There are many reasons why bears swing and scratch trees. Sometimes they communicate by scent marking trees, other times they are waxing and scratching that elusive itch. A new study posits an added bonus: smearing yourself with nature-made tick repellent.
As bears squirm against the bark, scratching posts release tars, resins, and sap. The thick tar from beech trees sticks to fur and skin longer, and is resistant to water, making it a strong contender for an effective tick repellent.
Agnieszka Sergiel, a bear biologist at the Polish Academy of Sciences and author of the study published last month in the journal of zoologyHe said animals rarely engage in complex behaviors like rubbing against trees for only one reason.
“We see a lot of examples among mammals using self-medication,” he said. So she and her colleagues decided to study whether rubbing against trees might 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 lure of the trees is so strong that scientists use the strong, sticky odors of beech tar to attract bears for study or to call them into zoos.
To test the hypothesis that beech tar is a tick repellant, Dr. Sergiel found herself looking at tube after tube of tar and caught Dermacentor reticulatus, a widespread hard tick known to feast on bears. She watched to see if the ticks escaped the beech tar at one end and made their way to the clean, safe water at the other end of the tube.
And run they did!
“It was really obvious that they hated beech tar,” said Agnes Blaise, a biologist at the University of Strasbourg in France and an author of the study. “Some were really fast, running and hiding underwater.”
The researchers also tested turpentine, a bear attractant, and the ticks also despised it.
The only ticks that didn’t count, Dr. Sergiel added, were the ones that managed to escape the tube entirely.
“There were some Houdinis,” he said, “but they were good laboratory animals.”
The researchers focused on ticks for their study because they are geographically widespread and environmentally flexible, and due to climate change, they spread farther and stay active longer. Ticks are also disease vectors, although scientists are still learning which pathogens they transmit to bears.
The simple result that beech tar is not popular with ticks provides the first experimental evidence to support the idea that tree resins could act as an insect repellent.
The researchers “had a nice, neat experiment that provided pretty clear evidence” that ticks avoided 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 rubs, but it certainly could be an added benefit.”
Hannah Tiffin, an entomologist whose graduate research at Pennsylvania State University focused on ticks and bears, hadn’t heard of the idea of tree tar as an insect repellent.
“I think it’s a really interesting route and it could make sense,” he said.
Other animals in the wild use insect repellants provided by nature; for example, capuchin monkeys carefully rub their skin with citrus, and dolphins may treat their skin with coral. Your cat can even use catnip as a mosquito repellent. So it’s perfectly plausible that bears can do it, too, said Dr. Tiffin, who was not involved in the study and is now a postdoctoral researcher at the US Department of Agriculture.
Much remains to be learned about tar as a tick repellent, Dr. Sergiel noted. Developing the sparse data on parasites found on bears in the wild (including ticks) will be one of the most important steps in advancing this work, the researchers said. It might also be useful to collect skin and resin samples from bears and test parasite responses to those materials, Dr. Tiffin added.