SoakingUptheSun

Imaginetakinganaturehikewithfriendsalongacreek.It’sasunnyspringmorning.Thewarmthofthesunfeelsgood,buttheairis chilly.


Alargegrayrocksticksoutofthemuddybank.Aflashofcolorcatchesyoureye.Youstepcloseranddiscoveralizardislyingontherock.Itsskinshimmersinthesun.Itisasstillasastatue.Youwonder:Isitsleeping?Thelizardtilts itshead,butitdoesn’tmoveofftherock.Why?Thelittlelizardis sunbathing!

Energyfromthesunhelpsthisdesertlizarddartand dash. 

Lizards,snakes,andotherreptilesareectotherms.Fish,insects,worms,andamphibians,likefrogs,areectotherms,too.Anectothermisananimalthatproducesverylittleheatinsideitsbody.Towarmup,theanimalhastogetheatfromits environment.


That’swhythelizardissunbathing.Thesunlightstrikesitsbody,warmingit.Therockabsorbsheatfromthesun,whichwarmsthelizard’s underside.

Duringthenight,thelizard’sbodytemperaturedrops.Itbecomesascoolastheairaroundit.Alowbodytemperaturemakesanectotherm’smusclesslow down.


Aftersoakingupsomemorningsunshine,however,thelizard’stemperaturerises.Itwillthenhavetheenergytoscurryaround.Itwillbecomefastenoughtohuntandtoescape predators.

Thisdesert lizardbasksinthesuntowarm itself.

HeatFromtheInside

Asyoucontinueyourhike,youhearasound:Clickety,clickety,clickety.It’sthedistinctsoundoftinyclawsontreebark.Twosquirrelsarechasingeachotheraroundatree trunk.


Unlikethelizard,thesefurryrodentsdon’tneedsunlighttogetgoinginthemorning.That’sbecausesquirrelsandothermammalsare endotherms.

Anendothermisananimalthatproducesenoughbodyheattokeepitsinsideswarm.Thesquirrelsdon’thavetowaitonthesun.Theycanbeactivethemomenttheywakeup.Socanbirds.Theyareendotherms,too.Andsocan you. 

That’sright!Youareanendotherm.Itdoesn’tmatterwhetheryou’replayingoutsideonahotdayorswimminginacoldlake.Theinsideofyourbodystayswarmatabout 37oC (98oF).Endothermsarealsocalledwarm‑blooded.That’sbecauseanendotherm’sbloodisalwayswarm.Ectothermsarecalledcold‑blooded.Thistermismisleading,though.Asanectothermwarmsup,sodoesits blood.

What’sInaName?

Ectothermandendothermmaysoundlikestrangewords.Yet,theymakesensewhenyoubreakthem down.


EctothermcomesfromtheGreek words ektos,meaning“outside,”andthermos,meaning“warm.”Anectothermgetswarmthfromoutsideits body.


EndothermcomesfromtheGreekword endo, meaning“within.”Anendothermgetswarmthfrominsideits body.