A、The squirrels’ breeding season is extended. B、The female squirrels’ productivity falls. C、The squirrels’ survival rate reduces

Winter is a pain in the animal kingdom. Birds can flee it by migrating to warmer regions but grounded beasts, including mammals, have no choice but to stick around.(23)To cope, 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.(24)Global warming should mean that the creatures have longer ice-free periods in which 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.(25)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.
23. How do many species of animals spend the winter?
24. What should global warming mean to the animals?
25. What might explain the reduced squirrel population according to Dr. Lane?