Of energy and one more migrating animals maintain an

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of energy. And one more: migrating )/animals maintain an intense attentivenessto the greater mission, which keepsthem undistracted by temptations andundeterred by challenges that would turnother animals aside.An arctic tern, on its 20,000 km flightfrom the extreme south of South Americato the Arctic circle, will take no noticeof a nice smelly herring offered from abird-watcher's boat along the way. Whilelocal gulls will dive voraciously for suchhandouts, the tern flies on. Why? The arctictern resists distraction because it is drivenat that moment by an instinctive sense ofsomething we humans find admirable:larger purpose. In other words, it isdeterrnined to reach its destination. Thebird senses that it can eat, rest and matelater. Right now it is totally focused onthe journey; its undivided intent is arrival.Reaching sorne gravelly coastline in theArctic, upon which other arctic terns haveconverged, will serve its larger purposeas shaped by evolution: finding a place, atime, and a set of circumstances in which itcan successfully hatch and rear offspring.But migration is a complex issue, andbiologists define it differently, dependingin part on what sorts of animals they study.Joe! Berger, of the University of Montana,who works on the American pronghornand other large terrestrial mammals, preferswhat he calls ª-simple, practica! definitionsuited to his beasts: 'movements froma seasonal home area away to anotherhome area and back again'. Generally thereason for such seasonal back-and-forthmovement is to seek resources that aren'tavailable within a single area year-round.But daily vertical movements byzooplankton in the ocean - upward bynight to seek food, downward by day toescape predators - can also be consideredmigration. So can the movement of aphidswhen, having depleted the young leaveson one food plant, their offspring thenfly onward to a different host plant, withno one aphid ever returning to where itstarted.Dingle is an evolutionary biologist whostudies insects. His definition is moreintricate than Berger's, citing those fivefeatures that distinguish migration fromother forms of movement. They allowfor the fact that, for example, aphids will69
Test 3become sensitive to blue light (from thesky) when it's time for takeoff on theirbig journey, and sensitive to yellow light(reflected from tender young leaves) whenit's appropriate to land. Birds will fattenthemselves with heavy feeding in advanceof a long migrational flight. The value of hisdefinition, Dingle argues, is that it focusesattention on what the phenomenonof wildebeest migration shares withthe phenomenon of the aphids, andtherefore helps guide researchers towardsunderstanding how evolution has producedthem all.

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