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AWarm-BloodedWhenitcomestophysiology,theleatherbackturtleis,insomeways,morelikeareptilianwhalethanaturtle.Itswimsfartherintothecoldofthenorthernandsouthernoceansthananyotherseaturtle,anditdealswiththechillywatersinawayuniqueamongreptiles.Awarm-bloodedturtlemayseemtobeacontradictioninterms.Nonetheless,anadultleatherbackcanmaintainabodytemperatureofbetween25and26°C(77-79°F)inseawaterthatisonly8°C(46.4°F).Accomplishingthisfeatrequiresadaptationsbothtogenerateheatintheturtle’sbodyandtokeepitfromescapingintothesurroundingwaters.Leatherbacksapparentlydonotgenerateinternalheatthewaywedo,orthewaybirdsdo,asaby-productofcellularmetabolism.Aleatherbackmaybeabletopickupsomebodyheatbybaskingatthesurface;itsdark,almostblackbodycolormayhelpittoabsorbsolarradiation.However,mostofitsinternalheatcomesfromtheactionofitsmuscles.Leatherbackskeeptheirbodyheatinthreedifferentways.Thefirst,andsimplest,issize.Thebiggertheanimalis,theloweritssurface-to-volumeratio;foreveryounceofbodymass,thereisproportionatelylesssurfacethroughwhichheatcanescape.Anadultleatherbackistwicethesizeofthebiggestcheloniidseaturtlesandwillthereforetakelongertocooloff.Maintainingahighbodytemperaturethroughsheerbulkiscalledgigantothermy.Itworksforelephants,forwhales,and,perhaps,itworkedformanyofthelargerdinosaurs.Itapparentlyworks,inasmallerway,forsomeotherseaturtles.Largeloggerheadandgreenturtlescanmaintaintheirbodytemperatureatadegreeortwoabovethatofthesurroundingwater,andgigantothermyisprobablythewaytheydoit.Muscularactivityhelps,too,andanactivelyswimminggreenturtlemaybe7°C(12.6°F)warmerthanthewatersitswimsGigantothermy,though,wouldnotbeenoughtokeepaleatherbackwarmincoldnorthernwaters.Itisnotenoughforwhales,whichsupplementitwithathicklayerofinsulatingblubber(fat).Leatherbacksdonothaveblubber,buttheydohaveareptilianequivalent:thick,oil-saturatedskin,withalayeroffibrous,fattytissuejustbeneathit.Insulationprotectstheleatherbackeverywherebutonitsheadandflippers.Becausetheflippersarecomparativelythinandblade-like,theyaretheonepartoftheleatherbackthatislikelytobecomechilled.Thereisnotmuchthattheturtlecandoaboutthiswithoutcompromisingtheaerodynamicshapeoftheflipper.Theproblemisthatasbloodflowsthroughtheturtle’sflippers,itriskslosingenoughheattolowertheanimal’scentralbodytemperaturewhenitreturns.Thesolutionistoallowtheflipperstocooldownwithoutdrawingheatawayfromtherestoftheturtle’sbody.Theleatherbackaccomplishesthisbyarrangingthebloodvesselsinthebaseofitsflipperintoacountercurrentexchangesystem.Inacountercurrentexchangesystem,thebloodvesselscarryingcooledbloodfromtheflippersruncloseenoughtothebloodvesselscarryingwarmbloodfromthebodytopickupsomeheatfromthewarmerbloodvessels;thus,theheatistransferredfromtheoutgoingtotheingoingvesselsbeforeitreachestheflipperitself.Thisisthesamearrangementfoundinanold-fashionedsteamradiator,inwhichthecoiledpipespassheatbackandforthaswatercoursesthroughthem.Theleatherbackiscertainlynottheonlyanimalwithsuchanarrangement;gullshaveacountercurrentexchangeintheirlegs.Thatiswhyagullcanstandonanicefloewithoutfreezing.Allthisapplies,ofcourse,onlytoanadultleatherback.Hatchlingsaresimplytoosmalltoconservebodyheat,evenwithinsulationandcountercurrentexchangesystems.Wedonotknowhowold,orhowlarge,aleatherbackhastobebeforeitcanswitchfromacold-bloodedtoawarm-bloodedmodeoflife.Leatherbacksreachtheirimmensesizeinamuchshortertimethanittakesotherseaturtlestogrow.Perhapstheirrushtoadulthoodisdrivenbyasimpleneedtokeep

