Less sleep, more trouble Study finds bears hibernating less with warmer winters, access to human food


Warmer winters and access to human food are keeping black bears awake longer — and the resulting increase in bear-human conflict is not good news for the future of Ursus americanus.

Research wildlife biologist Heather Johnson was working for Colorado Parks and Wildlife when the department began to notice bear-human conflicts on the rise. The number of conflicts totaled more than 1,200 in 2015 and is growing by about 4 percent every year.

“The state didn’t really know if bear-human conflicts were increasing because bears were changing their behaviors or if we had more bears or some combination of those,” said Johnson. “Understanding what’s driving those conflicts, we hope to help frame how a management agency should respond.”

Using GPS collar data from 131 adult female black bears over a six-year period in Colorado, Johnson and fellow researchers found that for every one degree Celsius minimum temperatures increase during winter, bears hibernate for six fewer days. With climate models predicting continued global warming, by 2050 the average length of bear hibernation could decline by 15 to 39 days, according to the study.

“If hibernation length is declining, that means bears will be awake longer in the year to interact with people and that means we should expect that bear-human conflicts will increase and that we will probably see more human-caused bear mortalities,” said Johnson.

The study also found that access to more natural food and human food reduced hibernation time.

“The idea behind that is that hibernation is a response to food shortage,” explained Johnson. “So bears hibernate when there is nothing left to eat, and when there is a lot of human foods on the landscape they can keep eating.”

A golden cub in Lake Tahoe. Paul Hamill / www.paulhamillphotography.com


In Tahoe, access to human food has drastically changed the winter denning habits of black bears.

Toogee Sielsch is a trained responder for the Bear League, a Lake Tahoe organization focused on educating the public on cohabitating with bears, and co-director of Sierra Wildlife Organization. His specialty is getting bears out from underneath houses.

This winter Sielsch handled 18 bear “evictions” from under homes.

“We do have a great number of bears that are not denning,” said Sielsch. “I personally had eyes on seven this winter that didn’t den. All of these bears were born within the city limits and live within the city limits.”

Video by Toogee Sielsch

By his estimates, there are around 19 bears that live within the boundaries of South Lake Tahoe. On his early morning walks he finds them opening dumpsters and sifting through garbage cans.

“If they’ve become used to eating these kinds of foods and they’ve become successful at doing it, then they are going to keep doing it,” said Sielsch. “And if the mother is having cubs she is going to teach her cubs to do it, too. Last year there were cubs in the Tahoe Keys going down the streets trying to open every car door.”

Heather Reich, a bear biologist for the Nevada Department of Wildlife, said they see bears coming out of their dens in Tahoe during the winter to access the food supply left by humans.

Though black bears do experience physiological changes when they hibernate — their heart and metabolic rates drop, breathing slows, and they do not eat, drink or defecate — they are not considered true hibernators because they can wake up at any time and be fully functional.

“The Tahoe Basin was actually the first place that was documented in the United States where bears would come out of their dens on garbage day specifically to hit the trash cans,” explained Reich.

Ninety-five percent of human-bear conflicts are associated with trash, according to NDOW.

Video by Toogee Sielsch


This year two wildland bears —versus urban-interface — collared by NDOW and living in the Pine Nuts and Wassuk Range did not hibernate at all this winter.

“Those two bears are very much wildland bears. It’s not like they had access to human food,” said Reich. “The thought is that the pine nut crop was so great last fall that it actually provided a good resource for them throughout the winter. They didn’t actually have to den.”

On the flip side, Johnson’s study points out that food shortages caused by changes in weather patterns can result in an increase in bear-human conflict.

“One of the things that causes food shortages is a late freeze that kills all the flowering bodies and then you don’t have berries in the summer,” explained Johnson. “With climate change we expect the frequency of these type of food shortages to increase over time. With climate change there is more variation, and weather variability is associated with these types of events.”

A large bear lumbers through Lake Tahoe. Paul Hamill / www.paulhamillphotography.com

As bears prepare to hibernate, they up their food intake to more than 20,000 calories a day. Berries make up a large part of that diet.

“If there’s a complete berry crop fail in the fall, that’s when we will see crazy conflict because the bears will be desperate for those calories and they aren’t going to get them from the berries,” added NDOW’s Reich.

Between 1987-1991, NDOW received an average of 20 calls a year. That number continued to rise over the years with an average of 103 calls between 1997-2001 and 178 between 2002-2006. In 2007 there was a large spike — 1,500 calls — but the most recent data for 2013-2014 has the calls in around 601 per year.

NDOW credits drought years with limited natural foods, increasing human development and the growth of Nevada’s bear population for the continued increase — a factor that the Colorado study did not attribute to increased conflicts.

There are an estimated 600-700 bears in Nevada compared to Colorado’s 17,000.

A pair of cubs in a wooded area of South Lake Tahoe. Paul Hamill / www.paulhamillphotography.com


Johnson acknowledged that the hibernation study only examined weather patterns over a short period of time, but said it doesn’t undermine the importance of its findings.

“What we know from climate change is these long-term trends that minimum winter temperatures will increase and the landscape will get more developed with increased opportunities for bears to get human foods, we know with these trends that we can expect bear hibernation to continue to decrease,” said Johnson.

The study’s findings could impact how Colorado manages bears going forward.

By disproving the idea that the bear population is growing, leading to more conflicts with humans, it could impact Colorado’s current approach of increasing bear hunting in a given area based on the number of conflicts.

The study also found that bears that eat human trash do not become “food conditioned” and will return back to berries and nuts when possible, conflicting with the state’s current two-strike euthanizing policy for bears. In Nevada, there is a three-strike policy for “nuisance bears.”

“The major question,” said Johnson, “is how do we effectively manage for that?"

Cover photo: Paul Hamill / www.paulhamillphotography.com

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