A Hypothetico-Deductive Approach to Assessing the Social Function of Chemical Signalling in a Non-Territorial Solitary Carnivore

1. What they did

Bears are mostly solitary and range widely, so they are thought to rely heavily on scent to communicate. Brown bears claw, bite, urinate on and rub against trees, and some trees are used over many generations. Yet little was known about which bears mark these trees, which bears only sniff them, and why.

The authors tested four ideas about what tree marking is for:

• Self-advertisement for mate attraction: adult males (or adult females) advertising themselves to the other sex.
• Communicating dominance: adult males signalling their status to other bears.
• Competitor assessment: bears sniffing marks to size up rivals.
• Infanticide avoidance: mothers with young keeping away from marking trees, because males may kill young.

At Glendale Cove, Knight Inlet, British Columbia, Canada, they searched the forest along wildlife trails and an old logging road for marking trees. They looked for claw and bite marks, hair left from rubbing, and wounds from bark stripping. Old claw and bite scars showed a tree had been used for a long time. Fresh marks were shown by oozing sap, torn bark, the colour of the exposed wood and loose hair. They monitored 21 such trees.

They set up trail cameras (7 in 2009, 17 in 2010) facing marking trees along the trails, recording day and night. Breeding season data came from June and July 2009 and 2010, and non-breeding season data from August to early October 2010, when the pink salmon were running.

For each “event” (a bear caught on camera), they recorded age and sex class and what the bear did: scent marking (rubbing, clawing, biting or licking the tree, rolling or sitting next to it), investigating (sniffing the tree or the ground near it, stretching the neck, changing course or hesitating towards it), or simply walking past. Direct watching from boats and viewing platforms helped them identify individual bears and place them in the right class. They then compared how often each class marked and sniffed trees with how often that class used the trails.

2. What they found

The sample

• 1,050 events were captured. 733 could be assigned to an age and sex class (159 in the breeding season and 574 outside it), and 694 of these were identified to the individual bear.
• Leaving out dependent cubs, 25 individual bears were identified in the breeding season and 51 outside it; 11 were seen in both.
• The mix of bears on the trails matched the mix of bears seen in the area, so the trail bears were a fair picture of the local population.

Adult males

• Adult males marked trees more than expected from how often they used the trails, in both seasons.
• In the breeding season they marked in 38 of 77 events; outside it, in 70 of 168 events.

Adult females

• Adult females marked less than expected in both seasons (7 of 37 events in the breeding season; 11 of 143 outside it).
• They did not sniff marking trees more during the breeding season, and sniffed less than expected outside it.

Subadults

• Subadults marked less than expected in the breeding season (3 of 23 events), but as expected outside it.
• In the breeding season they sniffed trees (7 events) more often than they marked them (3 events).

Females with young

• No mothers with cubs under a year old were caught on camera in the breeding season, although such families were seen near areas with marking trees.
• Mothers with yearlings did use the trails then, and marked and sniffed trees about as expected.
• Outside the breeding season, families (including cubs under a year) were caught on camera for the first time. They had the highest number of trail events of any class, the most sniffing, and as many marking events as adult males (71 compared with 70), even though there were about half as many of them in the population. They marked and sniffed about as often as their trail use predicted.

The four ideas

• Communicating dominance: supported for adult males. The matching prediction that adult females do not mark to signal dominance was also supported. Partly supported for subadults.
• Self-advertisement: partly supported for adult males; not supported for adult females. Subadults avoided advertising themselves during the breeding season.
• Competitor assessment: not supported for any class.
• Infanticide avoidance: not supported overall, though mothers with young cubs did avoid the marking trails in the breeding season.

3. What they concluded

• Tree marking by brown bears seems mainly to be a way for adult males to communicate dominance to other males, in both the breeding and non-breeding seasons. Outside the breeding season it may help reduce fights where bears gather at food.
• Marking trees do not seem to be how adult females advertise that they are ready to mate. Females might use other kinds of scent marks, such as marks on the ground.
• What subadults gain from marking is unclear, but it may be linked to the behaviour of adult males. Sniffing more than marking during the breeding season may help subadults learn who is around while avoiding drawing the attention of adult males. Subadult sexes were pooled, so splitting them may show more.
• Sniffing marks to assess rivals is unlikely to be the main purpose on its own, but may come before dominance marking.
• The authors propose a new hypothesis: mothers may introduce their young to marking trees outside the breeding season, a relatively “safe” time of year, so the young learn the scents of adult males. This could help them avoid infanticide as cubs and competition as subadults. They report possible imitation of mothers by young cubs at trees (unpublished), and say more evidence is needed.
• The patterns may apply to other non-territorial solitary carnivores with a similar social and spatial structure.

4. How tracking was or was not used

Tracks and signs identification was used to find the study trees. The researchers searched the forest along wildlife trails and an old logging road for marking trees, recognising them from claw and bite marks, rubbed hair, and bark stripping. They separated long-used trees (old claw and bite scars) from those with recent activity (oozing sap, torn bark, the colour of the exposed wood, loose hair). Cameras then recorded marking that leaves little or no visible sign, such as cheek rubbing. No trailing was reported.

5. Tracker or observer certification

Not reported. The paper does not say who identified the marking trees or how fresh marks were judged, or describe their experience. The choice of the 21 monitored trees rests on this sign interpretation.

6. Other technologies, skills or innovations

• Reconyx passive digital still-image cameras (model RC55 in 2009, plus model PC85 in 2010) with infrared flash, placed opposite marking trees and facing along the trail to catch bears approaching and leaving.
• Identification of individual bears from camera images and direct observation, adapted from earlier work: scar patterns were used as a stable feature within a season, and coat colour as a changing feature, because bear coat colour can change over time and looks darker when wet. Other clues included size, presence of young, behaviour around boats, people and other bears, habits, and features such as a short snout or pale claws. A new bear was only counted when caught on camera twice, or once on camera and once seen directly.
• The authors note they are the first to publish camera trap data on scent marking by wild bears, with more trap nights than earlier camera studies of bears (493 to 1,200).