Grizzly Bears Can Conserve Energy By Using Human Hiking Trails
Conserving energy is important for bears, especially in the fall, when they're getting ready for hibernation. According to new research, their travel habits have a lot to do with that goal.
"If a bear's traveling faster around a road versus further into the wilderness, we can say, 'well, they're traveling faster,' but what does that mean for the bear? And so our main goal with this project was to be able to actually put a currency to that behavior, i.e. energy expenditure," said Washington State University PhD student and lead author Tony Carnahan.
Carnahan and his team put nine grizzly bears from Washington State University's Bear Research, Education, and Conservation Center onto a horse treadmill with a special enclosure built around it to measure oxygen exchange. Food rewards like slices of apple and hot dogs motivated the bears to enter the enclosure, and, eventually, walk.
"We'd slowly turn on the treadmill. At first, they couldn't understand why they were or how they were moving away from their food without actually physically moving. And so some of them kind of get into this little crawl and try to get back up to the food and finally, we were able to get the treadmill up to speed where they actually had to start walking to keep in contact with the food," said Carnahan. "That was probably the hardest thing for the bears, but once they caught on, they were just fantastic at doing it. As long as they were receiving a food reward, they're more than happy to do what we asked. Bears are highly, highly food motivated."
The team took measurements from the bear walking on the flat and at various slopes uphill and downhill. These they plugged into various equations to get the optimal speed for bears to travel to conserve energy.
"We calculated the most efficient speed that we would expect them to go on the horizontal, uphill, and downhill, and that was about 4.2 kilometers an hour," said Carnahan.
But the team wanted to know if this number was applicable in the real world. So they compared data from GPS collared grizzlies in the Greater Yellowstone Ecosystem over the last 70 years.
"We expected that that should be the rate at which bears in the Greater Yellowstone Ecosystem would travel and it really wasn't. It was about a little more than half of the optimal speed, or the most efficient speed for them. We found that they traveled on average about 2.2 kilometers an hour," he said.
They also found that the bears appeared to prefer slopes that were 10 percent grade or less. If a hill was steeper, they would meander back and forth in a switchback pattern to stay moving at that grade.
"The fact that they didn't move at the most efficient speed made us re-examine why they might be moving at a slower speed," said Carnahan. "Bears are omnivores, and so as they're traveling across the landscape, different vegetation is maturing at different rates.
So it made sense that they might forego efficiency of speed for being able to take in the landscape and see what was available or what was becoming available to them as a forage source."
This could also explain why bears are often spotted on hiking trails - they're relatively clear of debris and they tend to be under a 10 percent grade.
According to Carnahan, this important data can be used for future conservation projects, like giving insights into how much and how fast bear populations can recover from recent population drops based on local resources.
"Because we know the energy acquired from different food sources and the energy they will need to make it through hibernation, then we can start making some inferences into what they get off the landscape, how their movement behaviors can impact their ability to basically make a living off the landscape," he said.
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