根据下列文章,请回答 31～35 题。
How Animals Keep Warm
Man has invented ways to keep warm, but how do animals defend themselves? They cannot reason in the sense that man can, but nature has taken care of the animal kingdom by providing animals with special instincts (本能)．One of these instincts is known as hibernation (冬眠).
'Sleeping like a dormouse （榛睡鼠) is not only a common saying but is a reality．When winter comes, the dormouse and other hibernating animals have reached a well-nourished state．They eat very well in warmer days laying down fat in the tissues of their bodies and during hibernation this keeps them alive．Safe in their nests, or burrows-(地洞), they sleep soundly until the warmth of spring arrives.
Bats, tortoises, snakes, frogs, even insects like butterflies, hibernate more or less completely．Some, like the squirrels (松鼠), sleep during coldest weather but are roused(弄醒) by a warm spell (暖流)．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 regions, have summer and winter quarters．The Arctic deer of North America, as well as the reindeer (驯鹿) 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.
There are animals which do not attempt to leave at the first sign of winter cold．Their instinctive means of defense is to dig out a deep burrow, made it soft and warm by padding (填塞）out with straw, leaves, moss and fur．In it they have a 'secret place' containing food which they hope will last the winter through! Animals which fall into this class include the Arctic fox, the rabbit and the little field-mouse．
第 31 题 How does the dormouse defend itself against cold in winter?