The effect of predation risk on group behaviour and information flow during repeated collective decisions

Abstract

Making fast and accurate group decisions under uncertain and risky conditions is a fundamental problem for groups. Currently, there is little empirical evidence of how natural selection (such as environmental predation risk) has shaped the mechanisms of group decision making. We repeatedly tested individually marked guppies, Poecilia reticulata, from the upper and lower reaches of the Aripo and Turure rivers in a Y-maze and found that populations that had evolved under different predation regimes differed in their decision-making speed. All groups decreased decision time over successive trial rounds, but fish from low-predation environments did so at a greater rate than fish from high-predation environments. This effect was only significant when fish were tested in groups of eight, not when tested individually. Decision-making accuracy was not significantly different between high- and low-predation populations. Group behaviour differed according to predation risk and trial round, most notably with low-predation groups reducing the amount of positional switching over successive trial rounds at a greater rate than high-predation groups. While fish from both predator risk environments had repeatable within-shoal positioning behaviour as they swam down the maze, only fish from the high-predation groups had repeatable finishing positions. Meaningful measures of information transfer (mean pairwise transfer entropy) were only recorded in low-predation groups, and here there was a significant effect of source river on information flow within groups. Aripo groups increased information flow over successive trial rounds, while Turure groups did not. Variation in mean pairwise transfer entropy increased over successive trial rounds for both low-predation populations, suggesting that information used to make movement decisions became less homogeneously distributed and increasingly directed through a subset of individuals. Predation pressure is a ubiquitous selection force, and these findings may apply to a variety of natural social groups.