by Jessica Marshall
“Some are moderately even-tempered, and some irritable, some brave and some timid; some volatile and some phlegmatic,” wrote A.T.A. Ritchie before his death in 1962. But he was not discussing the personalities of children, soldiers, or mothers-in-law. Ritchie, who served as the chief game warden of Kenya from 1923 to 1949, was referring to wild black rhinoceroses (Diceros bicornis).
|The more scientists look, the more they find that black rhinoceroses and other animals display marked personality differences.|
The notion that rhinos have personalities might well seem a dubious one, but Ritchie is not alone in his attributions. People have described personality in everything from bears to birds to bugs.
Can animals really have personalities? To anyone with a pet dog or cat, the answer is, emphatically, yes; to them, the existence of animal personalities seems preposterously obvious. But many scientists are more wary. They warn against the dangers of anthropomorphism, projecting human thoughts and feelings onto animals without the ability to test objectively whether these exist. Just because an animal’s mouth is wide, for example, does not mean it is smiling, which does not in turn imply that it is happy—even though we may be programmed to interpret the expression that way. For years, scientists have tread carefully around the idea that an animal might have a personality.
That was not always the case. The idea that animals’ resemblance to humans goes further than just anatomy and physiology was fairly mainstream until the 1940s. Charles Darwin argued for emotions in animals in The Expression of Emotions in Man and Animals, published in 1872. “Many personality traits involve emotions. It’s a very short leap from emotions to personality,” says psychologist Sam Gosling, who heads the Animal Personality Institute at the University of Texas at Austin. But a negative view of animal personality research was well entrenched by the 1990s when Gosling was pursuing his graduate work at the University of California, Berkeley. His advisor actively discouraged him from studying animal personality, calling it “goofy” and reiterating that his peers would consider the work anthropomorphic.
Despite such warnings, Gosling and others have breathed legitimacy into the topic by showing that personalities can be measured in a scientific way. Personality is not just observable in monkeys, dogs, and humans. Even fish and spiders have shown predictable personalities. The more scientists look, the more they find that animals have measurable personality differences: bold, shy, curious, sociable, aggressive, and more.
“I started out thinking animals did not have personality,” Gosling says. “But the more I thought about it, the more I couldn’t think of a reason why they shouldn’t. I began realizing that maybe they do have personality, and maybe that can lead us in interesting directions.”
It is easy to describe an individual’s personality, but a scientific definition is tougher. Scientists agree that personality means more than just a single behavior, and is instead a pattern of consistent behaviors. A bold animal, for example, might consistently be quick to try new foods, explore new environments, and recover quickly after a threat. But an animal’s overall personality is the combination of several such traits—its boldness, its aggressiveness, and its sociability, for example.
In humans, personality is often assessed using a five-factor test, which indicates where an individual lies along the continuums of introversion versus extroversion, conscientiousness versus impulsivity, aggression versus agreeableness, neuroticism versus emotional stability, and being open versus closed to new experiences.
By finding similarly objective measures of personality, scientists have persuaded their peers to take research on animal personality seriously. But since animals can't fill out personality questionnaires, scientists need other methods—namely behavioral observations—to make objective assessments. To do this, they must show that an animal has a reproducible set of behaviors consistent with a particular personality trait, and that its observed personality can be used to predict its behavior in a new setting.
|Handlers of spotted hyenas in a captive colony consistently agreed on the animals' personalities.|
Gosling was one of the first to prove that personality can be measured reliably in a verifiable way. He conducted experiments in the 1990s that showed that four handlers of a colony of captive spotted hyenas (Crocuta crocuta) consistently agreed on the animals’ personalities. He extended the work to domestic dogs, using a four-factor test he developed that rates them in terms of energy versus slothfulness, affection versus aggression, trainability versus stupidity, and anxiety versus calmness. Using this scale, he found that strangers’ ratings of the dogs’ personalities matched those of the dogs’ owners. Likewise, psychologist John Capitanio’s group at the University of California, Davis, demonstrated a different four-factor test for primates, which included degree of sociability as one of the factors.
