How do bears select spots for winter dens? New UNR research provides answers

Nov. 18, 2024

How do bears select spots for winter dens? New research provides answers

Team led by University of Nevada, Reno finds safety is key as black bears prepare to hibernate

RENO, Nevada – As northern Nevada’s growing population of black bears looks for places to create dens to spend the coming winter, it’s all about location, location, location.

And, like their human neighbors, the region’s black bears are searching for winter homes that are safe, warm and good places to get youngsters started in life.

Newly published research from a team led by scientists in the College of Agriculture, Biotechnology & Natural Resources at the University of Nevada, Reno shines new light on the decision making of black bears as they search for dens to spend the winter in the Sierra Nevada and western Great Basin.

Kelley Stewart, one of the lead investigators on the project and a professor in wildlife ecology in the Department of Natural Resources & Environmental Science, said better understanding of bears’ decision making is becoming more important as the population of humans and bears both increase in the region, increasing the number of encounters between the two.

Climate change, meanwhile, may affect the locations that black bears select for their dens or change their hibernation routines.

Among the team’s findings published in the journal Ecology and Evolution:

• Female black bears in the region enter their dens earlier than males, typically around Dec. 6, compared with Dec. 16 for males. Females also typically stay in their dens longer — leaving somewhere around March 28, about 16 days later than the typical departure for males. Females may be staying longer to care for the cubs that are born in the den.

• Male black bears apparently begin to stir out of their dens when day and nighttime temperatures remain above freezing, but temperatures don’t appear to be a big factor in female decisions about departure.

• Rugged, steep terrain is favored for dens, probably because it provides better hiding places.

• Black bears aren’t picky about the materials that provide shelter to their den. In the wooded Carson Range of the Sierra Nevada, hollow trees or rock piles commonly provide winter dens. But farther east, where the Pine Nut Mountains and Virginia Range are sparsely wooded with smaller pinyon pine and juniper trees, bears are more likely to find dens in piles of rocks.

• Den locations aren’t reused from one year to the next. Instead, bears select a new den each year.

• Female bears appear more likely than males to select den locations on steeper slopes closer to highways. That’s possibly because male bears sometimes kill young cubs in the spring, and females may want to give birth in locations where males are reluctant to go.

Heather Reich, co-author of the study and a former bear biologist with the Nevada Department of Wildlife, said black bears have been steadily reestablishing themselves in the region since about 1980. (The name “black bears” is sometimes misleading because more than half the black bears in the West are brown, cinnamon, blue-gray or even white.)

Black bears largely had disappeared from the region by the early 1900s, Reich said, after forests were clear-cut to meet the demands of the mining industry, and bears were hunted indiscriminately. Today, black bear populations are established as far east as the Hawthorne area.

The foundation for the new analysis of bear dens came from data painstakingly collected by the Nevada Department of Wildlife from 2011 to 2022. It identified 116 den sites in the region after putting GPS collars on bears and tracking when they stopped moving about during the winter and stirred again in the spring.

The University researchers visited 26 den sites in the Sierra Nevada, Pine Nut Mountains and Virginia Range, carefully detailing the topography, vegetation and concealment at each location.

Stewart, who also conducts research as part of the University’s Experiment Station, said the ongoing close cooperation between the University’s Department of Natural Resources & Environmental Science and the Nevada Department of Wildlife is among the elements that have built the department’s strong reputation.

“Our students have the opportunity for a lot of hands-on experience that supports what they learn in the classroom from our excellent faculty,” she said. “When our students present at conferences, we often hear that their work is some of best.”

Contributors to the bear-den research included Morgan Long, a graduate student, and Kevin Shoemaker, an associate professor, in the University’s Department of Natural Resources & Environmental Science; Carl Lackey, a bear biologist with the Nevada Department of Wildlife; and Jon Beckman, an adjunct faculty member of the University and a wildlife supervisor with the Kansas Department of Wildlife & Parks.

Funding for the research was provided by a grant from the Nevada Wildlife Record Book Foundation.

Black bears are reestablishing themselves in northern Nevada, increasing the number of encounters between bears and people. Photo by Derek Reich, Nevada Department of Wildlife: