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
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 to 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.
18.Which of the following best describes the relationship of the second paragraph to the first？
（A）The second paragraph relies on different evidence in drawing a conclusion similar to that expressed in the first paragraph.
（B）The second paragraph provides furtherelaboration on why an assertion made at the
end of the first paragraph proves to be true inmost cases.
（C）The second paragraph provides additionalinformation in support of a hypothesis stated in the first paragraph.
（D）The second paragraph provides an example of a case in which the assumption described in the first paragraph is unwarranted.
（E） The second paragraph describes a phenomenonthat has the same cause as the phenomenon described in the first paragraph.
19.It can be inferred from the passage that the author of the passage would be most likely to agree with which of the following assertions about vigilant behavior？
（A） The larger the group of animals， the higher the probability that individuals in the interior of the group will exhibit vigilant behavior.
（B） Vigilant behavior exhibited by individuals in small groups is more effective at warding off predators than the same behavior exhibited by individuals in larger groups.
（C） Vigilant behavior is easier to analyze in species that are preyed upon by many different predators than in species that are preyed upon by relatively few of them.
（D） The term “vigilant,” when used in reference to the behavior of animals， does not refe exclusively to behavior aimed at avoiding predators.
（E） The term “vigilant，” when used in reference to the behavior of animals， usually refers to behavior exhibited by large groups of animals.
20.The passage provides information in support of which of the following assertions？
（A） The avoidance of predators is more important to an animal's survival than is the quest for food.
（B） Vigilant behavior aimed at predators is seldom more beneficial to groups of animals than to individual animals.
（C） Different species of animals often develop different strategies for dealing with predators.
（D） The size of a group of animals does not necessarily reflect its success in finding food.
（E） Similar behavior in different species of animals does not necessarily serve the same purpose. |
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