A North Carolina State University professor emeritus told the Ely Tuesday Group that understanding where animals go isn’t enough. Researchers need to understand how animals think about their world.
Roger Powell, who has spent over 50 years studying animal behavior, discussed his recently published book “Home Ranges of Animals” during a presentation that drew on his extensive field research with weasels, fishers, and black bears. His wife Consie provided the book’s illustrations.
“A home range is where an animal lives,” Powell began, before spending the next hour explaining why that simple definition fails to capture the complexity of how animals navigate and remember their environments.
From weasels to black bears
Powell’s fascination with animal behavior began in 1970 at a field station on Basswood Lake, where he took a course on vertebrate ecology and encountered his first live weasel. Two years later, while conducting research on coyote predation in Colorado, a female long-tailed weasel brought her brood of 13 youngsters into the middle of camp.
“My wife decided she was going to catch one,” Powell recalled. “He ended up living with us for a year. He was a wonderful fellow. He taught us about the color change. He taught us about how weasels kill prey.”
Over the years, Powell and his wife have housed many weasels at different times — six simultaneously at one point — including one litter born in their home. His research on weasels included an unusual experiment at Chicago’s Brookfield Zoo, where he painted a large white patch atop the lion house and worked with trained hawks to prove that the black-tipped tail distracts predators from the weasel’s body.
When Powell moved to North Carolina State in 1979, he wanted to continue weasel research but couldn’t secure funding. The state offered money for black bear studies instead, leading to over 20 years of productive research in the southern Appalachians.
“Handling black bears is not quite the same as handling weasels, especially big black bears,” Powell noted dryly.
Challenging traditional definitions
Powell’s book challenges the standard definition of home range established by William Burt in 1943, which described it as “that area traversed by the individual in its normal activities of gathering food, mating, and caring for young.”
The problem, Powell argues, is Burt’s caveat that “occasional sallies outside the area, perhaps exploratory in nature, should not be considered part of the home range.”
“I don’t think it should be excluded from what an animal finds important,” Powell said. Those supposedly occasional trips often represent critical survival knowledge, he explained.
He described how one of his radio-collared female black bears, designated as bear 260, suddenly disappeared one year during a poor acorn crop. A graduate student eventually located her signal far down the Blue Ridge Parkway, where she had taken her cubs to a ridge she remembered — possibly from her own mother — that had reliable oak trees.
“That’s one of those occasional sallies, and it’s important,” Powell said.
The cognitive map
Powell’s research focuses on what he calls the “cognitive map,” the mental representation of their environment that animals maintain in their hippocampus, a region of the brain found in all vertebrates.
“My definition of a home range is that part of the animal’s cognitive map that the animal keeps up to date, and it includes places that aren’t visited recently but that are held in memory for future specific uses,” he said.
This cognitive approach recognizes that different animals perceive the same landscape entirely differently. Powell illustrated this with his family’s cabin in northern Wisconsin, where he spent 65 years becoming intimately familiar with the land.
While Powell knew where he had cut oak for firewood and planted red pines, a fisher passing through the same area would focus on red-backed voles and carcasses it could smell from hundreds of yards away, plus rest cavities in large oaks and the bog where it hunted snowshoe hares. A bear would concentrate on blueberry patches, their renewal rates, and which other predators frequented those areas.
“Different animals view the world differently,” Powell said, showing contrasting images of how humans, wolves, and weasels would perceive the same winter landscape.
Evidence from brain science
Powell cited multiple lines of evidence supporting the cognitive map concept. Brain scans show that specific cells in the hippocampus “light up” to create literal maps as animals move through space. The hippocampus contains place cells, grid cells, time cells, and head direction cells that work together to track location and plan movements.
Recent research on Egyptian fruit bats using implanted electrodes revealed that the bats plan trips to different food sources while flying, thinking ahead to where they’re going and thinking back to where they’ve been, exactly what the cognitive map theory predicts.
“This back and forth in the hippocampus is going on all the time. I think this is really cool,” Powell said.
He also described Stella, a dog trained by speech pathologist Christina Hunger to communicate using buttons that speak words. With over 40 buttons for nouns, verbs, and adverbs, Stella can form rudimentary sentences and express information about past walks to the beach, demonstrating the forward and backward thinking associated with cognitive mapping.
Powell uses mathematical models to generate testable hypotheses about animal behavior. For example, when should a black bear return to a blueberry patch?
The model considers two competing factors: berry abundance increases over time as the patch renews, but the bear’s information about that patch decreases the longer it’s been away. Other animals might eat berries, renewal rates might vary, or the best foraging spots might shift.
The optimal return time occurs before abundance peaks when the bear still has good information about the patch. The model predicts bears should return sooner in good berry years than poor ones, which is exactly what Powell’s data showed.
Similar models explain why red squirrels maintain individual territories rather than sharing them. With a central food cache, each squirrel must travel to the edges of its territory to gather food. Sharing a territory would require twice as much food, necessitating a larger area and longer, more energy-intensive trips.
“It’s really more energy efficient to have individual territories instead of shared territories,” Powell said. “And indeed, that’s exactly what red squirrels do.”
The book’s purpose
Powell said that his book isn’t filled with natural history stories but is written for researchers to provide new approaches for understanding animal home ranges. It’s currently available in the United Kingdom and will be released in the United States early next year.
“The goal is to give people ideas on different ways to understand why animals do what they do,” Powell said.