Since August 1, the bears at the Washington State University Bear Research, Education, and Conservation Center have added around 20 percent body fat to their frames by eating voraciously.
Soon, they will stop eating altogether, and settle down for a nice long rest.
“That weight gain is their energy source,” said Heiko Jansen, an associate professor in WSU’s College of Veterinary Medicine. “It’s the metabolic fuel that gets them through the five-plus months of hibernation.”
Grizzlies and other bears rely on internal and external cues to let them know when to go into and come out of hibernation, Jansen said.
“Falling temperatures and less daylight hours provide some cues, but they also rely on the hormone leptin,” he said. “That tells them how much fat they have on board, and it increases to about 10 times the normal levels in October.”
By the end of October, the bears are ready to hibernate.
Not all sleep
During hibernation, the bears don’t sleep the entire time. They move around a little and do some stretching. They’re awake for about 15 minutes a day, Jansen said.
At the start of hibernation, Bear Center staffers and volunteers wait until the bears are settled in for a day or two, then go in and clean up any waste and add straw to make the bears more comfortable in their dens. Bears don’t defecate or urinate during hibernation.
“They have a beautifully efficient body system,” Jansen said. “They don’t drink much or eat anything, but burning fat actually produces water. Their bladder can re-absorb water, so the only waste they create is the carbon dioxide from burning fat.”
As opposed to smaller hibernating mammals, like squirrels or marmots, bear hibernation doesn’t mean they’re unconscious. Instead, it’s a very deep, slow-wave sleep, Jansen said.
Work at the Center
While the bears are sleeping, the people at the Bear Center will be preparing for the spring. That includes center manager Brandon Hutzenbiler, who helped start a new enrichment program for the bears along with the previous manager.
“We’ll build some new puzzles and other items for them to play with,” Hutzenbiler said. “I’ve been wanting to build an elevated structure that will allow them to swing about a foot off the ground, like a hammock.”
The center relies on donations for most supplies, including what they use for the enrichment program.
“The fire department has been great, they gave us old fire hoses that hold up fairly well to grizzly teeth and claws,” he said. “But we’ve found that they really like to lay on very big tires, with a fire hose woven through the middle. So if anybody has any big tractor tires, let us know.”
Hutzenbiler is also hoping to build a few activities in the bears’ exercise yard to challenge their intellect in getting food.
He’ll be working with WSU student volunteers to put the hibernation period to good use and benefit the bears when they wake up.
Studying the bears
If humans tried to do what bears do every year, they wouldn’t survive. Bears reach weight levels that would be considered morbidly obese in humans, then don’t do anything for more than five months.
In humans, obesity can affect the body’s ability to process glucose with insulin, possibly leading to diabetes. But bears lose their insulin sensitivity during hibernation, and regain it as soon as they come out of hibernation and start eating.
“Right now, they’ve got a lot of fat and don’t move much, which is a really bad situation for a human,” Jansen said. “But the bears don’t have any problems with that and even thrive on it.”
Humans would see their muscles waste away and their bones weaken if they tried to sleep for five months with little movement. And we would suffer from congestive heart failure if our heart rate dropped to the levels bears reach during hibernation.
“Bears don’t have any of these problems in hibernation, like diabetes and heart problems,” Jansen said. “They’ve evolved to tolerate things that would be fatal for us. So we’re trying to figure out how they do it to hopefully help solve some of these problems in humans. Studying our bears continues to provide important data on how to help humans as well as bears in the wild.”