Several times in August, a cougar was sighted prowling the suburbs of New Canaan, Conn., and not for the first time. Ten years ago, a young mountain lion found its way to Connecticut, prowling more than 1,500 miles from the Black Hills of South Dakota, before being killed crossing a highway.
What stands out about these incidents is the exploratory personalities of the tawny felines, Malcolm L. Hunter, Jr., professor emeritus of wildlife ecology at the University of Maine.
“You can bet that the individual young mountain lion who left South Dakota and ended up being run over by a car in Connecticut was not a stay-at-home timid fellow,” he said.
Wildlife biologists have traditionally studied factors such as prey abundance, habitat quality and behavior to assess the role of animals in particular ecosystems. But a growing number of scientists argue that a crucial component is missing: the range of personality traits of individual animals, whether grizzly bears, squirrels or earthworms. Some scientists claim that even bacteria are unique.
“Personality is found in all taxa,” said Alessio Mortelliti, a rodent personalities expert at the University of Maine and a recipient of a career grant from the National Science Foundation.
The five common animal personality traits are daring, aggressiveness, activity, inquisitive tendency, and sociability. To be eligible, these properties must be present “over time and in different contexts”.
Wild animals “are not just little, double automatons doing what they do; they do them in different ways,” said Dr. Hunter, a co-author of a paper urging scientists to include such studies in the field of ecology. “It’s important to know and appreciate that personality can be a consideration when managing these systems.”
It’s no different than a litter of puppies or a box of kittens, with each animal being different. In the wild, however, those personalities ripple through natural processes.
dr. Hunter noted in his article that in his last book, “The Formation of Vegetable Mold Through the Action of Worms,” published in 1881, Charles Darwin variously referred to earthworms as timid, neat and tidy, and untidy. A 2018 study found that earthworms were very different at solving problems.
One personality trait “that crosses many species is the extent to which some individuals are more curious, inquisitive, and more willing to stick their necks out,” said Dr. Hunter. “Well, I guess an earthworm doesn’t really have a neck,” he laughed.
Understanding animal personalities gives a more complete picture of the natural world, some experts say, and digs into individual differences much deeper than behavioral ecology. It plays a role in every aspect of their lives, be it mating or forest regeneration.
dr. Michael Goldstein, a psychology professor at Cornell University who studies the social importance of babbling in bird and human babies, caged 48 zebra finches to see how different personality types mate and how that affects parenting. “It turns out they mate through exploration,” he said. “Low-exploring men and low-exploring women came together, and high-exploring men and high-exploring women came together.”
While the idea that earthworms or bacteria have personalities may seem a bit far-fetched, it’s clear that more complex animals, including wolves, bears, dolphins, whales and many birds, have highly evolved personalities that reflect human traits.
Take the case of the famous Giefer grizzly, with a penchant for housebreaking. For several years in the 1980s, the big bear, which weighed more than 500 pounds, smashed windows or doors in unoccupied summer cabins along the North Fork of the Flathead River, near Glacier National Park in Montana, and helped itself flour, sugar, or whatever food was left behind. The bear searched about two dozen huts.
“That bear was incredibly tricky,” said Chris Servheen, a retired American Fish and Wildlife biologist who had attempted to capture the bear. “He didn’t want to be trapped and he was never seen. He was like a ghost. We did everything we could to catch this bear, but it was impossible to catch.”
The Giefer grizzly was eventually killed by a hunter in British Columbia.
In wolves, pack cohesion is very dependent on the alpha female. A strong leader in these matriarchal societies will track down and discipline wandering wolves, but less aggressive leaders will not. And when an alpha female dies, the pack can often disintegrate.
Personalities play a key role in ecosystem dynamics, and knocking out one type of individual could affect evolution, scientists say. A daring grizzly or cougar can help a population adapt to new terrains and conditions — a trait that may be especially important as climate change forces some species to cut out for more suitable living conditions.
Animal personalities can also have profound effects on ecology.
Rodents play a key role in forest regeneration, for example, a topic that Dr. Mortelliti studies voles, mice and squirrels in the Penobscot Experimental Forest in Maine. If a mouse running across a forest floor encounters a seed, its personality may cause it to immediately gobble up the seed, meaning it will not germinate into a plant. If the mouse buries the seed, a new plant can develop. Tough animals, for example, spread seeds further than shy individuals. But a cheeky animal is also more likely to fall into the clutches of a hawk or owl.
Rodent personalities are determined with various tests, which are animal versions of the Rorschach. In one test, a mouse is placed in a large box. Shyness can be indicated by a mouse’s tendency to spend time near the corners or walls, while boldness can be shown by those who move towards the center. The mouse is placed in a pouch to measure the stress response or is studied in an open field for its response to novelty.
The captive animals are marked with a microchip and ear tag and released. They’re offered seeds, and when they go past an antenna, it reads the microchip “so we know exactly who got the seeds and what they did with them,” he said. More than 3,200 rodents had their personalities assessed for these studies.
How a forest is managed can change the balance of personality types.
“With unmanaged forests or a forest that is as relatively natural as possible, you have a very nice distribution of bold and shy,” said Dr. Mortelliti.
As plants shift their range to keep up with a changed climate, mice and squirrels will be introduced to new, strange-looking seeds, and they will be needed to propagate them, an important ecosystem service.
“An individual’s boldness will have effects on how or not to handle that seed,” said Dr. Mortelliti. “That can affect a plant’s chance of adapting to climate change.”
Adapting landscapes in a certain way favors certain types of personalities and changes the course of evolution. And it could affect whether forests and other ecosystems can adapt to a changing world. What’s important, he said, is a diversity of personalities in a landscape.
“Natural selection has promoted this variation in personalities because in some cases, in some years and in some contexts, it is more beneficial to have different personality types,” he said. “The greater the diversity, the better the populations can adapt to change.”
Aggression plays a role in ecosystems by determining the territory between species. To measure the level of aggression, an animal is placed in front of a mirror to see how it reacts to what it thinks is a rival. The winner of the most aggressive rodent competition?
“The American red squirrel,” said Dr. Mortelliti, who has conducted personality tests on rodents around the world. “They are incredible. They’re the only species you can take measurements on, you let them go, then they go up a tree and come back to you and yell at you. They are really feisty, territorial individuals.”
Research into the ecological role of animal personalities still has some way to go, said Dr. Mortelliti, before specific forest management recommendations can be made.
Iain Couzin is a researcher at the Max Planck Institute of Animal Behavior at the University of Konstanz in Germany who studies swarm intelligence — how fish come together to form a school or birds float and dive together in murmurs. He thinks the idea of animal personalities is exaggerated.
Biologists “are all aware of individual differences,” he said in animals. “Darwin was well aware of individual differences. But I don’t like the term personality. The whole process is quite subject to anthropomorphization.”
dr. Mortelliti sees it as a useful prism to view animal behavior. “Ultimately, we all understand that we measure individual differences in behavior,” he said. “I have no qualms about using the term personality because it’s intuitive and people can identify with it.”