Winter is here.The weather gets colder and days get shorter.People live in warm houses and wear over

题目

Winter is here.The weather gets colder and days get shorter.People live in warm houses and wear overcoats outside.But what happens to the animals?
Some animals travel to warm places,such as butterflies,whales and many fish.S____ do some birds.Birds can fly very long distances.For example,the Arctic tern(北极燕欧) lives c________ to the North pole(北极) in summer.In autumn,it flies to the Southern Hemisphere(南半球)!Each spring it r______ north again.How do they know when to travel?Scientists are still studying this.
Some animals make c_______ in their behavior or bodies.To keep warm,many animals grow thicker hair in the autumn.Others gather a lot of fodd in the autumn for them to e______ in the winter.
Some animals hibernate(冬眠) for part or all of the winter.This is a very d_____ sleep.The animal's body temperature drops,and its heartbeat slows down.For example,the heart rate of a hibernating woodchuck(美洲旱獭) slows f_____ 80 to 4 beats each minute,and its temperature drops from 37℃ to as low as 3℃.If its temperature falls too low,it will wake up a little and shiver(抖动) to get a little warmer.

答案

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