SCIENCE LAB SCAMPER : BEARS & BUGGS

Naturally Repellent

Why do bears rub against trees?

Maybe because they're bugged by ticks.

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 reason : They're slathering on natural tick repellent.

When bears wriggle against bark, the trees exude resins and saps. These substances stick to fur and skin and are water-resistant, useful properties in an effective tick repellent.

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

But ''we see plenty of examples among mammals using melf-medication.'' So, she and her colleagues decided to study whether rubbing against trees could protect bears against parasites.

To test the hypothesis, Dr. Sergiel and her colleagues placed ticks known to feast on bears in tubes with water at one-end and beech tar at the other. The ticks invariably ran toward the water.

The researchers also tested turpentine, another strong-smelling, tree based substance known to appeal to bears.

The ticks despised that, as well. [Rebecca Dzombak]