FAIRBANKS (AP) -- In his almost 30 years as a wildlife biologist in Alaska, Pat Valkenburg has never seen a caribou die of old age.
''The oldest one I know of was 15 years old, a cow from the Delta herd,'' Valkenburg said. ''We collared her as a 10-month-old calf and followed her all the way up to 15 and she was killed by a wolf.
''Caribou don't die of old age,'' the biologist said. ''Caribou get killed by wolves.''
Bears have the longest life span of any big game animal, probably in part because they spend the harshest part of the year sleeping. Cow moose and caribou cows live longer than bull moose and caribou bulls. You won't find too many old wolves in Alaska.
''When you find a wolf that's 10 years old, the teeth are pretty much gone,'' said biologist Mark McNay, who tracks wolves for the Alaska Department of Fish and Game. ''When they get old, they can't feed themselves and they just run out of energy. They can't keep up with the pack and they just die.''
The same thing happens to caribou. Once a cow caribou lives 12 years, its chances for survival drastically decline, said Valkenburg.
''They get old and slow,'' he said. ''Wolves are pretty adept at picking out anything old and slow.''
Cow moose, on the other hand, can live to be 20-plus years old, said Vic Van Ballenberghe, a biologist who has studied moose in Denali National Park and Preserve. Bulls, however, very rarely make it past 12.
''I did have one bull that made it to 17,'' Van Ballenberghe said. ''He was shot three years ago outside the park.''
Biologists use teeth to determine the age of animals.
Aging the tooth of a grizzly bear or moose is not much different from counting rings in a tree, said zoologist Gary Matson, who owns a laboratory in Montana that specializes in what is known as ''cementum aging.''
The animal's tooth is soaked in acid to decalcify it, revealing a fibrous layer of tendon called collagen. The collagen has dark rings of protein in it that can be counted under a microscope to determine the age of the animal.
Matson's lab has aged nearly 1 million teeth from 34 different species of wild animals in the last 32 years.
Most of the teeth come from biologists at agencies like Alaska Fish and Game, U.S. Fish and Wildlife Service and the National Park Service.
Matson, who studied zoology at the University of Montana in Missoula, got the idea for a tooth-aging laboratory more than 30 years ago, when a friend showed up one day with an elk tooth and asked him to age it.
''My friend said, 'I betcha if you set up and did this, you could make a living at it,''' Matson said.
Recognizing the fact that ''Montana is a pretty gamy place,'' Matson decided to give it a try. Things started out slowly. But over the years, Matson's client list has grown to include customers in Alaska, the Lower 48, Canada, Finland and Spain.
Today, orders vary from 2,000 moose teeth from Finland to a single tooth from a hunter interested in finding out more about the trophy he shot, Matson said.
Bears are definitely the longest-living animals in the wild, according to Matson's statistics. The record age for a grizzly bear is 37, and the oldest black bear on record is 35.
The oldest bear state wildlife biologist Harry Reynolds has seen in his 27-year career with Fish and Game was 30 years old but he hasn't seen her since the fall of 2000 when she went into her den with a pair of 2-year-old cubs.
''She's probably dead,'' Reynolds said. ''We had some of our best people searching for her this spring and we couldn't find her.''
Reynolds first captured the bear in 1983 and a tooth extracted showed her to be 13 at the time.
Reynolds knows of a grizzly bear on the North Slope that lived to be 35. The bear's collar was found near the village of Noatak in 1995 and there were wolf tracks a short distance away. Reynolds also knows of a 34-year-old grizzly sow that was killed by a hunter in the Brooks Range in 1990.
There are several bears in Denali National Park and Preserve that have reached the 30-year mark, according to park biologist Pat Owen.
The oldest bear Owen has found was 32 but biologists lost track of her two years ago and she's probably dead, Owen said. Of the 21 bears the Park Service is now tracking by radio collar, the oldest is 31, he said.
The primary reason ungulates such as moose, caribou and sheep die is that their teeth wear out, Valkenburg said. ''They just don't eat as efficiently,'' he said.
Moose spend eight months of the year eating wood in the form of branches and twigs. Both Dall sheep and caribou feed relatively close to the ground, gobbling up rocks and lichens.
''Sheep eat a lot of coarse vegetation and ingest a lot of grit that they have to sort through in their mouth,'' said biologist Steve Arthur, who tracks sheep in the Alaska Range for Fish and Game.
Dall sheep have the shortest life span of Interior ungulates. ''The oldest record that I've seen is 14 for a ram,'' said Arthur.
The oldest caribou tooth Valkenburg has encountered was 17 and came from a large sample of teeth taken from the Western Arctic Caribou Herd in the 1960s.
While predation, hunting and starvation are probably the top three causes of death for most animals in Alaska, there are other ways to die.
Hundreds of moose are killed in Alaska each year in collisions with vehicles or trains. Other moose fall through thin ice and drown or die as a result of fights with other bulls during the rut.
Dall sheep and mountain goats fall off the steep cliffs they inhabit and get buried in avalanches.
Wolves kill each other, as do bears to a lesser degree. Other wolves get kicked in the head trying to kill moose.
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