Not just scratching an itch: Grizzly bears might be using 'rub spots' to find mates, says study

If you've ever been lucky enough to see a grizzly bear rubbing itself against a tree, you may have thought it was simply scratching an itch in a hard to reach spot.

But new research indicates the rubbing also plays an important role in the mating game.

Andrea Morehouse, the lead author of a new study, The Smell of Success, says there's a link between the bears' rubbing and their reproductive success.

"Rubbing is probably about communicating dominance and competitive ability but also signaling for mates," Morehouse told the Calgary Eyeopener.

The study was co-authored by Anne Loosen, Tabitha Graves and Mark Boyce.

The finding came about during an earlier study where the researchers wanted to get more insight into the habitat and population density of grizzly bears.

They started by attaching strands of barbed wire on trees and other objects, like phone poles, that bears rubbed on. This helped them collect DNA samples from individual bears, said Morehouse.

There were about 900 rub spots in southwestern Alberta.

Researchers then collaborated with biologists in British Columbia, as well as Montana, and with a large data set, they were able to put together a family tree for grizzly bears.

"We're able to look and see, you know, which bears had mated with which other bears," she said.

"For each bear that we detected, we were able to look at the number of offspring they had, the number of mates they had, the number of different rub objects, where we detected them, and the number of different sampling occasions during which we detected them."

More rubbing, more mates

What they found was the more bears rub, the more they mated and had offspring.

This confirmed the suspicion that one reason for bears rubbing on trees is to attract mates and signal their virility.

"We've sort of long assumed that rubbing has to do with some form of communication, but our's is just sort of the first study to really provide that link," Morehouse said.

Morehouse says they suspect there's some chemical deposits from their scent glands that are being left behind on the rub spots too.

"If you've seen one of these rub trees, ones that are used quite a lot often (have) bear trails that go up to them. So bears in some ways, they're kind of creatures of habit, and so they step in the exact same spot each time they're approaching these trees," she explained, adding it leaves behind footprints.

"Stomping is also a really common behaviour at these rub objects … we don't know exactly what is being left behind from these scent glands, but certainly know that they do have these scent glands and that these behaviours are likely leaving some chemical deposit that is being interpreted by other bears."

Morehouse says they found the same patterns for both males and female grizzly bears in rubbing.

Kin detection still unknown

Mark Boyce, a population ecologist at the University of Alberta, and one of the researchers behind the new study, says the scents left at the rubbing objects — some of which were "rubbed smooth" by the bears — might be an important method for how female grizzly bears pick their mate.

"This olfactory communication about the condition, and genetic makeup of the males may well be very important information for female choice," Boyce said.

"One of the questions we'd love to be able to sort out is whether or not females can identify males that are genetically related or not to avoid inbreeding depression."

Boyce says though that happens in some species, it's not been shown in bears. He says they would also like to find out whether a male grizzly bear is able to recognize if a cub is its offspring.

"You would expect that males would not kill their own cubs. But we know that they certainly [have] killed a lot of cubs. And we presume — but it's not been shown yet — that they will not kill their own cubs," Boyce said.

"It would be just a wonderful thing to sort out to help us understand some of the behaviour."