单项选择题

Bats, tortoises, snakes, frogs, even insects like butterflies, hibernate more or less completely. Some, like the squirrels, sleep during coldest weather but are aroused by a warm spell. During hibernation, the temperature of an animal’s body drops drastically. Breathing and heart-beats almost cease.
Another instinctive method of avoiding intense cold is to escape by means of migration. Wild swans, seagulls, swallows and cuckoos are a few of the very many kinds of birds which fly thousands of miles, twice a year, to avoid cold. Many animals, especially those of the Arctic reindeer of Europe, move southward towards the forests when winter approaches. They return to the northern area when the warmth of spring begins to be sensed.
Arctic reindeers of Europe avoid the severe cold in winter by