Why do bears rub against trees? New explanations are here

NEW YORK: There are many reasons bears shimmy and scratch against trees. Sometimes they communicate by scent-marking trees, other times they’re removing hair and scratching that hard-to-reach itch. A new study posits an additional perk: slathering on nature-made tick repellent. When bears wriggle against bark, the tree scratching posts leak out tars, resins and saps. The thick tar of beech trees sticks to fur and skin the longest, and it is water-resistant, making it a strong contender for an effective tick repellent.

Agnieszka Sergiel, a bear biologist at the Polish Academy of Sciences and an author of the study published last month in the Journal of Zoology, said animals seldom engage in complex behaviors such as rubbing against trees for a single reason.

“We see plenty of examples among mammals using self-medication,” she said. So, she and her colleagues decided to study whether rubbing against trees could protect bears against parasites.

For years, biologists have observed that brown and black bears have an affinity for certain types of trees — especially beech trees. The trees’ appeal is so strong that scientists use the sticky, strong scents of beech tar to attract bears for studies or to call them inside in zoos.

To test the hypothesis that beech tar is a tick repellent, Dr. Sergiel found herself staring at tube after tube of tar and trapped Dermacentor reticulatus, a widespread hard tick known to feast on bears. She watched to see if the ticks would run away from beech tar on one side and toward safe, plain water at the other end of the tube. And run they did!

“It was really obvious they hated the beech tar,” said Agnes Blaise, a biologist at the University of Strasbourg in France and an author of the study. “Some were really speedy, running around and hiding under the water.”

The researchers also tested turpentine, a bear attractor, and the ticks despised it as well. The only ticks that didn’t count, Dr. Sergiel added, were the ones that managed to escape the tube entirely. “There were some Houdinis,” she said, “but they were good lab animals.”

The researchers focused on ticks for their study because they are geographically widespread and environmentally flexible — and because of climate change, spreading farther and remaining active longer. Ticks are also disease vectors, although scientists are still learning about what pathogens they spread to bears.

The simple result of beech tar not being popular with ticks provides the first experimental evidence supporting the longstanding idea that tree resins could act as a bug repellent. The researchers “had a nice, tidy experiment that provided some pretty clear evidence” that ticks were avoiding beech tar, said Andrea Morehouse, an independent wildlife biologist in Alberta whose work focuses on bear-human interactions and was not involved in the study. “Repelling parasites is probably not the primary function of tree rubbing, but it certainly could be an additional benefit.” Hannah Tiffin, an entomologist whose graduate research at Penn State University focused on ticks and bears, hadn’t heard of the idea of tree tar as insect repellent. “I think it’s a really interesting route to go and could make sense,” she said.