如果您无法下载资料，请参考说明：
1、部分资料下载需要金币，请确保您的账户上有足够的金币
2、已购买过的文档，再次下载不重复扣费
3、资料包下载后请先用软件解压，在使用对应软件打开
好学教育：HYPERLINK"http://www.5haoxue.net"http://www.5haoxue.net2013职称英语考试理工a常用短语模拟题及答案2ADog’sDilemmaFindingababysitterwhileyougoouttoworkis,forexample,aninconvenience.FortheAfricanwilddog,oneofthecontinents’smostendangeredcarnivores,it’samatteroflifeanddeath.Newresearchshowsthatoncepacksfallbelowacertainsize,theyarenotenoughanimalstobothhuntfoodandstayathomeprotectingtheyoung.TheAfricanwilddoghasdeclineddrasticallyoverthepastcentury.Habitualloss,persecutionandunexplainedoutbreaksofdiseasehaveallbeenblamed.Only3,000to5,000animalsremain,andthespeciesisexpectedtogoextinctwithindecadesifthetrendcontinues.Otherlargecarnivoressuchasthespottedhyenafacesimilarpressures,yetarenotdeclining.NowFranckCourchampofCambridgeUniversityhasfoundareasonwhy.Thedog’sweaknessliesinitssocialorganization.Withineachpackofupto20adultsandpups,onlyhedominantmaleandfemalebread.Theremaininganimalshelpraisethepups,cooperatingtohuntpreyanddefendthekillfromothercarnivores.Becausepupscan’tkeepuponahunt,largepacksleaveanadultbehindtoprotectthemfrompredators,whichincludelionsandhyenas.Butleavingababysitteralsocarriescosts.Asmallerhuntingpartyislessabletotacklelargepreyandtodefendthekill.Thereisalsoonelessstomachinwhichtocarryfoodbacktotheden,andonemoremouthtofeedwhentheygetthere.Courchampinvestigatedthisawkwardtrade-offbymodelinghowthecostsofababysitterchangewithdecreasingpacksize.Thisshowedthatpacksofmorethanfiveadultsshouldbeabletofeedallthepupsandstillspareababysitter.Butwithsmallerpacks,eitherthehuntingorthebabysittingsuffers,ortheanimalshavetocompensatebyincreasinghenumberofhuntingexcursions—whichitselfcarriesacosttothepack.FieldobservationsinZimbabwesupportedthemodel.Packsoffiveanimalsorfewerleftpupsunguardedmorefrequentlythanlargerpacksdid.Therewasalsoevidencethatwhentheydidleaveababysitter,theywereforcedtohuntmoreoften.Apackwhichdropsbelowacriticalsizebecomescaughtinaviciouscircle,saysCourchamp,whoisnowatParis-SudUniversity.“Poorreproductionandlowsurvivalfurtherreducespacksize,c
立即下载