Other animals, like birds, rodents, and fish, also show consistent personality traits. Those that tend to be more aggressive toward their peers tend also to be bolder in exploring their environment, says evolutionary ecologist Niels Dingemanse of the University of Groningen in the Netherlands, who has studied personality in great tits (Parus major) and three-spined stickleback fish (Gasterosteus aculeatus).
One of the classic but surprising examples of animal personality is octopuses, which are solitary and are invertebrates, unlike the more obviously personable chimpanzees or golden retrievers. Giant Pacific octopuses (Octopus dofleini) at the Seattle Aquarium, for example, have had such dramatically different personalities that their handlers have given them descriptive names. “Lucretia McEvil” tore up her tank nightly, chewing through plastic cable ties and digging up the water filter. “Leisure Suit Larry” was highly aggressive and would immediately begin groping with his tentacles anyone who opened his tank. “If he had been a person, he would have been cited for sexual harassment,” says Roland Anderson, a biologist at the aquarium.
To show in a quantitative way why this was so, Anderson and psychologist Jennifer Mather of the University of Lethbridge in Alberta, Canada, studied smaller, East Pacific red octopuses (O. rubescens). The scientists tested the octopuses’ responses to three of Anderson’s actions: opening their tank and looking at them, poking them gently with a bottle brush, and feeding them a crab. They found that individual octopuses’ responses were consistent from trial to trial, and their personality traits could be grouped by “activity,” “reactivity,” and “avoidance.”
|Octopuses' responses to three actions convinced scientists that they could easily group the invertebrates by documenting consistent and quantifiable behavior. (Jessie Cohen/NZP)|
Anderson’s and Gosling’s studies illustrate different disciplines’ approaches to understanding animal personality. Anderson’s comes from animal behavior research, in which scientists quantify specific behaviors such as how long it takes an animal to approach a new object or how often it strikes at an intruder. These behaviors are then tallied to make a quantitative assessment of personalities. Gosling’s originates in psychology studies: Ask those familiar with the animal to judge the extent of certain traits. “Humans are extraordinarily good at interpreting information in context,” Gosling says, so his approach can be more straightforward than trying to count behaviors, which sometimes fall into gray areas. But such an approach may not work as well for species like fish or water striders, which show less variety in their behavior than primates or dogs.
With the data from these and other studies, scientists are beginning to understand why evolution has favored the persistence of different personalities. Evolution depends on variation, Gosling says, but in many ways, it selects out variation. There are no short-necked giraffes roaming the savannas anymore, he points out. “So the question is, why do we have differences in personality?”
On one level, personality is uniform among members of a species: “Stump-tailed macaques [Macaca arctoides] are easygoing. Bonnet macaques [M. radiata] are super affiliative. Pigtail macaques [M. nemestrina] can be very aggressive,” Capitanio says. This suggests that all individuals in a species may benefit from having a particular personality trait to a certain degree—which might be different from the degree that is beneficial to another species.
Nonetheless, there must be advantages to animals having different personalities at different times, because there is also individual variation within species. Renée Duckworth, an evolutionary ecologist at Harvard University in Cambridge, Massachusetts, is investigating this question in bluebirds. Duckworth has found that individual western bluebirds (Sialia mexicana) exhibit consistent differences in aggressiveness, based on how often they attack, fly by, or hover near a model of a tree swallow (Tachycineta bicolor) introduced into their territory. These differences, she finds, are an asset in some situations and a liability in others.
