MISSOULA — A large grizzly bear can weigh up to 800 pounds and for a large animal to take such steps a lot of energy must be used to carry all that weight around. But until recently little was known about the energy expenditures grizzlies had as they moved around their habitats.
Usually, the fattest females produce the fastest-growing cubs that also have the best chance of survival. Similarly, the biggest males do most of the breeding. Therefore, bear foraging and which decision they make on where to walk are probably closely tied to maximizing energy intake while minimizing energy expenditures.
Researchers at Washington State University decided to measure the animals' metabolic rates as they walked on different slope types to find out how much energy they use on a daily basis and using captive bears to measure their physiological differences.
The study published in the Journal of Experimental Biology discovered that grizzly bears prefer to walk on shallow paths to save energy and helps explains why the animals often appear on human hiking trails. The study combined their research with the ongoing work on grizzlies in Yellowstone - a study that has been around since the 70s.
Combining measurements on both wild and captive bears allows researchers to understand the ecological context of the bear’s foraging and habitat use decisions. The biologists used a treadmill that is normally designed for a horse to walk on and trained the captive grizzly bears to walk on the equipment.
As you can imagine, bears don’t work for free, so they fed them their favorite treats while they were exercising on the treadmill.
Now, if they were studying energy expenditure in humans, they would’ve used masks to collect respired air, but of course, grizzly bears will not wear a mask. So, they encased the treadmill in a polycarbonate box and continuously pulled air through the treadmill and box. Small samples of that air were taken to determine energy expenditure.
The front and back of the treadmill can be raised or lowered to help them measure the costs of bears walking on level ground and up and downhills. They also monitored the bear’s heart rate with the same implanted cardiac monitors used on people, evaluating activity with Fitbit-type sensors. They determined that ascending and descending slopes were quite costly for the bear's energy.
Researchers also found that the grizzlies did not have a lot of energy to maintain long high-speed chases. Grizzlies consume similar amounts of energy to climbing humans, wolves and wild cats, in contrast to fleeing elk and deer, which use 46% less energy than grizzlies over mountainous terrain.
GPS collars on the wild bears in Yellowstone National Park showed that the bears walk at a natural speed of 0.6 meters per second -- similar to that of humans. It also turned out that grizzlies tend to select flatter paths that wind along shallower gradients to save energy, instead of tackling steep ascents and descents - the bears chose to do a lot of side-hilling.
Coincidentally, the bears' preference for shallow gradients is the same as the US National Park Service's recommendation for the maximum gradient of trails, which explains why they often use the same trails we hike on. Additionally, it’s more than likely if a bear hears a person coming on a trail they will move away. And the chances of seeing that bear are slim.