英语听力：冰河绿洲—14

Some of this ice age dung was found beneath the skull in the Aucilla. The dung contained plantremains that tell us what the mastodon was browsing on 13,000 years ago or more—a mixture of trees and grasses.

This suggests that ice age Florida was drier than it is today—a mix of woodlands and savanna rather than swamps. And mastodon teeth found in the Aucilla held a more important revelation. The enamel contains chemical signatures of a local soil, passed by the plants the mastodons ate. But some of the chemicals found in the Aucilla teeth could only have come from soil hundreds of miles further north. The inescapable conclusion is that these mastodons migrated, making a round trip of more than 400 miles every year. And since their dung also contains remains of summer fruits from the Aucilla region, they must have travelled north for the winter.

The question is, why? Why leave a place which as we have seen was abundant with food and a refuge from the cold? And why go north for the winter when most migrants move south? Perhaps the present climate can provide a clue.

Florida has a peculiar seasonal quirk. Although the winter months are cooler, they are also drier, much drier. Between October and February, there's almost no rainfall at all.