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cellscannotremainaliveoutsidecertainlimitsoftemperatureandmuchnarrowerlimitsmarktheboundariesofeffectivefunctioning.enzymesystemsofmammalsandbirdsaremostefficientonlywithinanarrowrangearound37c;adepartureofafewdegreesfromthisvalueseriouslyimpairstheirfunctioning.
eventhoughcellscansurvivewiderfluctuationstheintegratedactionsofbodilysystemsareimpaired.otheranimalshaveawidertoleranceforchangesofbodilytemperature.
forcenturiesithasbeenrecognizedthatmammalsandbirdsdifferfromotheranimalsinthewaytheyregulatebodytemperature.waysofcharacterizingthedifferencehavebecomemoreaccurateandmeaningfulovertime,butpopularterminologystillreflectstheolddivisioninto"warm-blooded"and"cold-blooded"species;warm-bloodedincludedmammalsandbirdswhereasallothercreatureswereconsideredcold-blooded.
asmorespecieswerestudied,itbecameevidentthatthisclassificationwasinadequate.afencelizardoradesertiguana-eachcold-blooded----usuallyhasabodytemperatureonlyadegreeortwobelowthatofhumansandsoisnotcold.
thereforethenextdistinctionwasmadebetweenanimalsthatmaintainaconstantbodytemperature,calledhome0therms,andthosewhosebodytemperaturevarieswiththeirenvironments,calledpoikilotherms.butthisclassificationalsoprovedinadequate,becauseamongmammalstherearemanythatvarytheirbodytemperaturesduringhibernation.
furthermore,manyinvertebratesthatliveinthedepthsoftheoceanneverexperiencechangeinthedepthsoftheoceanneverexperiencechangeinthechillofthedeepwater,andtheirbodytemperaturesremainconstant.