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The Bear Facts


Senate Minority Leader Harry Reid has said at least twice that climate change is causing bears in the Sierra Nevada mountains to change their hibernation patterns. There is no evidence that climate change is actually having such an effect.

On July 29, Reid, a Democrat from Nevada, spoke on the Senate floor (at the 5:30 mark) about some of the damaging effects of a warming climate:

Reid, July 29: The drought that’s affecting all across America, but especially in the West. It’s so bad, Mr. President, so bad in the West that in the Sierras there are hot bears who don’t even hibernate anymore. It’s not cold enough.

In another Senate floor speech on July 15, he made a similar claim: “We have things happening that have never been recorded before. In the Sierras some bears are not hibernating.”

We could find no published evidence regarding changes to black bear hibernation, and biologist Jesse Garcia of the California Department of Fish and Wildlife told us that “we have no data or specific evidence” that black bears in the Sierras have changed their hibernation patterns.

Black bears are not “true” hibernators, in that their metabolism and body temperature do not drop as dramatically as those of other animals, such as chipmunks and squirrels, according to the CDFW. And in spite of the ongoing drought and recent extremely warm years, Garcia told us in an email that black bears’ activity has not changed:

Garcia, Aug. 13: We have no reason to believe that the drought or climate change is significantly changing hibernation behavior in California black bear populations. This statement is mainly due to the fact that we observe great variability of black bear denning or hibernation chronology throughout different regions of our vast state. And even in specific areas (such as around Lake Tahoe), black bears will often display some mid-winter activity even during non-drought years.

He explained further that black bear hibernation patterns vary based on location, with some hibernating for up to eight months in the far northern parts of the continent and others in more southern areas not hibernating at all. “For hibernating sub-adults and adults,” Garcia explained, “if the winter is mild enough, they may wake up and forage for food.”

SciCHECKinsertThere is no specific data, however, showing bears are waking up more in recent years than they had in the past. A spokeswoman for Reid sent us two news articles as support for the claim, but neither offers more than anecdotal evidence of any particular increase in winter bear activity.

Though evidence is lacking on any changes to hibernation for bears in the Sierras, there are studies indicating climate change is having an effect on other hibernating animals. For example, a 2000 paper published in Proceedings of the National Academy of Sciences found that yellow-bellied marmots in the Rocky Mountains in Colorado emerged from hibernation an average of 38 days earlier than 23 years before, “apparently in response to warmer spring air temperatures.”

Climate change can have unexpected effects on specific areas and species. A 2012 paper published in the journal Nature showed that Columbian ground squirrels living in Alberta, Canada, actually emerged from hibernation later over a period of 20 years, by an average of 0.47 days per year. The researchers still blamed this on climate change: There was no particular trend in the region with regard to temperature, but there was an “increasing likelihood of late-season snowstorms” and thus delayed snowmelt. The squirrels wait for the melt before emerging.

In Reid’s July 29 speech, he got a lot of things right about climate change: Arctic ice caps are indeed melting; wildfire seasons, such as the one currently devastating Alaska, are indeed getting worse; and moose in New Hampshire are suffering thanks to changes to the tick population. The evidence on the bears of the Sierras, though, doesn’t measure up.

Editor’s Note: SciCheck is made possible by a grant from the Stanton Foundation.

– Dave Levitan