The western bluebird lost habitat in Montana to logging and agriculture in the late 1930s, but is now returning to its original range thanks to nest-box introduction programs. These nest boxes are colonized more quickly by competing mountain bluebirds (S. currucoides), but the western bluebirds tend to win out in the end. Duckworth has found that among western bluebird males, aggressive individuals are more successful in booting mountain bluebirds out of nest boxes and colonizing new areas. However, these aggressive males invest less in caring for their young, leading to high mortality of their offspring. This puts them at a disadvantage compared with less aggressive males once colonization is complete, because less aggressive males raise more offspring.
Duckworth learned that over time, populations become less aggressive overall, because the situation in which aggression is favored has passed. She proposes that the nest boxes may ultimately change the overall distribution of western bluebird personalities by providing highly stable nest sites. The birds’ traditional nest sites—cavities found in dead trees—are much more ephemeral compared with nest boxes, requiring birds to seek new areas to colonize every few years. It makes sense, then, that evolution would have favored maintenance of both personalities in such an environment—where aggressiveness sometimes pays off and sometimes doesn’t.
Still, it seems it would be better for individuals to be totally adaptable—to be bold when boldness is called for and timid when that offers an advantage—so why do consistent personalities that get animals in trouble persist? “I don’t think we have a really good answer to that yet,” Duckworth says. “I think what we might find eventually is that there is some set of developmental constraints in how the hormone system or the nervous system is constructed—that somehow physiologically it can’t be accomplished to change personality based on context.” Capitanio agrees that so far there is no answer, but his work with macaques suggests that different personalities are important in successful social groups. “Some of our evidence suggests that the groups that work best together are ones where some animals are high in sociability and some animals are low,” he says.
It follows from what we know intuitively about human personality that past experiences—growing up in a particular culture or family situation—and inherited tendencies can affect one’s personality. Not surprisingly, there is evidence in animals for a role of both nature and nurture in determining personality. Certain dog breeds are known for being high-strung or intelligent, indicating a genetic component to their personalities, but a dog’s training and history also make a difference. Researchers are working to put a finer point on the relative importance of both learned and heritable traits. A study by David Sinn at the University of Tasmania in Australia shows that in dumpling squid (Euprymna tasmanica), personality traits are passed from parent to offspring in varying degrees. Since squid do not care for their young, this family resemblance must be inherited, not learned from parents.
If personality can be inherited, then there must be genes responsible for personality traits. Scientists have recently found a candidate gene that may contribute to novelty-seeking or curiosity. Previous work had identified a gene in humans—DRD4—that appeared to be associated with novelty-seeking behavior, but the link was inconclusive, perhaps because it is confounded by environmental factors or because gene interactions in humans are especially complex. Animal experiments, on the other hand, allow researchers to control for these factors. So far, three groups of scientists have shown that variations in the DRD4 gene are associated with novelty-seeking in monkeys, great tits, and domestic horses, respectively. These findings suggest that the role of DRD4 in novelty-seeking predates the divergence of avian and mammalian lineages, according to Bart Kempenaers of the Max Planck Institute for Ornithology in Seewiesen, Germany, and colleagues, who reported the relationship between DRD4 and personality in great tits in the Proceedings of the Royal Society B last year.
But genes are not the whole story. Dingemanse recently showed that three-spined stickleback fish demonstrate personality only in certain environments. Stickleback exposed to predators in the wild show correlations among three traits—activity, exploratory behavior, and aggression—but this connection disappears among those living in ponds where no predators are present. Also, Duckworth examined the degree of aggression throughout the family tree of the western bluebirds in her study and found that neither genes nor environment alone explained the variance in personality she observed. “There are definitely contributions from both,” she says.
|Nasty cat? Putting knee-jerk impressions aside, scientists wouldn't judge personalities of cheetahs (Acinonyx jubatus) or other animals without lengthy investigates that assess behaviors. (Jessie Cohen/NZP)|
Although evidence indicates that animals have personalities, it is important to recognize that they do not have the same personality dimensions as humans. Anthropomorphism is still a concern, Gosling says. “One of the traits that people rate their dogs on is ‘philosophical,’” he says. “I do not believe dogs are philosophical. We still have to be tremendously careful.” Moreover, human personalities are much more complex than those of other animals. “Humans have all kinds of other things: values, goals, motives, identity,” he says. There probably are also differences between the complexity of personality in nonhuman primates and, for example, stickleback fish, says Capitanio. “A lot of what is studied in fish or rodents or invertebrates is what’s often characterized as ‘shy-bold’ continuums.” By contrast, the macaques he works with have many personality dimensions. In other words, the extent of what it means for species to have personality may differ.
