By E. Kirsten Peters, College of Agricultural, Human & Natural Resource Sciences
Grizzly bears put my weight gain to shame. In late summer, they eat some 50,000 calories per day and gain more than 100 pounds. When they hibernate, they fast and live on their body fat. While sleeping the winter away, they don’t pee or poop. They conserve energy by having heart rates around 15 beats per minute.
While hibernating, the sows give birth and nurse their young – activities fueled by what they ate in the fall. When they emerge from their dens in the spring, the bears are much slimmer. In short, their “before” and “after” pictures are quite different.
In – and out – of diabetes
Here’s the simple version of how grizzlies manage their huge weight transition. They first succumb to diabetes and then reverse slipping into that state. We know when they do this; researchers are now investigating how they manage the trick.
Lynne Nelson and Charles Robbins of Washington State University work with grizzlies kept in the only research-based grizzly colony in the country. They study the bears as they go through their annual transformations. In the fall, when the bears are packing on the pounds, they are fed commercial kibble supplemented by such things as salmon, venison and apples. The bears also have access to a grassy meadow.
“Grizzlies are grazers,” Nelson told me. “People don’t always think of that, but they eat a fair amount of grass.”
One secret to how grizzlies manage to stay healthy while becoming obese is that they have a lot of “good” cholesterol. And their cholesterol levels don’t change much when they pack on the pounds. Studying how they do that could one day help with interventions in human medicine.
Hearts at rest
A number of things the bears do while they hibernate are fascinating. The animals have a four-chambered heart, just like we do. But when they sleep the winter away, only two of the chambers keep working while two are at rest.
“Working on two of four cylinders makes sense because the demands on the heart are low,” Nelson said.
Even with that reduced cardiac output, grizzlies can stand up and move around during hibernation. Humans would black out in a similar situation. Again, studying what bears can do may help spur advances in human medicine.
As the winter months tick by, the grizzlies’ hearts lose muscle mass. Up to 25 percent of their hearts can atrophy. This change is then naturally reversed in the spring when they come out of their dens and begin a more active life.
Human exposure from the start
Of course, doing cardiac research on grizzlies requires some special approaches.
“We start training the bears when they are cubs for exams we’ll want to do on them throughout their lives,” Nelson said. “It’s easier to start on an animal that’s 4 pounds rather than one that’s 400 pounds.”
Nelson, Robbins and those who work with them use positive reinforcement and “clicker training,” much like that used with dogs today. Food is used as the ultimate reward.
“Bears are faster learners than dogs,” Nelson said. “They are problem solvers.”
The goal is to have bears trained so that researchers can draw blood from them and administer exams like electrocardiograms and echocardiograms (an ultrasound test). To facilitate the research, the bears are taught to go into a crate.
“They sometimes fight to get to go into the crate first,” Nelson said.
Problem bears sheltered
The bears raised from cubs at the WSU facility are used to a lot of interaction with people.
“They need entertainment or work,” Nelson said. “Left to their own devices, they will dig up the sprinkler system (in their yard) or pull down the security cameras.”
The WSU bear colony has 11 animals. About half of the bears were raised at the center while the other half were wild bears that started posing problems or a danger to humans and were brought to WSU rather than being destroyed.
As I struggle with my extra 5 pounds, I marvel at the weight transitions grizzlies naturally go through each year – and I wish the WSU researchers well as they study bear metabolism, weight transitions and cardiac function.
Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University.