06年GRE模拟试题第3部分
文章作者 100test 发表时间 2007:02:25 20:09:21
来源 100Test.Com百考试题网
One explanation for the tendency of animals to be more vigilant in smaller groups than in larger ones assumes that the vigilant behavior-looking up， for example-is aimed at predators. If individuals on the
（5） edge of a group are more vigilant because they are at greater risk of being captured， then individuals on aver- age would have to be more vigilant in smaller groups， because the animals on the periphery of a group form a greater proportion of the whole group as the size of the
（10）group diminishes. However， a different explanation is necessary in cases where the vigilant behavior is not directed at predators. J. Krebs has discovered that great blue herons look up more often when in smaller flocks than when in larger
（15）ones， solely as a consequence of poor feeding conditions. Krebs hypothesizes that the herons in smaller flocks are watching for herons that they might follow to better feeding pools， which usually attra
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