（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 attract larger numbers of　the birds.

　　17.It can be inferred from the passage that in species in　which vigilant behavior is directed at predators， the　tendency of the animals to be more vigilant in　smaller groups than in larger ones would most likely　be minimized if which of the following were true？

　　（A） The vigilance of animals on the periphery of a　group always exceeded that of animals located　in its interior， even when predators were no　　in the area.

　　（B） The risk of capture for individuals in a group　was the same， whether they were located in　the interior of the group or on its periphery.

　　（C） Animals on the periphery of a group tended to　be less capable of defending themselves from　attack by predators than animals located in the　interior of the group.

　　（D） Animals on the periphery of a group tended t　　bear marks that were more distinctive to　predators than animals located in the interior　of the group.

　　（E） Animals on the periphery of a group tended to　have shorter life spans than animals located in　the interior of the group.