The effect of disparate information on individual fish movements and emergent group behavior

Abstract

Within a group, the level of knowledge held by any individual often differs from that held by other members. Such heterogeneity can be advantageous, potentially allowing groups to cope with situations novel to the majority. It can also affect group integrity, inciting sorting or group fission. To better understand how heterogeneity affects school structure and the physical expression of decision making, we manipulated the ratio of knowledgeable-to-naive fish within groups of giant danios, Devario aequipinnatus, and examined the relationship between collective knowledge, individual behavior, and emergent group properties. Specifically, we varied the proportion of naive fish within groups of 15 individuals, quantified horizontal trajectories of individual fish, and calculated various individual and group swim metrics. When presented with a learned signal (red light) associated with the presence of prey, groups of all-knowledgeable fish exhibited searching behavior (fast, broad turns resulting in diffusely polarized groups), whereas groups of all-naive fish remained unaffected and continued to mill (slow, tight turns resulting in greater packing and lower polarity). In heterogeneous groups, influences of knowledgeable and naive fish were unequally weighted across each measured swim metric, favoring knowledgeable behaviors (fast, broad turns), but incorporating elements of naive behaviors (greater packing). However, at a minority threshold of 20%, knowledgeable individuals conformed to the behavior of the majority and group response echoed all naive. A null model, composed of independent behaviors of knowledgeable and naive fish, predicted significantly different swim metrics from those observed, suggesting actual heterogeneous groups were performing integrated rather than separate behaviors.