Black bear marking behaviour at rub trees during the breeding season in northern California

1. What they did

Bears leave scent in many ways: rubbing against trees, rocks or the ground, marking with their feet as they walk, and urinating. Rub trees seem to matter a great deal for bear communication, but very few people have actually watched bears using them in the wild. The authors set out to film American black bears at rub trees during the breeding season, to see which marking behaviours they use, how often and for how long, and whether bears of different sex and age mark differently.

They worked in Redwood National Park and neighbouring timber company land in northwestern California. They found rub trees by walking along bear trails and looking for a combination of hair, bite and claw marks, wounds on the tree, and a worn line of tracks leading to and from the tree. At six rub trees they set infrared video cameras, mostly at knee to shoulder height (one was later moved 5 m up because bears kept knocking it), recording 60-second clips from 25 April to 9 August 2013, for 529 trapping nights in total.

From the videos they built an ethogram (a catalogue of behaviours) with eight marking behaviours, including rubbing with the back or face, biting, scratching, sniffing, stance, orientation and “pede marking”, where a bear walks stiff-legged, places its hind feet in its front footprints and twists or shuffles its feet into the ground. Where possible, they recorded each bear’s sex, age class and individual identity.

2. What they found

Who marked, and when

• They filmed 49 bear visits, including 31 marking events.
• All 16 marking bears whose sex could be seen were males; the sex of the other 15 could not be seen. Adults made 29 of the 31 marking events. They could confidently tell apart 12 individual bears from coloration, size and patterns of hair loss.
• Marking happened between 26 April and 27 July, with a median date of 12 June; 45.1% of events fell between 1 and 15 June.
• Marking happened between 05:45 and 20:45, with a median of 10:43, and 53.8% of events were between 08:01 and 12:00. By time period, 83.9% were in daylight, 16.1% around dawn or dusk, and none at night.

How they marked

• Rubbing against the tree was the most common behaviour (83.9% of events), more frequent than sniffing (41.9%), pede marking (38.7%), scratching (22.6%) and biting (9.7%). Facial rubbing (neck, head or cheeks) occurred in 54.8% of events.
• When bears rubbed, they stood on their hind legs in 92.3% of events and on all fours in 23.1%. They always rubbed with their back to the tree; only once did a bear also rub facing the tree.
• When bears pede marked, they always did it after rubbing (100% of these events), and only sometimes before (16.7%).
• Five of the six rub trees had a series of ground marks from earlier pede marking. The sixth had none and was marked only once during the study.
• Three bears that did not mark spent longer sniffing the ground marks than the tree. Some males pede marked past the tree, turned around, and pede marked back in the same ground marks before rubbing. On one occasion, pede marks from a male stretched about 400 to 500 m from the tree.
• All six rub trees were on bear trails used often enough to form a visible path through the forest.

A new behaviour: “groin marking”

A young bear (estimated at 2 years old or less) rubbed the tree on all fours, then stretched its hind legs back almost flat, as in a push-up, apparently pressing its lower body or groin to the ground while rubbing its hip against the tree. It did this twice in one video. It was the only young bear seen marking.

3. What they concluded

• During the breeding season, rub tree marking in this area is done mainly by adult males, in daylight, with a peak in early June, which suggests the breeding season peaked then. This fits earlier studies of black bears and brown bears.
• Bears seldom investigated rub trees without marking them, which may mean the communication is mainly between males.
• Pede marking may be nearly as important as rubbing for leaving scent, and may have been undercounted because the cameras were sometimes slow to trigger. The authors suggest it may be a moveable, short-lived way to leave scent along a route, while the rub tree is a more lasting site. Whether black bears have scent glands between their toes is not known.
• The video could not show whether bears were leaving urine or saliva. A high rate of facial rubbing could point to saliva, but biting was rare; urine would be expected with more rubbing while facing the tree, which was seen only once.
• Groin marking is probably a rare, hidden behaviour.
• Further studies should follow bears of known sex, age and reproductive status through the whole active (non-denning) season, and map and watch all rub trees within home ranges.

4. How tracking was or was not used

The researchers found rub trees by walking bear trails and reading tracks and signs: hair caught on the tree, bite and claw marks, wounds in the bark, and a worn line of tracks leading to and from the tree (citing tracks and signs sources including Elbroch 2003). They also recorded series of ground marks from pede marking at five of the six trees, and on one occasion noted a male’s pede marks running about 400 to 500 m from a tree. No trailing to find bears is reported.

5. Tracker or observer certification

Not reported. The paper does not say who identified the rub trees and ground marks or how experienced they were. This matters for which trees were chosen for filming.

6. Other technologies, skills or innovations

• Infrared remote video cameras (Truthcam 35, Primos) recording 60-second clips with a 10-second delay, running 24 hours a day. One camera was mounted 5 m high pointing down to stop bears moving it. Condensation caused some camera failures and one camera was stolen.
• Individual bears were identified on video from sex, size, place and time of capture, and the amount of hair loss on the back.
• Sex was judged from visible genitals, a patch of hair around the vulva, swollen teats, or the presence of cubs. Age class was judged from size, but was too uncertain to use in the statistics.

7. Implications for conservation (from the paper)

• Biologists use DNA from hair collected at natural rub trees to monitor bear populations. Because mainly adult males mark trees in the breeding season, these samples may be biased towards adult males.
• Black bears are managed as a game species, so managers need methods that measure population structure and numbers correctly, and need to understand bear behaviour, when setting hunting seasons and quotas.