Abstract
Predation is thought to shape the macroscopic properties of animal groups, making moving groups more cohesive and coordinated. Precisely how predation has shaped individuals' fine-scale social interactions in natural populations, however, is unknown. Using high-resolution tracking data of shoaling fish (Poecilia reticulata) from populations differing in natural predation pressure, we show how predation adapts individuals' social interaction rules. Fish originating from high predation environments formed larger, more cohesive, but not more polarized groups than fish from low predation environments. Using a new approach to detect the discrete points in time when individuals decide to update their movements based on the available social cues, we determine how these collective properties emerge from individuals' microscopic social interactions. We first confirm predictions that predation shapes the attraction–repulsion dynamic of these fish, reducing the critical distance at which neighbours move apart, or come back together. While we find strong evidence that fish align with their near neighbours, we do not find that predation shapes the strength or likelihood of these alignment tendencies. We also find that predation sharpens individuals' acceleration and deceleration responses, implying key perceptual and energetic differences associated with how individuals move in different predation regimes. Our results reveal how predation can shape the social interactions of individuals in groups, ultimately driving differences in groups' collective behaviour.
Highlights
Predation is often considered to be the major selective force driving the origin and maintenance of group living [1,2,3]
Evidence that predation drives the formation of larger and more cohesive groups has come from a number of comparative studies between populations or groups exposed to varying degrees of predation pressure [10,11,12,13,14,15,16]
In many moving animal groups, these social decisions are characterized by simple interaction rules, such as attraction and alignment with near neighbours, that allow individuals to remain cohesive and coordinated while on the move together [17,18,19]
Summary
Predation is often considered to be the major selective force driving the origin and maintenance of group living [1,2,3]. Cohesive and coordinated group behaviours emerge, from the decision rules that individuals use to interact in groups, and how predation has shaped these fine-scale social decisions is still unclear. In many moving animal groups, these social decisions are characterized by simple interaction rules, such as attraction and alignment with near neighbours, that allow individuals to remain cohesive and coordinated while on the move together [17,18,19]. Using high resolution trajectory data on the movements of fish originating from both high and low predation environments, here we quantify how predation has shaped the social decisions that produce these differences. We modelled all response variables using generalized linear mixed effects models These were fitted with predation regime (high or low), sex, subgroup size (where applicable), and body size (see electronic supplementary material, figure S1) as fixed effects. Full details of all statistical models, analyses, and tables can be found in the electronic supplementary material
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