley, pulled from more than 100 potentially relevant reports on nonhuman animals—including monkeys, rats, and hyenas—19 studies that mentioned traits, such as boldness, avoidance, and fearfulness, that could be easily mapped onto the most widely accepted human personality assessment model, the Five-Factor Model. He argued that it was time for personality psychologists to understand the biological roots of their subject, and what better way than via comparative studies of nonhuman animals?

Gosling puzzled about why scientists were reluctant "to ascribe personality traits, emotion, and cognition to animals," when they accepted that human and animal anatomy and physiology are similar. "[T]here is nothing in evolutionary theory to suggest that only physical traits are subject to selection pressures," he observed, echoing the words of animal physiologist Donald Griffin, who asserted in the late 1970s that scientists should recognize the cognitive side of animals.

Evolutionary biologists and behavioral ecologists agreed. "They've led the charge," says Gosling. If being bold, shy, or passive didn't confer some survival benefit, natural selection would have weeded these characteristics out. "Natural selection acts on variation, whether it's color, size, or personality type," explains Alison Bell, a behavioral ecologist at the University of Illinois at Urbana-Champaign who studies personality in stickleback fish. Bell is exploring how selective pressures—such as predators, environment, and social setting—produce and maintain different personality types.

An Adaptive Advantage

As Hilltop Male discovered to his mortal detriment, a personality type that succeeds one day may cost its owner the next. "Certain personality types are associated with survival risks," says Sih, who studies the dispositions of insects, spiders, crayfish, crabs, fish, and salamanders. "We see this in people. Bold people do crazy stuff; in some cases that works well, but not in others. But being timid and shy also carries risks. You may miss opportunities." The two traits are at opposite ends of one personality dimension, usually called "boldness." Whatever their benefits and risks, both personality types—shy and fearless—are found in every animal population scientists have studied.

Either strategy can pay off from an evolutionary standpoint, Sih says, because a shy individual—say a timid crab who spends more time than bold crabs hiding under a rock—is less likely to be eaten by a predator and so may survive longer. But, if the shy crab stays hidden too long, she'll never encounter a mate. She may live long, but she will have failed in what evolutionary biologists consider life's biggest contest: passing on genes. Meanwhile, a bolder crab might get lucky at a young age, meeting a mate and laying her eggs before her curiosity leads to her demise. Natural selection maintains both personality traits because both strategies—being shy or bold—pay off often enough that both fearful and fearless crabs successfully reproduce, thus ensuring that their dispositions are passed on to their progeny.

Other strategies, such as frequency-dependent selection and life-history trade-offs, are also thought to play a role in maintaining various personality traits in a population. Frequency-dependent selection comes about in populations that have become skewed toward a particular trait; for instance, if most members of a population are scroungers, relying on others to find food (as do some pigeons), then the few producers who search for their own food may be at an advantage (and produce more offspring) just because they face little competition in doing so. Some personality traits are also associated with particular life-history strategies. For instance, it may be best to be highly aggressive and bold if you're a member of a species that lives fast—maturing and reproducing early—and dies young. Yet many species, such as the three-spined stickleback, have both shy and bold members.

"There are examples in many species of these two extremes, shy and bold," Bell says. "So we know this variation is more than noise; it is real and it has consequences—that is, it affects how individuals live their lives and their reproductive success." To explore how these types come about and why such strong differences exist within the same species, Bell set up experiments with sticklebacks, small fish that can be found in the coastal marine waters and streams and lakes of the Northern hemisphere.

Sticklebacks are ideal for studying the origin of personality types because scientists know a great deal about their evolution. The oceanic species have changed little over the last 7 to 12 million years; they've also invaded freshwater habitats, colonizing lakes and streams and producing new species. "They give us a variety of parallel populations to study," says Bell, who works with fish she collects from various sites in Scotland and California.

Some of these populations are regularly eaten by predators, such as pike and trout, while others live in environments with few enemies. Another researcher discovered that if a juvenile male stickleback acts particularly boldly in the presence of a pike, he will be especially aggressive toward other sticklebacks as an adult. "It's a 'behavioral syndrome,'" Bell says. "Bold sticklebacks are aggressive—you see that correlation again and again."

Bell's tests indicate that whether or not the bold-aggressive personality evolves in a stickleback population depends on the number of predators in a given body of water. Sticklebacks from high-predation areas have this personality, whereas those that live without the fear of being eaten don't—on average, they seldom chase or nip one another. "There is this correlation in high-predation populations: Aggressive individuals are also bold," Bell says. "But with the low-predation populations, this correlation doesn't exist. You can't predict how bold an individual will be—even if it is more aggressive toward other fish, it may not be bold in the presence of predators."

Most intriguing, sticklebacks from low-predation areas can be turned into bold-aggressive types simply by introducing a predator. In an experiment, Bell allowed rainbow trout to feast on a stickleback population that wasn't aggressive. After the trout gobbled up half of the sticklebacks, Bell reassessed the behavior of the survivors and discovered the same correlation she'd spotted in sticklebacks from high-predation areas; that is, they were now bold-aggressive. "So predation is one selective factor that favors the evolution of this personality type," she says. That conclusion leads Bell to wonder how much of our human personality has been shaped by the predators we faced in the past.

Too Much Of A Good Thing?

Predators may set the stage for a definitive, predictable personality to emerge, but curiously, once an animal has gained its disposition, it rarely shakes free of it, even if the personality type leads to a bad or unproductive end. That's what Sih has found among male water striders, insects with long, Lunar Lander-like legs that let them do what their name says: walk on water. Male water striders are aggressive and bold; they spend their time fighting other males and harassing the females for sex. "There's no courtship," Sih says. "They jump on the females and bother them so much that the females hide at the edge of the pools."

