A、They sleep for a long time. B、They migrate to warmer areas. C、They eat more food to keep warm. D、They store up food for the ne

Winter is a pain in the animal kingdom. Birds can flee it by migrating to warmer regions but land beasts, including mammals, have no choice but to stick around. To cope with it, many species have learned to spend the winter in a long sleep. Some, like the Columbian ground squirrel, spend up to nine months of each year in their shelters. This conserves energy but leaves them with only three months to become well-rounded for the next winter and, crucially, to reproduce.
    To make matters worse, climate change is leading them to emerge from winter sleep later than usual. Global warming means that creatures have longer ice-free periods to go about their evolutionary tasks. But it can also disturb weather patterns, which may have the opposite effect.
    Because female squirrels remain in their place of birth, the researchers were able to tag and observe them and their offspring each year for the past two decades. A typical female would bear three babies. On average, only 30％ of them survive the first winter, enough to sustain population numbers since female squirrels can expect three or four litters in their lifetime. If the proportion falls, however, the population reduces.
    In the first decade of the study, the number of squirrels dropped just once. But it fell in four of the past ten years. Dr. Lane, an evolutionary biologist at the University of Edinburgh, speculates that this might be explained by the fact that over the past 20 years the late snow has delayed the melting of ice by half a day each year, on average, shortening the squirrels’ breeding and feeding season by several days and disrupting their life cycles.
    Questions 13 to 15 are based on the passage you have just heard.
    13. How do many land animals spend the winter according to the passage?
    14. What does global warming mean to animals?
    15. What might explain the reduced squirrel population according to Dr. Lane?