When to use social information: the advantage of large group size in individual decision making

Abstract

Correct decision making is crucial for animals to maximize foraging success and minimize predation risk. Group-living animals can make such decisions by using their own personal information or by pooling information with other group members (i.e. social information). Here, we investigate how individuals might best balance their use of personal and social information. We use a simple modelling approach in which individual decisions based upon social information are more likely to be correct when more individuals are involved and their personal information is more accurate. Our model predicts that when the personal information of group members is poor (accurate less than half the time), individuals should avoid pooling information. In contrast, when personal information is reliable (accurate at least half the time), individuals should use personal information less often and social information more often, and this effect should grow stronger in larger groups. One implication of this pattern is that social information allows less well-informed members of large groups to reach a correct decision with the same probability as more well-informed members of small groups. Thus, animals in larger groups may be able to minimize the costs of collecting personal information without impairing their ability to make correct decisions.

Figure 1

Relative quality of social versus personal information. Plot of quality of social information (I_s), i.e. the probability that the majority of the group is correct, against the quality of personal information (I_p), i.e. the probability that the information possessed by an individual is correct. Lines plotted are for odd group sizes (n) up to 51. To the left-hand side of the dashed line through the origin (n=1) are represented all scenarios where an individual should use social information; to the right-hand side of the line, an individual would do better to rely on its own personally acquired information.

Figure 2

The advantage of large group size in individual decision making. Plot of quality of personal information (I_p), i.e. the probability that the information possessed by an individual is correct, against group size (n). Lines are plotted for odd n (3<n<21), where social information (I_s) is correct with probability 0.5, 0.6 and 0.9.