Ever since I saw Baloo the Bear [wiki] in Disney's adaptation of Kipling's The Jungle Book, I've always thought bears just like to scratch their backs on trees.
Turns out, they may be doing more than just scratching an itch - they do it to communicate with each other:
Nevin, who will present his findings at the Sept. 10 annual meeting of the British Ecological Society, thinks that by marking the trees with their scent, the male bears get to know each other better, which could reduce fighting among the bears over female mates.
"Big male bears can seriously injure or even kill each other when they get into a fight," Nevin said. "If one recognizes 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."