Tree rubbing by Yellowstone grizzly bears Ursus arctos

1. What they did

Bears rubbing on trees has been seen across the northern hemisphere, but it has rarely been studied, and earlier writers disagreed about why bears do it: for hygiene, for relief from insect bites, for stretching or massage, to find their way, or to communicate by scent.

The authors recorded tree rubbing by grizzly bears in the Yellowstone region from 1986 to 1992, as part of a larger study of how radio-marked bears fed. Bears were captured, measured (including their body length from nose to tail) and fitted with radio collars, then located from the air about every 4 days. Field crews visited these locations, and also recorded any grizzly bear signs they came across on the way there and back.

At each site they described the landscape and forest: slope, aspect, landform, distance to the forest edge, amount of deadfall, and the tree species present. At each rubbed tree they recorded the tree species and diameter, any clawing or biting, how recently it had been rubbed, which way the rubbed surface faced, the highest point of rubbing, how much of the trunk was rubbed, and any bear trails or game and recreation trails nearby. They judged how recently rubbing had happened from the brittleness and bleaching of hair caught on the bark, how runny the sap was, and the freshness of other signs such as scat, urine and tracks.

They then used statistical models to ask which sites and which trees bears chose for rubbing, and when.

2. What they found

Rubbing sites

• Crews visited 1,225 forested sites where radio-marked bears had been located, and found feeding, bedding or rubbing by unknown grizzly bears at 779 more.
• At 93 sites, at least one grizzly bear had rubbed something. Bears rubbed 116 trees, one trail marker post, the side of a backcountry barn and the porch posts of two backcountry cabins.
• Seven radio-marked bears were known to rub trees: four females and three males.
• Of the 116 rubbed trees, 76 were lodgepole pine, 18 Engelmann spruce, 12 Douglas-fir, seven subalpine fir and three whitebark pine.

When and where bears rubbed

• Rubbing peaked in late spring and early summer, around 7 June, during the mating and moulting seasons, and declined after that.
• Bears chose gentle, south-facing slopes, sites close to the forest edge, and places with little deadfall. Rubbing sites were a median of 10 m from the forest edge, against 50 m for other bear sites.
• Rubbing was most likely in stands dominated by Douglas-fir or lodgepole pine, and least likely where whitebark pine dominated.

Which trees, and how

• Bears did not seem to choose particular tree species, but they strongly chose larger trees. Rubbed trees averaged 45.0 cm in diameter, against 33.8 cm for trees nearby that were not rubbed.
• 70% of rubbing sites were on recreation trails and 16% on game trails. 63% of rubbed surfaces faced towards the trail.
• Bear trails, short lines of deep pad-shaped prints leading up to the rubbed tree, were found at 58% of rubbing sites, whether or not there was a human trail. The 46 bear trails measured averaged 6.17 m long, with about two pad prints per metre, each print about 3.2 cm deep.
• Every tree marked by a grizzly bear had been rubbed; only 9% showed clawing or biting.
• On average about half of the trunk’s circumference was rubbed. The highest point of rubbing averaged 1.73 m, close to the body length of adult bears caught in the study (1.88 m for males, 1.70 m for females).
• Most rubbed surfaces faced east (37%) and fewest west (14%). Almost all trees were free of branches where they were rubbed, and many leaned slightly towards the rubbed side or had a burl or rounded bump at the centre of the rubbed surface.

3. What they concluded

• Yellowstone grizzly bears mainly rubbed their backs and shoulders while standing upright. This differs from reports of American black bears, which often claw and bite trees, although brown bears in Europe and Asia have also been described clawing and biting. More study is needed to explain these differences.
• Bears seemed to choose large trees, smooth or with burls and a slight lean, that made rubbing more pleasurable, and sites that were easy to reach. The authors suggest south-facing trees may release more scent in the sun, but this is speculation.
• Rubbing is strongly linked to travel routes, which explains its link to forest edges. Some of this could be observer bias, because the crews often walked on trails, but the authors argue the bias was partly controlled for.
• Fewer rubs in whitebark pine stands probably reflect where bears were feeding at that time of year.
• The repeated use of the same trees, and the bear trails worn into the ground leading to them, do not fit rubbing simply to relieve itching. Together with the seasonal peak, this is consistent with earlier conclusions that rubbing is a form of scent communication between bears, though rubbing may also relieve irritation and serve as landmarks.

4. How tracking was or was not used

The study rests on finding and reading bear signs in the field. Crews identified rubbing, feeding and bedding by grizzly bears, including at sites where the bear was unknown. They recorded clawing and biting on bark, and found and measured bear trails: lines of entrenched pad prints leading up to rub trees, with their length, number of prints per metre and the depth of each print. They estimated the age of rubbing from hair caught on bark, sap, and the freshness of nearby scat, urine and tracks. No trailing of individual bears is reported; bears were found by radio telemetry.

5. Tracker or observer certification

Not reported. The paper says only that field crews collected the data, and names people in the acknowledgements, with no information on their sign-reading skill. The findings depend on crews telling grizzly bear signs and on aging signs.

6. Other technologies, skills or innovations

• Radio collars, with bears located from fixed-wing aircraft about every 4 days.
• Body measurements of captured bears used to check whether rubbing height matched body length.
• Plots centred on each rubbed tree, with trees weighted to correct for the fact that smaller trees are harder to detect.

7. Implications for conservation (from the paper)

• If tree rubbing is a form of marking, disturbing or removing traditionally rubbed trees could disrupt communication among bears.
• Bears moved by managers to a new area may have to learn where the rub trees are, which could affect their chances of finding mates, especially for males.
• Rubbing is probably more important in low-density populations, where finding mates is harder.
• Where managers aim to reduce conflict with people and keep bear populations socially healthy, traditionally rubbed trees should be considered in management decisions.