Grizzlies and Wolves in the Northern Rockies

by Jessica Edberg – International Wolf Center (2005)

The grizzly bear and the gray wolf, two of the most powerful predators in the great, wild West, are demonstrating a unique association. In places where grizzlies and wolves coexist, it has been documented that bears often follow wolf packs with the foresight that the wolves will lead them to a food source. In general, grizzlies usurp carcasses from wolves by using their large size to intimidate. Now there is evidence that this interaction between wolves and grizzlies is associated with a little-known phenomenon that may change the way we think about grizzly bears and hibernation.

Hibernation is an adaptation that many mammals utilize to help them survive the season that poses the most difficulty in finding food: winter. Most often, this state of inactivity is brought about by short day lengths, cold temperatures, and limited food. To many people, the word hibernation elicits a mental image of a bear, black or brown, curled up, deep in sleep in a cozy den for the long, cold winter. In reality, hibernators such as deer mice, black and grizzly bears, skunks and raccoons are not “true hibernators (long periods of inactivity and body temperatures dropping to 5 degrees C). Instead they exhibit a less profound inactive state called torpor.

Torpor can be very short-lived (the cold hours of the night, for example) and involves the drop of the animal’s body temperature to no less that 15 degrees C. An animal in torpor is also capable of relatively quick arousal. A major advantage of torpor is that the animal can be active right up until winter conditions become excessively stressful and can even be active all winter long if the season is particularly mild.

Some grizzly bears in Glacier National Park, in northwest Montana, are foregoing the traditional form of winter survival for a more active alternative. James C. Halfpenny states in his book Yellowstone Wolves in the Wild that “it appears that some grizzlies may avoid hibernation by feeding on carcasses provided by the increasing wolf population. Researchers are watching the wolf-grizzly interactions in Glacier and Yellowstone National Parks closely to obtain more information on this interesting phenomenon of bear hibernation cessation.

Time and research will tell if recovering wolf populations in the West will affect grizzly bear hibernation patterns, and possibly answer questions such as “What factors make one bear chose hibernation and another chose winter activity? “Will all grizzly bears eventually forego hibernation in favor of taking advantage of wolf predation as wolf populations increase? “What other factors (besides daylight, cold temperatures and prey availability) may limit or lengthen grizzly bear hibernation?

Predator-prey interactions are complex, and this curious phenomenon adds one more piece to the puzzle of how species interact and affect one another.