Do I look scratchy to you?
Credit: FWS / Image Library
It’s part of Far North lore: Brown bears will back up to a particularly craggy spruce and rub their massive rumps back and forth, up and down. They will scratch and bite. Leave behind hair and claw marks. Same trees, same bears, year after year, generation after generation.
Hmmmm, you imagine them grunting. That feels so good.
But science has once more ruined another common sense notion and made things much more complicated. It turns out that bears don’t do the arboreal cha-cha-cha just because all that rank, greasy fur has gotten itchy. Or even because they want to mark territory, exactly.
A British scientist who monitored several of these “rubbing trees” in British Columbia via remote infrared cameras and satellite uplink says he’s found the real reason.
As you probably guessed, it’s all about scratching that other itch.
The bears, it turns out, massage the spruce as a way to woo willing sows into the devil’s club and avoid all that wear-and-tear that comes from those tooth-and-claw battles with rival boars.
“Big male bears can seriously injure and even kill each other when they get into a fight,” explained biologist Owen Nevin of the University of Cumbria. “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.”
Here are the details from a British Ecological Society online story:
Speaking at the British Ecological Society’s Annual Meeting in Glasgow next week, (Nevin) will reveal that adult male grizzly bears use so-called “rub trees” as a way to communicate with each other while looking for breeding females, and that this behaviour could help reduce battles between the bears.
Many theories have been advanced as to why bears rub trees: some thought females might rub trees as they came into oestrous, and others that bears might be giving their backs a good scratch to get rid of parasites or pick up sap to act as insect repellent.
Source: Cumbria University
Until now these ideas have been extremely difficult to test because bears usually live at low densities and rubbing is relatively rare behaviour. Because bears use the same rub trees for generations, and because he has logged rub trees over almost a decade working on bears in British Columbia in Canada, Nevin has been able to gain a unique insight into grizzlies’ behaviour.
Over the past two years, he used four digital cameras with infra-red trips set up opposite rub trees to collect data on which bears used the trees and when. He coupled this with satellite telemetry equipment to track individual bears’ movements.
“The cameras show that adult male bears are the most likely to rub trees, and the satellite telemetry tells us that males move from valley to valley in large loops, marking trees as they go, while looking for breeding females,” Nevin says.
Nevin believes that by marking trees, adult males may be getting to know each other better, and that this scent familiarity could act as a way of reducing fighting among adult male bears.
Like other species, male bears will sometimes kill a female’s offspring to get a chance to mate with her; scientists refer to this as sexually-selected infanticide.
Nevin’s work suggests that very young bears may be using the rub trees to help them trick potentially killer males.
“It’s really hard to document, but on several occasions the cameras caught cubs who are being chased away from their mother by a large male visiting and rubbing on trees which he has marked. They can visit the tree two or three times in a day, sometimes within an hour of the big male, so it may be that smelling like him makes them safer — related animals smell similar and animals are less aggressive towards relatives,” Nevin says.