While aggressive males usually have the most mating success, sometimes a hyper-aggressive male will dominate a pool, "harassing everyone—jumping on females, males, juveniles," Sih says. The result: All the other striders leave the water; all mating ceases. "Nobody mates, not even him. He's too busy wrestling other males to even see a female swimming by," he explains.

Such overly aggressive behavior runs counter to the idea of "optimality theory," which was once common among biologists, Sih notes. "It was thought that animals would do optimal foraging, mating, and so forth. But in reality, they're not that smart. They don't have as much behavioral flexibility as an optimal forager would need." Instead, animals tend to follow what their personality type dictates.

Female fishing spiders, for instance, practice sexual cannibalism. Best case scenario: They eat the male after mating—a strategy that helps them nourish more eggs. (Evolutionary biologists interpret the male's sacrifice as a form of parental care.) But often, females consume males before getting down to business. Some become such avid male killers that they never mate.

What Primate Personality Reveals

How different are human personalities from those of other animals? It's difficult to say—in part because behavioral studies typically focus on only one or two traits, such as shyness and boldness, behaviors that are easy to see and evaluate. Human personality questionnaires, in contrast, examine numerous traits that reflect five key domains: neuroticism, extraversion, openness to experience, agreeableness, and conscientiousness.

Still, researchers have discovered striking similarities—as well as intriguing differences—between our personalities and those of our close cousins, the great apes and monkeys. From these, scientists are beginning to sketch the basic disposition of some of our earliest ancestors.

"We share certain clusters of traits with other primates," says Weiss, who codeveloped a Hominoid Personality Questionnaire with 54 behaviors for observers to assess. It can be used for people as well as for monkeys and apes. "For instance, humans and chimpanzees have the conscientiousness factor. But not orangutans." Conscientiousness is about planning and being predictable; people and chimps that rate high in conscientiousness follow through on plans and are generally predictable. Weiss suspects that orangutans lack such a dimension, perhaps in part because of their social structures. Orangutans are largely solitary animals, while chimps and humans live in complex societies where individuals regularly join groups, then split off for some activities. "Conscientiousness may evolve in such fission-fusion societies," he says.

Indeed, Weiss and his colleagues recently discovered supporting evidence for their idea after assessing the personality types of capuchin monkeys and rhesus macaques. Intriguingly, capuchin monkeys have personality types almost identical to those of chimpanzees, including a factor similar to conscientiousness, but the macaques do not. Capuchins, Weiss notes, although more closely related to macaques than to chimpanzees, share certain traits with the apes. Capuchins and chimps are bright, tool-using species that live in complicated social groups. Rhesus macaques, while equally intelligent, live in relatively stable societies where individuals spend their days together and don't head off to do their own thing. "It's not the final answer, but it is suggestive," Weiss says. "We think discoveries like this will eventually help us explain why different traits emerged in different species."

Many nonhuman primate species, including chimpanzees and macaques, also have a personality domain that humans lack—dominance. "It's related to competitive prowess," Weiss says. "Individuals rated as high on this dimension will probably be higher up in the dominance hierarchy." In primates other than humans, dominance is a full-fledged trait, just like agreeableness. (In humans, traits related to dominance are spread across multiple factors; someone who scores high on extraversion and low on agreeableness is also likely to be dominant.)

Unlike with conscientiousness, scientists have not yet been able to link dominance to a particular mechanism—that is, to social or environmental factors that would cause it to evolve. "It's variable in how it's expressed," Weiss says. When rating chimpanzees, for instance, those that score low in fear and cautiousness—but high in confidence and aggressiveness—are dominant. But orangutans that rate high on dominance are described as being "primarily disagreeable; they're aggressive but not necessarily any less fearful or cautious," he says.

Some scientists, such as Harvard primatologist Richard Wrangham, have argued that the human evolutionary path took us away from that of our aggressive, hierarchy-focused relatives and toward more egalitarian social groupings. In that shift, they suggest, dominance became a diffuse rather than a single personality trait.

Still, we've kept personality factors from our past, Weiss says. Researchers have known for years that people who are agreeable and happy, and low on neuroticism, tend to have more successful lives, live longer, and have better immune systems. Meanwhile, people who rate high in neuroticism and low in extraversion are generally less happy and have poorer immune systems. Weiss and his colleagues have identified the same correlation in rhesus macaques, chimpanzees, orangutans—even domestic cats. "It shows that the connection between well-being and personality goes back a long way phylogenetically," Weiss says.

It's likely been conserved because there are common underlying genes linked to these traits. "Being emotionally stable and extraverted means that you're likely to be happy," he adds. If so, then happiness may be, in part, a sexual signal, like a male peacock's train, advertising that you have good genes and therefore would make a good mate, he suggests.

In time, Weiss and others expect to find more explicit links between genes and personality, although he notes that our dispositions are also shaped by experiences in life—as are those of chimpanzees. As humans age, our reserves of agreeableness and conscientiousness increase, extraversion and neuroticism decline, and openness increases, then later decreases. Scientists have debated whether this is due to biological processes or social and cultural factors. Weiss and his colleagues tackled the question by rating the personality of 202 chimpanzees of both sexes, ranging in age from juvenile to elderly. The results indicate that even dominance-obsessed male chimpanzees become more agreeable as they age. Like many other aspects of personality, it seems that this change is "biological," Weiss says. For both man and chimpanzee, experiences have the power to shape the brain—and, as a result, shift personality.

Personality, it turns out, does not separate us from the other members of the animal kingdom—but rather strengthens our biological ties. Our temperaments have evolved from the shy and the bold, the gentle and aggressive, the good-natured and ill-tempered. The very same mix of personality types that we'd expect to find in human societies is there, too, among the other creatures of our world.