A helicopter is not necessarily a match for an angry moose.

Instead of lying down after being shot with a tranquilizer dart, a moose charged a hovering helicopter used by a wildlife biologist, damaging the aircraft's tail rotor and forcing it to the ground.

Neither the pilot nor the biologist was injured, but the moose was maimed by the spinning rotor and had to be euthanized, wildlife officials said.

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"It just had to be one of those quirky circumstance. Even dealing with bears and goats and moose and wolves, this is pretty unusual and truly a very unique situation," said Doug Larsen, regional supervisor for the Division of Wildlife Conservation.

Biologist Kevin White was aboard the chartered helicopter on Saturday for a study of moose near Gustavus, a community of 459 people about 50 miles northwest of Juneau in southeast Alaska.

Moose outnumber humans there 2-to-1, White has written in an essay for the Department of Fish and Game Web site.

He shot the animal with a tranquilizer dart, Larsen said, and the pilot maneuvered the helicopter to keep the animal from slipping into a tight space or collapsing in water and drowning.

"The moose would start to move, and then the helicopter would back off and try to keep the moose out in the open," Larsen said.

But instead of moving toward open space, the moose charged the helicopter.

"As the animal got closer and closer to going down, an animal sort of loses its thinking — its ability to rationalize what's in its best interest," Larsen said.