Collective decision-making appears more egalitarian in populations where group fission costs are higher.

Collective decision-making is predicted to be more egalitarian in conditions where the costs of group fission are higher. Here, we ask whether Trinidadian guppies (Poecilia reticulata) living in high or low predation environments, and thereby facing differential group fission costs, make collective decisions in line with this prediction. Using a classic decision-making scenario, we found that fish from high predation environments switched their positions within groups more frequently than fish from low predation environments. Because the relative positions individuals adopt in moving groups can influence their contribution towards group decisions, increased positional switching appears to support the prediction of more evenly distributed decision-making in populations where group fission costs are higher. In an agent-based model, we further identified that more frequent, asynchronous updating of individuals' positions could explain increased positional switching, as was observed in fish from high predation environments. Our results are consistent with theoretical predictions about the structure of collective decision-making and the adaptability of social decision-rules in the face of different environmental contexts.

Predatory fish select for coordinated collective motion in virtual prey.

Movement in animal groups is highly varied and ranges from seemingly disordered motion in swarms to coordinated aligned motion in flocks and schools. These social interactions are often thought to reduce risk from predators, despite a lack of direct evidence. We investigated risk-related selection for collective motion by allowing real predators (bluegill sunfish) to hunt mobile virtual prey. By fusing simulated and real animal behavior, we isolated predator effects while controlling for confounding factors. Prey with a tendency to be attracted toward, and to align direction of travel with, near neighbors tended to form mobile coordinated groups and were rarely attacked. These results demonstrate that collective motion could evolve as a response to predation, without prey being able to detect and respond to predators.

Ecological consequences of the bold-shy continuum: the effect of predator boldness on prey risk.

Although the existence of different personality traits within and between animal populations has been relatively well established, the ecological implications of this variation remain neglected. In this study we tested whether differences in the boldness of pairs of three-spined sticklebacks led to differential predation risk in their prey, Chironomidae larvae. Bolder pairs, those that left a refuge and crossed the tank mid-line sooner, ate a greater proportion of prey in 10 min than less bold fish (therefore prey were at a greater per capita risk). Fish crossed the mid-line more rapidly when a larger number of prey were presented, suggesting they accepted greater risk in return for a larger foraging reward. Perception of predation risk also affected the differences between fish in boldness, as larger fish crossed the mid-line sooner after leaving the refuge (larger fish are less at risk from predation). Hence, an interesting trophic interaction occurs, where the risk experienced by the chironomid larvae is determined by the risk perceived by their predators. Through the variation generated by boldness, a form of behaviourally mediated trophic cascade can occur within (as well as between) communities.