英语听力：冰河绿洲—12

They can grow more than two metres long and weigh as much as three men. But the giant tortoises of ice age North America were even larger. Florida does have tortoises today, but on a much smaller scale. This is the gopher tortoise.

Tortoises are cold-blooded animals. They rely on external temperature to keep themselves warm. Although Florida is plenty warm enough for much of the year, during the winter months, it can get cold. So to survive the winter, gopher tortoises must burrow and hibernate underground. The colder it gets, the deeper into their burrow they go.

Giant tortoises, however, can't burrow. And they sleep above ground. They need relatively warm temperatures all year round. The fact they were in Florida during the ice age means that paradoxically the climate must have been more stable and even milder than it is today. So we know something of the climate, but what about the vegetation and the landscape. How did that look 13,000 years ago?

Today Florida is one of the wettest parts of the continent, especially the vast swampy area known as the Everglades. Flooded grasslands stretches as far as the eye can see.