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The Rub on Bear Rub Trees

Shepherd directs me to the work of Kate Kendall, a bear researcher with the Northern Divide Bear Rub Project in Montana. Kendall is investigating why bears rub trees, and how this behaviour might help us understand population trends.

I Google her and find that not only does she have photos, but video as well. As I watch a video of a bear rubbing on a tree, it’s hard to find the neutral objectivity that – according to my university professors – is required of a good biologist. I am completely enamoured with the rhythmic side-to-side motion as the bear rolls its shoulders across the bark. At one point it drops into a squat, arms out in front, and then does a Beyoncé-like chest shimmy. Then it thrusts its head back, stretching its neck around the tree and moves up and down like a pole dancer. It has got to be enjoying itself. 

Of course, there is more to this behaviour than simply scratching an itch. In fact, scratching is likely only a minor part of the reason bears tree-rub. 

“The thinking is that [the behavior] is primarily for chemical communication with other bears.”

Kendall notes that bears are solitary except when mothers travel with cubs. They must, therefore, have a system of communicating to space themselves out on the landscape. For example, females with cubs avoid males, especially during the breeding season. Juvenile bears avoid more dominant bears to some degree, although dominance does not appear to play a major role in which bears use a tree over the course of a season. 

Kendall once found a heavily used rub tree just up the trail from a bear that was feeding on an elk carcass. She felt it was the bear’s way of letting other bears know that they were on its turf, and to take a hike!

Rub trees are easily identifiable due to the fact that one or more bears will rub it repeatedly over the course of a season, and often over many years. 

And it’s not just trees. Kendall says bears often key in on anything that stands out. “Bears rub on almost every wooden sign post along forest roads and trails [in my study area].” A lot of those signs are at road or trail junctions. It might be as much a function of something that stands out and attracts their attention, either visually, or because it smells differently. “We have bears rubbing on metal phone boxes, and they are especially attracted to creosote treated lumber, whether it’s a power pole or bridge abutment.” 

Regardless of the structure, bear rub “trees” are usually quite accessible, without much shrubbery or debris around the base.

“A lot of these trees have trails leading up to them,” says Kendall. “[The bears] have a stylized way of grinding and placing the paws on the ground.” Because each bear plants their feet in the exact same position, over time bear-paw depressions are worn in. “They could be leaving behind a visual indication of their presence, but there’s probably some scent-marking involved in grinding their feet as well.”

One of the most identifiable features of a bear-rub tree is that bear hair collects in the bark on its surface. This feature is proving itself to be of great value in the long-term study of grizzly bear populations. DNA extracted from hair samples can identify individual bears that are using the area. “DNA is the best way to get a population estimate, [especially] where bears can’t be counted from the air” says Kendall. 

Furthermore, if you couple hair samples from bear-rub trees with a remote camera, you not only know which bear has been there, but when. “It’s always more powerful to have more than one way of sampling. It makes my population estimates a lot stronger, more precise and less biased than if I was just sampling with one method.”

Another advantage of using hair sampling to estimate population size is that it is easier on the bear. Traditional methods of estimating bear population size and trend require the capturing and collaring of individual bears. This is achieved using snares, traps, or helicopter darting.

“There is some concern about the long-term effects of handling bears,” says Kendall. She points to research that suggests bear activity rates were lower up to one month following a capture, and body condition was lower in bears that were repeatedly captured. “If we can remove the stress to bears, danger to people and cost associated with [capture], that would be great.”

Through the Northern Divide Bear Rub Project, Kendall continues to evaluate the effectiveness of collecting hair samples from rub trees to monitor population trends. So far, the results are encouraging. “We detected 335 unique grizzly bears this year – so a third of the estimated grizzly bear population.” Collecting hair from bear rubs promises to be a safe, reliable, and cost-effective way to sample bear populations and monitor their status over time. Says Kendall: “I’m very excited.”

To see video from the Northern Divide Bear Rub Project, visit: nrmsc.usgs.gov/research/KendallRemoteCamera.htm 

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