Hibernation-冬眠

　　Hibernationismorethansleep.Itisaverydeepsleep.Theanimal'stemperaturedropstojustoverzerocentigrade,anditsheartbeatsveryslowly.Peoplewhofindhibernatinganimalsasleepoftenthinkthattheyaredead:thebodyfeelsverycold,andthecreaturemaybreatheonlyonceeveryfiveminutes.Ahibernatinganimalcannotfeelanypain.Youcantouchit,orevenpullitstail,withoutcausingittomoveorwakeup.Initshibernatingstateitcanevenliveinapoisonousatmosphereforalongtimewithoutanyilleffect.

　　Hibernatinginthisway,theanimalcansleepallthroughthewinter.Youmightwonderhowitmanagestolivewithouteatingforsomanymonths.Theanswerliesintwofacts.Thefirstisthatithasstoredsuppliesoffatinitsbodyduringthesummerandautumn.Thesecondisconnectedwiththemainusethebodymakesorrood——tosupplytheenergyformovement.Wehaveseenthathibernatinganimalreducesmovementtofarbelowtheordinarylevel.Eventhemovementsortheheartandlungsaregreatlyreduced.Theanimalhardlymakesanymovement,hardlyuseanyenergy,andhardlyneedsanyrood.

　　Someanimals,includingsomebearsonlyhairhibernate.Thatistosay,theysleepduringthewinter,buttheirsleepisnotdeepandtheirbodytemperaturedoesnotdrop,astheyarewarmbloodedanimals.Intheantumnthebeareatsandeatsandbecomesveryrat.Hishairgrowslonger.Soonhehasathickcoveringorratandfur.InNovemberherindsaplacetoliedownandgoestosleep.Onwarmwinterdayshemaythinkspringhasarrived；hegetsupandwa[ksaround.Whenheseesthatthesnowisstillthickontheground,hequicklygoestosleepagain.

　　此文是一篇非常好的背景材料.有关动物冬眠我们或许有所了解,但也许是一知半解.文章先叙述了动物冬眠时的状况,而后详细说明了为什么有些动物能做到冬眠,最后还举例说明了半冬眠动物熊的情况,使读者丰富了这方面的知识.