演变睡眠(Evolution of sleep)

详细内容

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sleepisveryancient.intheelectroencephalographicsenseweshareitwithalltheprimatesandalmostalltheothermammalsandbirds:itmayextendbackasfarasthereptiles.

thereissomeevidencethatthetwotypesofsleep,dreaminganddreamless,dependonthelife-styleoftheanimal,andthatpredatorsarestatisticallymuchmorelikelytodreamthanprey,whichareinturnmuchmorelikelytoexperiencedreamlesssleep.

indreamsleep,theanimalispowerfullyimmobilizedandremarkablyunresponsivetoexternalstimuli.dreamlesssleepismuchshallower,andwehaveallwitnessedcatsordogscockingtheirearstoasoundwhenapparentlyfastasleep.

thefactthatdeepdreamsleepisrareamongpraytodayseemsclearlytobeaproductofnaturalselection,anditmakessensethattoday,whensleepishighlyevolved,thestupidanimalsarelessfrequentlyimmobilizedbydeepsleepthanthesmartones.butwhyshouldtheysleepdeeplyatall?whyshouldastateofsuchdeepimmobilizationeverhaveevolved?

perhapsoneusefulhintabouttheoriginalfunctionofsleepistobefoundinthefactthatdolphinsandwhalesandaquaticmammalsingeneraseemtosleepverylittle.thereis,byandlarge,noplacetohideintheocean.coulditbethat,ratherthanincreasingananimal'svulnerability,theuniversityoffloridaandraymeddisoflondonuniversityhavesuggestedthistobethecase.

itisconceivablethatanimalswhoaretoostupidtobequiteontheirowninitiativeare,duringperiodsofhighrisk,immobilizedbytheimplacablearmofsleep.thepointseemsparticularlyclearfortheyoungofpredatoryanimals.thisisaninterestingnotionandprobablyatleastpartlytrue.