What Drives the Group Size Effect in Ant-Predator Vigilance?

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9723437 Lima This research is directed at a new understanding of a widespread behavioral phenomenon: the vigilance group size effect, in which socially-feeding animals engage in less vigilance (alertness) with an increase in group size. Despite the fact that this is one of the most well established results in the study of animal behavior, many important issues remain unaddressed and even unrecognized. The proposed experiments are designed to help remedy this situation, and in doing so, will shed light on the reasons why so many types of animals are social in the first place. The proposed research focuses first on an assumption fundamental to all explanations of the group size effect: that an animal cannot detect predators while it is actively feeding with its head down. Recent work suggests that apparently non-vigilant (feeding) birds can detect approaching predator attacks at a considerable distance. A main goal is thus to characterize this ability in birds (as represented by the dark-eyed junco, Junco hyemalis) under realistic situations, and then explore its consequences for the group size effect itself. This proposed work will determine whether the group size effect represents a previously unrecognized interaction between poor quality vigilance (while actually feeding), and high quality vigilance (while overtly alert). The proposed research also explores two unorthodox explanations for the group size effect. The first is based on theoretical developments which suggest that the group size effect occurs only because group sizes change frequently over time (as animals come and go from the group). This idea will be tested by allowing captive juncos to feed in constant group sizes or in groups that vary over time. If the new theory is correct, only those birds experiencing the variable group sizes should exhibit the basic group size effect. The second unorthodox explanation for the group size effect concerns the effect of competition for food on vigilance. This view does not deny that animals are vigilant for predators, but maintains that they decrease their vigilance in larger groups because competition for food is more intense as group size increases. If this view is correct, then the group size effect should be much more pronounced when animals are feeding on a limited food supply; this prediction will be tested in flocks of house finches (Carpodacus mexicanus). Results from this research will allow a better understanding of group behavior and the evolution of sociality.