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 not
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.
