Credit: Melanie Clapham
Not long ago, the amusing habit of grizzly bears rubbing their backs against a tree was assumed to be because these bruins were scratching an itch. But research is revealing that back rubbing bears have more to say.
Wherever they go, and especially on certain trees that seemingly act as bulletin boards, bears are leaving chemical messages for each other. A recent study led by conservation biologist Melanie Clapham, formerly at the University of Victoria, now with Nanwakolas Council Society, took a close look at these chemical bulletins by analysing the volatile organic compounds in secretions from brown bears in south-eastern Alaska.
For most bear species, very little is known about scent chemistry. Giant pandas are an exception. Pandas use secretions from an ano-genital pouch to mark trees, and also mark with urine. For grizzly bears, Clapham began studying scent communication over a decade ago during her doctoral work.
At Glendale Cove on the Central Coast of British Columbia, Clapham had previously studied rubbing behaviours with cameras placed on trees. She found that bears were selective about which trees they used, how much time was invested in marking, and the postures and body parts used for rubbing. Some bears – particularly adult males – rubbed more than others.
For Clapham, that sparked questions like “What are the actual chemical messages that these bears are placing on the trees through their scent? And… for the bears investigating the scent, what are they gathering?”
Those questions motivated the current study, in which Clapham teamed up to make chemical sense of bear scents. On the research team were conservation physiologist Abbey Wilson, then at the University of Saskatchewan, now carnivore biologist for the Government of the Northwest Territories, conservation microbiologist Candace Williams at the San Diego Zoo Wildlife Alliance, and conservation biologist Agnieszka Sergiel at the Polish Academy of Sciences in Krakow, Poland.
The team took advantage of samples swabbed in 2014 by bear ecologist Anthony Crupi at the Alaska Department of Fish and Game, while capturing and affixing GPS units to 12 bears for other research. While the bears were immobilized, secretions were sampled with sterile gauze rubbed over each bear’s cheek, flank, paws, digits and hump – muscle protruding between grizzly shoulder blades. These are bear body regions known to exude scent.
The study included two mature males aged 15 and 19 years old, two females aged 14 and 19 years old, six subadult males aged 4 years or younger, and two young females aged 4 and 6. Scent samples were frozen and shipped to a Victoria-based lab for analysis using a gas chromatograph mass spectrometer to identify compounds. Results were referenced against the NIST 2012 Mass Spectral Library – a National Institute of Standards database for identifying compounds in mass spectrometer results – to assess incidence and abundance relative to bear age, sex and body site of origin.
Analysis detected 254 compounds, with only 24 that could be reliably matched to the library database, so unassigned compounds were reported as mass@retention time, referencing their molecular mass and time taken to pass through a chromatography column.
The team found 23 compounds contributing to variation in volatile profile by sex (Supplementary Table S3), and one compound (72.0@1.97) contributing to variation of volatile profiles by age. Females and young males displayed individual differences and had a greater number of compounds overall than mature males. The incidence and abundance of compounds did not differ between body sites.
“We think that the communication of age is probably quite important,” says Clapham. The team had predicted that mature male profiles would show more variance than mature females, as a means to advertise their dominance and distinct individual identity. That was not the case. Compound incidence and abundance did not vary significantly between the two mature males. Males showed less variance than mature females and other age-sex classes.
“So it seems that the chemical profiles of mature males are very similar to each other,” says Clapham, which she finds fascinating. “Perhaps it’s just enough to smell like a mature adult male,” she hypothesizes.
Nuria Selva, a bear researcher based in Krakow, Poland, who was not involved in the study, notes that while the study is a nice contribution, the small sample size, particularly of adult males, merits caution for interpretation of results. Selva points to a need to study more populations, with a greater sample size across age and sex.
Grizzly bears are solitary animals, so what messages are they sending with scent? That remains a mystery, though Wilson suggests that sniffing marking trees, a visiting bear can probably tell the age and sex of other recent scent contributors.
Trees are not the only place that grizzlies leave chemical marks, however. They also use what Wilson calls a “cowboy walk,” squeezing paws into the ground to actively transfer scent. Interpreting this scratch and sniff trail is likely advantageous from a risk management perspective, so younger bears can avoid conflict with high status males, and mature bears can potentially detect mates.
With emerging technologies and extensive future validation work, Wilson wonders: “is there a way we could non-invasively track individuals using their pheromones? That would be really cool,” she says.
Credit: Melanie Clapham