The function of strategic tree selectivity in the chemical signalling of brown bears

1. What they did

Many large mammals leave scent marks on conspicuous objects such as trees. Making strong-smelling scent costs time and energy, so the authors asked whether brown bears choose their marking trees strategically, to make each mark work harder. They expected bears to pick trees in places they visit often, where a resource (such as food or mates) is worth defending, and to pick trees that stand out: rarer species, conifers over broadleaved trees, and larger trees.

The study ran from May to October, 2009 to 2011, at Glendale Cove in Knight Inlet, British Columbia, Canada. About 40 to 50 brown bears feed on spawning salmon there in autumn, and about 20 feed on sedge in the tidal marshes in spring, during the breeding season.

• Finding marking trees. The team searched 13 fixed transects (2.6 km in total, 200 m each, running straight up the hillside from the shore), 16 transects along wildlife trails (2.16 km in total), and also searched the forest unsystematically. A tree counted as a traditional marking tree only if it showed rubbing (such as hair and bark worn smooth) and visible claw and bite wounds at about the height of a standing bear.
• Measuring the trees. For each marking tree they recorded species, trunk diameter at breast height, lean, the height of the tallest mark, a description of the marks, and signs of overmarking (marks of different ages).
• Measuring what was available. They recorded every tree within 5 m of each marking tree, every tree over 2 m tall along the fixed transects (the wider landscape) and along the trail transects.
• Confirming with cameras. Seventeen camera traps watched 22 marking trees, confirming that the marks were made by brown bears and not black bears.

2. What they found

Where the trees were

• All marking trees were at forest edges bordering the estuary and river, clustered near rich food: sedge in spring and the salmon river in autumn.
• Marking trees were found on game trails; only 2 of the 50 were found on the fixed transects. Heavily marked trees were usually on more heavily used trails.
• Trees higher on the hillside showed some marking, but they were widely spaced and not marked enough to count as traditional marking trees.

Which trees bears chose

• Of the 50 marking trees, 23 were amabilis fir, 11 Sitka spruce, 8 Western hemlock, 7 Western red cedar and 1 red alder.
• Bears did not mark trees in proportion to what was available. Across the landscape, amabilis fir, Sitka spruce and Western red cedar were marked more than expected, and Western hemlock, the most common tree, much less. Close to the marking trees, amabilis fir and Western red cedar were again chosen more than expected and Western hemlock less.
• Marking trees were larger than unmarked trees nearby (average diameter 42.5 cm versus 31.7 cm).
• The lean of the tree made no difference.
• Tree species and size together predicted whether a tree was marked; the model classified 87.4% of trees correctly.
• The tallest marks were very similar in height, about 2 m (average 2.02 m on 30 trees).

What the marks looked like

• Marks from a bear rubbing with a wet coat were usually visible for less than 2 hours, while bite and claw marks stayed fresh-looking for the rest of the year.
• Fresh marks were recognized by resin oozing from wounds, torn bark with exposed fragments, the colour of exposed wood, and loose hair.
• Amabilis fir and Sitka spruce often lost large sections of bark to biting and clawing, leaving scars. Western red cedar and Western hemlock usually showed rubbing instead: worn bark with a changed texture. Rubbing darkened the smooth bark of the one red alder.
• Several marking trees had a line of depressions in the ground leading to them, made by bears grinding and stomping their feet as they walked. Some lasted the whole study, some only one season. When the authors watched stomping directly, it often left no lasting mark, depending on the ground, so trees with no visible trail may still have had stomp trails.
• Dead trees that had been marking trees when alive were sometimes still marked.

3. What they concluded

• Brown bears are selective about the trees they mark. Location comes first: trees along well-used trails near food and mates, where other bears are most likely to find the scent. Then they favour rarer species and larger trees that stand out.
• Amabilis fir and Sitka spruce, the most marked species, give off pungent resin when torn. The authors suggest this may help the scent last longer or draw other bears to the tree. Because strong long-lasting scents are costly to make, the authors speculate that bears may save energy by using aromatic trees.
• Lean does not seem to matter: bears seem to mark the side of the tree facing the trail, whatever the lean.
• Mark height is unlikely to be a main signal of size for brown bears, since mark heights varied little, cubs have been seen climbing and marking higher, and not all marking involves standing back rubs.
• Across mammals, whether a tree is chosen depends on context. The main function is to limit the energy cost of scent marking by placing marks where they are most likely to be found.

4. How tracking was or was not used

The study rests on reading signs on trees and on the ground:

• Identifying marking trees: bite and claw wounds, rubbed bark with a changed texture, and hair left on the trunk.
• Aging marks: resin oozing from wounds, torn bark, the colour of the exposed wood and loose hair showed fresh marks; marks of different ages on one tree showed overmarking.
• Trails: the team followed wildlife trails until they faded or met a road, river, shore or meadow, and found most marking trees along them. They also recorded the stomp trails of foot depressions leading up to marking trees.

Camera traps and daily checks of marking trees in the first week confirmed that the signs were made by brown bears.

5. Tracker or observer certification

Not reported. The paper does not say who identified the marking trees or how their skill was checked, although the definition of a marking tree depends on reading bite, claw and rub signs. Camera traps were used to confirm that the marks were made by brown bears.

6. Other technologies, skills or innovations

• Infrared camera traps (Reconyx RC55 and PC85) at 22 marking trees.
• A hipchain counter (Walktax, Haglöf) to measure transect length, a diameter tape and a clinometer to measure tree lean.