Personality research may only now be hitting its stride, but people have exploited heritable personality traits for centuries by selectively breeding domesticated animals. “You don’t want your milk cows to be crazy wild animals, so you select for docility. You want your dogs to be friendly,” Capitanio points out. A sheepdog should not be so aggressive that it hurts the sheep, nor so high-strung that it’s afraid of herding larger animals, he adds.
Not surprisingly, Gosling found in comprehensively reviewing personality research in dogs that most published work on the subject has been done with working breeds, like German shepherds. Experts in various breeds know even more about personality traits in dogs than is published, he says, but the highly competitive nature of dog breeding means they keep much of it secret.
Now Gosling is putting his research on dog personality to use. He is working with animal shelters in the Austin, Texas, area to try to figure out how best to test dogs and potential adopters to ensure a good match. He is also trying to figure out which breeds of dogs are best for sniffing out bombs. You might think that these are the animals that have the best sense of smell, he says, but “the real factor is whether the dog freaks out at helicopters and whether it can continue to look for roadside bombs under those chaotic conditions.” It is their personality that matters.
Zoos also select for certain personalities. “In a zoo, if you’re looking at the effects of visitors on animals, you have to take into account that certain individuals are going to respond differently from others,” says Kathy Carlstead, a biologist at the Honolulu Zoo. For example, Anderson found that the Seattle Aquarium octopus named Emily Dickinson was too shy for exhibiting, so visitors could never see her. The aquarium eventually traded her for a more gregarious tenant.
|Zoos must take into account an orangutan's or other animal's personality when planning breeding efforts and deciding which animals are best suited for public view. (Ann Batdorf/NZP)|
Carlstead investigated A.T.A. Ritchie’s claim that rhinos had personality in the 1990s while working at the Smithsonian’s National Zoo. “Anybody will tell you that rhinos, even in the wild, have strong differences in personality,” she says. To test this, she asked keepers at 19 zoos to rate the personalities of their 60 rhinos based on elements of their behavior. She then tested most of the same rhinos’ personalities by presenting them with a new object (a traffic cone) and a new scent (the urine of an unfamiliar female rhino) and scored them according to six personality traits. She and her colleagues found that the keeper’s ratings matched the scored behaviors, and that most personality traits remained consistent when the rhinos were retested two years later.
Carlstead says understanding rhinos’ personalities can also help zoos figure out how to get the animals to reproduce more successfully. “Any time you’re working with a small number of animals, individual differences are going to play a strong role.” So Carlstead also compared the extent of the observed traits with the breeding success of the rhinos in the study. For male rhinos, less sniffing and less dominance correlated with greater reproductive success. For females, better reproductive success was linked to less agitation or threat-associated behavior such as chasing or aimless, repetitive actions. The most successful match-ups for breeding were rhino pairs in which the female had a higher dominance rating than the male.
Denis Réale, an evolutionary ecologist at Université du Québec à Montréal, argues that such differences are important to consider when designing conservation efforts. Zoo breeding programs may select for bolder or shier animals or for those least stressed by living in zoos, creating selection pressures that could change the population over time, he says. “The problem is that these personality types may not be the ones that are most adaptive to the wild.”
Taken together, these findings suggest that personality is more than just a newly legitimate line of research, but an evolutionary force to be reckoned with. And it looks like the old expression is right: It takes all kinds.
—Jessica Marshall is a science writer in Saint Paul, Minnesota.