英语听力：猛玛象进城记—4
The same signs appear in young male mammoth tusks, but those living in North America at the end of Ice Age laid down their stress rings three years earlier than usual. In other words, it seems that young males were leaving the herd at an earlier age.
Some scientists believe the only thing that could cause such a major change in mammoth’s social structure would be hunting by humans. If hunting pressure was extreme enough to push the mammoths to extinction, then we would expect them to survive in areas that people couldn’t reach. And for a while, they did, here on Wrangel Island off the coast of Siberia.
There are no mammoths on Wrangel Island today, but it is home to another large animal, the polar bear. And there is evidence that polar bears and mammoths once lived side by side during the last Ice Age. They would have shared the island’s meager offerings.
This inhospitable and isolated place seems to have been a sanctuary for mammoths. The Wrangel Island herds were the last on the planet to survive. Remarkably, they survived here long after their North American relatives had all died out. But they were still on borrowed time. When people finally reached Wrangel Island 4,000 years ago, these last remaining mammoths also became extinct.
The events on Wrangel Island mirrored what had happened on the North American continent. Mammoths only became extinct after they came into contact with people.
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