GENOA, Nev. — There are certain axioms about the natural world we learn as children. The sea is salty. Plants grow toward light. Bears hibernate in winter.
But as climate change leads to warmer winters, later falls and earlier springs — which can disrupt both food supplies and biological rhythms — American black bears are changing their hibernation routines, scientists say. In some cases, bears are not hibernating at all, staying awake all winter. In others, bears are waking from their slumber too early.
For every 1 degree Celsius that minimum temperatures increase in winter, bears hibernate for six fewer days, a study found last fall. As global temperatures continue to rise, by the middle of the century black bears may stay awake between 15 and 39 more days per year, the study said.
A February visit to the Pine Nut Mountains of northwestern Nevada, near Lake Tahoe, provided a preview of what could lie ahead. The previous fall, regional temperatures were as much as 4.7 degrees Celsius (8.4 degrees Fahrenheit) warmer than the 20th century average. In January temperatures were 5.4 degrees Celsius warmer.
Ordinarily Rae Wynn-Grant, a large-carnivore ecologist based at the American Museum of Natural History, would be spying on Pine Nut's sleeping bears to better understand how they choose where to hibernate. But when she checked GPS signals from two bears that had been collared by the Nevada Department of Wildlife, the bears were moving — they were awake. That meant it was too dangerous to try to observe them up close. So on this outing Wynn-Grant was looking for sites where bears could have hibernated if they had gone to bed.
"Over the years we've had reports of bears hibernating under people's decks and in their garages and stuff, so we would have to wake them up in order to get them out," Wynn-Grant said. "But until this year, I had never known about awake bears."
Warmer winters deprive bears of a key signal they need to hibernate: cold weather. In a temperate climate, bears usually hibernate during winter when food is scarce, said Heather Johnson, a research wildlife biologist with the U.S. Geological Survey and an author of the hibernation study published last fall. It's more than just a deep sleep: When black bears hibernate, they do not eat, drink or defecate, their body temperature dips and their heart rate slows to as little as nine beats per minute.
But higher temperatures are not the only reason they may fail to fall asleep. Johnson's study, in the Journal of Applied Ecology, found that both higher temperatures and increased food supplies decreased the amount of time bears spent hibernating.
Researchers think both factors played a role as some of the Pine Nut bears stayed awake this year. The normal winter food shortage did not occur because of a sequence of events that began a year earlier, when an extremely snowy season provided plenty of moisture that led to a bumper crop of pine nuts last fall. This winter, the region, a popular ski area, experienced near-record low levels of snowfall until the end of February. That left the extra pine nuts on the ground, uncovered.
"What we think happened is the bears didn't really have any need to den because the food sources were still available," said Heather Reich at the Nevada Department of Wildlife, a specialist on human-bear conflicts.
A study published last month in Nature Climate Change predicted that this kind of weather, extreme droughts punctuated by the kind of extreme precipitation that led to the bumper crop of pine nuts, would happen more often in California — and by extension the Tahoe area — in the coming years. Researchers call it "precipitation whiplash."
Warmer temperatures do not always mean more food for the bears. In recent years of severe drought, like in 2014 and 2015, their food supplies collapsed. That sent bears in search of humans' food. In those years, Reich said, the bear complaints started early and kept on coming.
Often, the first signal that the bears are awakening comes in the form of someone registering a complaint with the Nevada Department of Wildlife.
In one complaint from 2015, a resident of Skyland, on Lake Tahoe, reported that a bear had entered her house through a closed but unlocked door.
According to a report filed with the department, the resident "found her kitchen door open and wet paw prints entering the house, going around the kitchen table and to the pantry door where the bear took a package of macaroni and a bag of Cheetos and exited."
In a typical year, the Nevada Department of Wildlife handles 69 bears that have wandered onto properties or have been hit by cars. But the department handled 143 bears in 2014 and 122 in 2015, during those drought years. The department tags and releases most of the bears, but it destroys a small number of them, to the consternation of some wildlife groups.
Researchers say climate change is increasing both the likelihood and intensity of drought in the region.
The bears are so close to people because of the expansion of what is known as the human-wildland interface, where developed land and wildlands meet. Researchers at Conservation Science Partners calculated that even in the less densely populated Western states, as of 2011 the average natural area was just 3.5 miles away from a developed area.
That means people's food is accessible when the bears' natural food sources run out.
It's almost as if bears were designed to get their hands on our food. The fur on their paws conceals appendages so similar to our own that meanspirited pranksters will occasionally leave a skinned bear paw in public to startle people who confuse them with human hands. Bears may not have opposable thumbs, but with their versatile and powerful paws they're able to get into trash cans, cars and even homes. Homeowners often complain about thousands of dollars in damage as bears rip off car doors and tear into garages in pursuit of food.
But accessing our detritus is often bad news for the bears. Once, Wynn-Grant and a colleague found a bear, a little over a year old, dead near Lake Tahoe. A field necropsy revealed that its stomach was engorged with dozens of individual ketchup packets. The packaging most likely killed it.
Yet black bears are something of a conservation success story. They rebounded from the edge of extinction in the late 1800s to an estimated 300,000 bears across the United States today. They returned to Nevada in the late 1980s when they crossed over from California via the Sierra Nevada.
Their recent arrival provided researchers an opportunity to study the animals with the goal of better understanding how to prevent human-bear conflicts — not just for black bears, but also for bears like grizzlies whose numbers are more tenuous.
When those conflicts increase, some people argue that there are too many bears, and that their numbers must be increasing. But the conflicts may be a sign of trouble for the bears. Johnson and colleagues were tracking female bears near Durango, Colorado, when the bears' natural food supplies collapsed in 2012— an event, like the California droughts, that is likely to increase with climate change. Complaints about bears in the area increased, but the female bear population plummeted: Of 203 bears tracked before the food failure, only 87 survived.
The relationship between humans and bears may grow even more fraught, since climate change could extend the time bears spend awake and disrupt their natural food supplies.
"My prediction is we will have fewer cubs survive the winter, and so many bear conflicts that residents will want them to be hunted off again just like the 1800s," Wynn-Grant said. "Except this time we have climate change to blame."