Volker Deecke of the University of Cumbria was on holiday in Alaska's Glacier Bay national park in July 2011 when he spotted a brown bear in shallow water pick up a small, barnacle-covered rock and rub it over its face for a minute, NewScientist.com reported.
The bear was moulting, Deecke said, and had big patches of fur hanging off its skin.
Bear that are moulting sometimes scratch themselves with their claws or rub up against trees.
"The barnacles may have given that exfoliating feeling," Deecke said.
After a minute or so, the bear dropped the rock, he said, but then it picked up another one and began rubbing itself some more.
Bears have large brains for their body size suggesting they are clever, Deecke said, although he acknowledged too few tests of bear cognition have ever been conducted to say for sure.