825-26攝氏度之間。要完成這一過程，棱皮龜積極游水的綠海龜身上的溫度可能比它所游區(qū)域溫度高12.6華氏度。MassCasesinwhichmanyspeciesbecomeextinctwithinageologicallyshortintervaloftimearecalledmassextinctions.TherewasonesucheventattheendoftheCretaceousperiod(around70millionyearsago).Therewasanother,evenlarger,massextinctionattheendofthePermianperiod(around250millionyearsago).ThePermianeventhasattractedmuchlessattentionthanothermassextinctionsbecausemostlyunfamiliarspeciesperishedatthattime.Thefossilrecordshowsatleastfivemassextinctionsinwhichmanyfamiliesofmarineorganismsdiedout.Theratesofextinctionhappeningtodayareasgreatastheratesduringthesemassextinctions.Manyscientistshavethereforeconcludedthatasixthgreatmassextinctioniscurrentlyinprogress.Whatcouldcausesuchhighratesofextinction?Thereareseveralhypotheses,includingwarmingorcoolingofEarth,changesinseasonalfluctuationsoroceancurrents,andchangingpositionsofthecontinents.Biologicalhypothesesincludeecologicalchangesbroughtaboutbytheevolutionofcooperationbetweeninsectsandfloweringplantsorofbottom-feedingpredatorsintheoceans.Someoftheproposedmechanismsrequiredaverybriefperiodduringwhichallextinctionssuddenlytookplace;othermechanismswouldbemorelikelytohavetakenplacemoregradually,overanextendedperiod,oratdifferenttimesondifferentcontinents.Somehypothesesfailtoaccountforsimultaneousextinctionsonlandandintheseas.Eachmassextinctionmayhavehadadifferentcause.Evidencepointstohuntingbyhumansandhabitatdestructionasthelikelycausesforthecurrentmassextinction.AmericanpaleontologistsDavidRaupandJohnSepkoski,whohavestudiedextinctionratesinanumberoffossilgroups,suggestthatepisodesofincreasedextinctionhaverecurredperiodically,approximatelyevery26millionyearssincethemid-Cretaceousperiod.ThelateCretaceousextinctionofthedinosaursandammonoidswasjustoneofthemoredrasticinawholeseriesofsuchrecurrentextinctionepisodes.Thepossibilitythatmassextinctionsmayrecurperiodicallyhasgivenrisetosuchhypothesesasthatofacompanionstarwithalong-periodorbitdeflectingotherbodiesfromtheirnormalorbits,makingsomeofthemfalltoEarthasmeteorsandcausingwidespreaddevastationuponimpact.OfthevarioushypothesesattemptingtoaccountforthelateCretaceousextinctions,theonethathasattractedthemostattentioninrecentyearsistheasteroid-impacthypothesisfirstsuggestedbyLuisandWalterAlvarez.Accordingtothishypothesis,Earthcollidedwithanasteroidwithanestimateddiameterof10kilometers,orwithseveralasteroids,thecombinedmassofwhichwascomparable.Theforceofcollisionspewedlargeamountsofdebrisintotheatmosphere,darkeningtheskiesforseveralyearsbeforethefinerparticlessettled.Thereducedlevelofphotosynthesisledtoamassivedeclineinplantlifeofallkinds,andthiscausedmassivestarvationfirstofherbivoresandsubsequentlyofcarnivores.Themassextinctionwouldhaveoccurredverysuddenlyunderthishypothesis.OneinterestingtestoftheAlvarezhypothesisisbasedonthepresenceofrare-earthelementiridium(Ir).Earth’scrustcontainsverylittleofthiselement,butmostasteroidscontainalotmore.Debristhrownintotheatmospherebyanasteroidcollisionwouldpresumablycontainlargeamountsofiridium,andatmosphericcurrentswouldcarrythismaterialallovertheglobe.AsearchofsedimentarydepositsthatspantheboundarybetweentheCretaceousandTertiaryperiodsshowsthatthereisadramaticincreaseintheabundanceofiridiumbrieflyandpreciselyatthisboundary.ThisiridiumanomalyoffersstrongsupportfortheAlvarezhypothesiseventhoughnoasteroiditselfhaseverbeenrecovered.Anasteroidofthissizewouldbeexpectedtoleaveanimmensecrater,eveniftheasteroiditselfwasdisintegratedbytheimpact.Theintenseheatoftheimpactwouldproduceheat-shockedquartzinmanytypesofrock.Also,largeblocksthrownasidebytheimpactwouldformsecondarycraterssurroundingthemaincrater.Todate,severalsuchsecondarycratershavebeenfoundalongMexico’sYucatanPeninsula,andheat-shockedquartzhasbeenfoundbothinMexicoandinHaiti.AlocationcalledChicxulub,alongtheYucatancoast,hasbeensuggestedastheprimaryimpactDavidRaupJohnSepkoski曾經(jīng)在大量的化石群里面研究了WalterAlvarez最先提出的小行星撞擊假說。根據(jù)這個假說，地球與一個直徑10公里的小行星或者總體積與之相當?shù)膸讉€小行星碰撞。碰撞的力量把Alvarez假說的一個有趣的檢驗是基于稀土元素銥的存在。這種元素在地殼中的Alvarez假說提供了有力支持。擊石英。尤卡坦沿海一個叫做Chicxulub的地方，曾被當做主要的撞擊點。GlacierGlaciersareslowlymovingmassesoficethathaveaccumulatedonlandinareaswheremoresnowfallsduringayearthanmelts.Snowfallsashexagonalcrystals,butonceontheground,snowissoontransformedintoacompactedmassofsmaller,roundedgrains.Astheairspacearoundthemislessenedbycompactionandmelting,thegrainsbecomedenser.Withfurthermelting,refreezing,andincreasedweightfromnewersnowfallabove,thesnowreachesagranularrecrystallizedstageintermediatebetweenflakesandiceknownasfirn.Withadditionaltime,pressure,andrefrozenmeltwaterfromabove,thesmallfirngranulesbecomelarger,interlockedcrystalsofblueglacialice.Whentheiceisthickenough,usuallyover30meters,theweightofthesnowandfirnwillcausetheicecrystalstowardthebottomtobecomeplasticandtoflowoutwardordownwardfromtheareaofsnowGlaciersareopensystems,withsnowasthesystem’sinputandmeltwaterasthesystem'smainoutput.Theglacialsystemisgovernedbytwobasicclimaticvariables:precipitationandtemperature.Foraglaciertogrowormaintainitsmass,theremustbesufficientsnowfalltomatchorexceedtheannuallossthroughmelting,evaporation,andcalving,whichoccurswhentheglacierlosessolidchunksasicebergstotheseaortolargelakes.Ifsummertemperaturesarehighfortoolong,thenallthesnowfallfromthepreviouswinterwillmelt.Surplussnowfallisessentialforaglaciertodevelop.Asurplusallowssnowtoaccumulateandforthepressureofsnowaccumulatedovertheyearstotransformburiedsnowintoglacialicewithadepthgreatenoughfortheicetoflow.Glaciersaresometimesclassifiedbytemperatureasfaster-flowingtemperateglaciersorasslower-flowingpolarGlaciersarepartofEarth’shydrologiccycleandaresecondonlytotheoceansinthetotalamountofwatercontained.About2percentofEarth’swateriscurrentlyfrozenasice.Twopercentmaybeadeceivingfigure,however,sinceover80percentoftheworld’sfreshwaterislockedupasiceinglaciers,withthemajorityofitinAntarctica.Thetotalamountoficeisevenmoreawesomeifweestimatethewaterreleaseduponthehypotheticalmeltingoftheworld’sglaciers.Sealevelwouldriseabout60meters.Thiswouldchangethegeographyoftheplanetconsiderably.Incontrast,shouldanothericeageoccur,sealevelwoulddropdrastically.Duringthelasticeage,sealeveldroppedabout120meters.Whensnowfallsonhighmountainsorinpolarregions,itmaybecomepartoftheglacialsystem.Unlikerain,whichreturnsrapidlytotheseaoratmosphere