When a male grizzly rubs a pine tree with his backside, it's not to satisfy an itch but to ward off competition while he looks for a female, according to unusual research.
By leaving his scent on a "rub tree," explained ecologist Owen Nevin of Cumbria University in Britain, a male bear may be seeking to avoid a tooth-and-claw fight with another male on the prowl.
When bears clash, it can lead to injuries or even death, so bark rubbing is a safe alternative to a showdown.
Bear buffs have bared claws in the past over rival theories to explain this unusual behavior.
Some experts have suggested that the tree rubbers are females coming into heat, while others have speculated that the friction could help the bear get rid of parasites or pick up sap as an insect repellent.
But two years of field research observing grizzlies in British Columbia led Nevin to conclude otherwise.
Digital cameras positioned in front of known "rub trees" showed that only males burnished their bottoms on the trees.
And GPS emitters attached to bears tracked by satellite showed that they traveled from valley to valley in large loops, marking trees as they searched for an intimate bear hug from the opposite sex.
"If one male recognizes another 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," Nevin said.
Nevin also noticed that very young bears were also rubbing the bark, leading him to speculate that they might be trying to ward off aggressive males who sometimes kill offspring to mate with the mother. (AFP)
根據特別研究指出，當一隻雄性灰熊用臀部磨蹭松樹，其實不是要止癢，而是要避開當牠在尋找女伴時的同類競爭。
英國坎布里亞大學的生態學家歐溫．內文解釋，雄灰熊可能藉由將自己的氣味留在「磨蹭樹」上，來避免與其他在附近遊蕩的公熊發生「尖牙利爪」大戰。
熊起衝突時，可能導致重傷，甚至死亡，因此磨蹭樹皮是比跟其他競爭對手一決高下更安全的選擇。
愛熊人士過去為了不同的理論解釋這個奇特的熊類行為而激烈爭辯。
一些科學家指出磨蹭樹木是雌熊發情的表現，而另一些科學家推測，摩擦可助於熊擺脫寄生蟲或是將樹汁塗在身上當作驅蟲劑來驅除蟲子。
但是經過兩年的實地觀察英屬哥倫比亞的灰熊，卻讓內文下了另外的結論。
架設在「磨蹭樹」前的數位相機顯示，只有雄性灰熊才會用屁屁磨蹭樹。
此外，熊身上黏貼了可用人造衛星追蹤的全球定位系統發射器，發現牠們繞著環形路線，從這個山谷走到另一個山谷，一邊在樹上做記號，一邊尋找親密的異性伴侶。
「如果一隻雄灰熊在該區的磨蹭樹上聞到另一隻雄灰熊留下的氣味，牠知道牠將面臨一場惡鬥，也就是說，牠現在正身處在別人的地盤裡，」內文說。
他還發現，非常年輕的熊也會磨蹭樹皮，因此推測牠們可能是在避開具有攻擊性的成年雄性灰熊，因為這些熊為了交配，有時會殺掉雌熊的後代。
（法新社／翻譯：楊鴻泰）
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