What-Do-Whales-Feel-剑桥4-test1-p2

Reading:2Student:WhatDoWhalesFeelAnexaminationofthefunctioningofthesensesincetaceans,thegroupofmammalscomprisingwhales,dolphinsandporpoisesSomeofthesensesthatweandotherterrestrialmammalstakeforgrantedareeitherreducedorabsentincetaceansorfailtofunctionwellinwater.Forexample,itappearsfromtheirbrainstructurethattoothedspeciesareunabletosmell.Baleenspecies,ontheotherhand,appeartohavesomerelatedbrainstructuresbutitisnotknownwhetherthesearefunctional.Ithasbeenspeculatedthat,astheblowholesevolvedandmigratedtothetopofthehead,theneuralpathwaysservingsenseofsmellmayhavebeennearlyallsacrificed.Similarly,althoughatleastsomecetaceanshavetastebuds,thenervesservingthesehavedegeneratedorarerudimentary.Thesenseoftouchhassometimesbeendescribedasweaktoo,butthisviewisprobablymistaken.Trainersofcaptivedolphinsandsmallwhalesoftenremarkontheiranimals’responsivenesstobeingtouchedorrubbed,andbothcaptiveandfreerangingcetaceanindividualsofallspecies(particularlyadultsandcalves,ormembersofthesamesubgroup)appeartomakefrequentcontact.Thiscontactmayhelptomaintainorderwithinagroup,andstrokingortouchingarepartofthecourtshipritualinmostspecies.Theareaaroundtheblowholeisalsoparticularlysensitiveandcaptiveanimalsoftenobjectstronglytobeingtouchedthere.Thesenseofvisionisdevelopedtodifferentdegreesindifferentspecies.Baleenspeciesstudiedatclosequartersunderwater–specificallyagreywhalecalfincaptivityforayear,andfree-rangingrightwhalesandhumpbackwhalesstudiedandfilmedoffArgentinaandHawaii–haveobviouslytrackedobjectswithvisionunderwater,andtheycanapparentlyseemoderatelywellbothinwaterandinair.However,thepositionoftheeyessorestrictsthefieldofvisioninbaleenwhalesthattheyprobablydonothavestereoscopicvision.Ontheotherhand,thepositionoftheeyesinmostdolphinsandporpoisessuggeststhattheyhavestereoscopicvisionforwardanddownward.Eyepositioninfreshwaterdolphins,whichoftenswimontheirsideorupsidedownwhilefeeding,suggeststhatwhatvisiontheyhaveisstereoscopicforwardandupward.Bycomparison,thebottlenosedolphinhasextremelykeenvisioninwater.Judgingfromthewayitwatchesandtracksairborneflyingfish,itcanapparentlyseefairlywellthroughtheair–waterinterfaceaswell.Andalthoughpreliminaryexperimentalevidencesuggeststhattheirin-airvisionispoor,theaccuracywithwhichdolphinsleaphightotakesmallfishoutofatrainer’shandprovidesanecdotalevidencetothecontrary.Suchvariationcannodoubtbeexplainedwithreferencetothehabitatsinwhichindividualspecieshavedeveloped.Forexample,visionisobviouslymoreusefultospeciesinhabitingclearopenwatersthantothoselivinginturbidriversandfloodedplains.TheSouthAmericanboutuandChinesebeiji,forinstance,appeartohaveverylimitedvision,andtheIndiansususareblind,theireyesreducedtoslitsthatprobablyallowthemtosenseonlythedirectionandintensityoflight.Althoughthesensesoftasteandsmellappeartohavedeteriorated,andvisioninwaterappearstobeuncertain,suchweaknessesaremorethancompensatedforbycetaceans’well-developedacousticsense.Mostspeciesarehighlyvocal,althoughtheyvaryintherangeofsoundstheyproduce,andmanyforageforfoodusingecholocation.Largebaleenwhalesprimarilyusethelowerfrequenciesandareoftenlimitedintheirrepertoire.Notableexceptionsarethenearlysong-likechorusesofbowheadwhalesinsummerandthecomplex,hauntingutterancesofthehumpbackwhales.Toothedspeciesingeneralemploymoreofthefrequencyspectrum,andproduceawidervarietyofsounds,thanbaleenspecies(thoughthespermwhaleapparentlyproducesamonotonousseriesofhigh-energyclicksandlittleelse).Someofthemorecomplicatedsoundsareclearlycommunicative,althoughwhatroletheymayplayinthesociallifeand‘culture’ofcetaceanshasbeenmorethesubjectofwildspeculationthanofsolidscience.