WASHINGTON (AP) -- Moose that had never seen wolves or grizzly bears didn't try to get away when the predators returned to parts of the Yellowstone basin of Wyoming, but it took only a few bloody encounters for the animals to wise up, researchers say.
Wolves and grizzlies are back in parts of the sprawling Yellowstone ecosystem, where they had been absent for more than 50 years. Wolf packs were reintroduced, while the bears have naturally colonized the Grand Teton National Park and adjacent forest areas.
The wolves and bears immediately feasted on the innocence of the moose in the areas, easily catching, killing and eating animals that had never known such predators.
At first, said Joel Berger, first author of a study appearing today in the journal Science, the wolf and bear merely had to walk up to a moose. There was little attempt by the moose to run, he said.
''We were like forensic scientists'' in studying the kills, said Berger, a professor at the University of Nevada, Reno, and researcher for the Wildlife Conservation Society. He said it was clear from tracks and other evidence that the predators at first made easy kills.
''We got direct evidence of grizzlies killing 10 adult moose,'' said Berger. An adult moose can weigh almost 1,000 pounds.
Other evidence was found of younger moose standing almost still as they were approached and killed by the wolves that ate them.
But that quickly changed. Within a season after the killing began, moose became alert and wary, startling at wolf howls and quickly moving on when they sensed danger.
Berger and his colleagues tested the predator alertness of the Yellowstone moose and compared it with that of moose in Alaska, where wolf and bear have been historic predators.
Before the Yellowstone moose had developed their wariness, Berger said, the animals would pause for only 30 seconds when researchers played recorded wolf calls, then nonchalantly return to their feeding.
After the predator attacks began, however, recorded wolf calls caused the moose to become alert and restless. Often they left a feeding site and retreated from the sounds. Moose mothers who had lost calves to wolves, for instance, became five times more alert, as measured by the time spent looking for danger, than unmolested moose mothers. The reaction of the mothers that had lost calves was very much like that of Alaska moose who have never known the absence of predators.
''Wyoming moose that have lost even one of their offspring to predators may become as savvy as their Alaskan cousins within a single generation,'' said Berger.
The finding is reassuring for many conservationists who had worried that the reintroduction of predators could lead to the extinction of whole herds of prey.
Such an eradication, called the ''blitzkrieg theory,'' is what some researchers believe happened when humans first arrived in North America some 50,000 years ago. About half of the American animal species weighing more than 100 pounds -- including the camel, horse, mammoth and some types of sloth -- became extinct after the arrival of man.
It could be, said Berger, that the eradicated animals were simply not smart enough ever to learn that humans were dangerous.
''Our data are consistent with the idea that some prey species may not recognize certain kinds of predators,'' said Berger. ''This is total speculation, but it may be that the animals that went extinct in the Americas when humans appeared were those that were not as smart as the moose.''
Berger said that bears also have been reintroduced in parts of Scandinavia, and researchers there also found that the moose, after a bloody season, quickly learned to avoid the new predators.
Researchers now are studying the reaction of elk, caribou and deer in areas where predators have been introduced in recent years. Berger said the elk is now the major menu item for wolves in the Yellowstone basin, but it, like the moose, seems to be learning either to avoid or to fight the wolf.
John L. Gittleman, a professor of biology at the University of Virginia, said Berger's study ''is an important work'' that lays to rest some worries that prey animals would be wiped out by reintroduced predators. Understanding this risk is important, he said, because predators are being reintroduced in at least 173 locations worldwide.
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