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Bears that rear up to scratch their backs against a tree trunk smear themselves with pungent resin, possibly keeping bloodsucking parasites at bay.

Brown bears (Ursus arctos) regularly scrape their necks, chests and backs against trees. The behaviour is often thought of as chemical communication with other bears, through the animals depositing their own odours or picking up a coating of aromatic resin. Bears will also gravitate towards smellier options for tree-rubbing, such as creosote-treated power poles.

But since many odoriferous plant oils and other substances have anti-fungal and anti-parasite effects, at the Institute of Nature Conservation of the Polish Academy of Sciences in Krakow and her colleagues wondered if the sticky stuff was acting as a protective pomade against parasites.

The team collected ornate dog ticks (Dermacentor reticulatus) from meadows outside of Wroc艂aw, Poland, and brought them into the laboratory for testing. The researchers placed a total of 52 ticks, one at a time, in the centre of a 12-centimetre-long transparent silicone tube. The tube had filter paper at one end with water dribbled onto it and either turpentine (which is made from tree resin) or beech tar on the other side. They then recorded the ticks鈥� behaviour over 3 minutes.

The ticks wanted nothing to do with the pungent tree substances, rapidly moving to the water side of the tube. Most of the ticks touched the watered filter paper, but not one dared touch the turpentine, for instance.

鈥淚t was pretty fun just to watch the videos and see the ticks really running away from the turpentine, just hiding under the water [filter paper],鈥� says Agnes Blaise at Strasbourg University in France, part of the team.

The researchers think the ticks鈥� strong aversion to the tree compounds may mean that tree-rubbing gives bears the benefit of a parasite repellent. Such a perk may make the energy-draining activity especially worthwhile for a bear while it spreads its scent.

at the Tokyo University of Agriculture and Technology finds the results convincing, noting that the bear species he studies 鈥� Asian black bears (Ursus thibetanus) 鈥� will rub their whole bodies on trees, possibly to treat against parasites as well.