By Rebecca Morelle
Science reporter, BBC News
The mystery behind the tree rubbing antics of North America's grizzly bears may at last have been solved.
First, the bear's back gets a good rub...
A few select trees are used by grizzlies to perform strange rubbing rituals, but for years the reasons for this behaviour have baffled ecologists.
Now, a study suggests that male grizzlies seeking mates are marking the trees to communicate with other males - possibly to dodge deadly bear battles.
The work will be presented next week at a British Ecological Society meeting.
Owen Nevin, a behavioural ecologist at Cumbria University, UK, who carried out the study, said: "A handful of trees ('rub trees') are used for years by different grizzlies who each approach the trees in exactly the same way.
"They will step into the footprints of other bears that have approached the trees, urinating as they approach.
...then the tree is bitten and clawed...
"Then they rub their back on the tree, turn around and then bite the tree and claw it. Then they give it a 'bear hug' by rubbing their chest against it, and then they rub it with their back again."
Many theories have been put forward as to why grizzlies are rubbing these trees: some thought they were using them to scratch an itch, others that they were trying to rub on tree sap to repel insects, while some thought they were using the trees to attract mates.
Dr Nevin told the BBC News website: "Until now, we haven't really known which bears use these trees and why they use them."
...next the bear grips the tree in a bear hug to rub its front...
To investigate the bears' behaviour, Dr Nevin looked at a grizzly population living in a 150 sq km (58 sq miles) valley in British Columbia, Canada.
He set up digital cameras, activated by infra-red sensors, at four frequently used rub trees and attached satellite collars to bears to track their movements.
The cameras revealed that large adult male bears were marking and carefully inspecting the rub trees, but female bears were ignoring the trees. The satellite telemetry showed that the grizzlies were moving around the area in large loops, marking trees along the way, while looking for females.
Dr Owen said he thought the male bears were using the trees to communicate with other males in the area and that this could be a way of reducing fighting amongst them.
...finally its back gets one more good scratch
He said: "For a large grizzly bear, the only real source of mortality is other big bears, so lots of strategies are adopted to reduce the likelihood of having to fight.
"If one recognises the other from the scent marks on the rub trees in the area, he knows he's in for a tough fight - he's on the other guy's patch so to speak - so it might be better to back away than make a serious challenge.
"When two males meet, the more information they have, the better for both of them."
The digital cameras also recorded that the rub trees were occasionally visited by young cubs that were being chased away by a male courting their mother.
Male bears can sometimes kill a female's cub in order to mate with her, explained Dr Nevin, and the cubs might be visiting the trees for self-protection.
He said: "In the animal world, related individuals recognise each other by the fact that they smell a bit like each other, so perhaps the cubs are seeking security by trying to smell like the males who have just chased them off."