Mechanisms and functions of collective anti-predatorresponses

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Animals that form groups or shoals benefit from a lower risk of predation than what they would face when being alone. Beyond the beneficial effects of simply being part of a group, several animal species - including bees, fish and ungulates - have been shown to actively perform collective behaviours in response to attacking predators. For example, individuals within a group may respond to attacking predators with a sudden startle movement away from the perceived threat. The startle responses of a few individuals may then spread to other members of the group, resulting in a cascading chain-reaction of the entire group. Startles usually function as a spatial escape strategy, but in some species individuals have been found to perform repeated, startle-like behaviours without leaving the area of danger. The wave-like propagation of these behaviours may confuse and inhibit attacking predators, but there is little empirical evidence that they effectively reduce predation rates, or how individuals collectively achieve such highly coordinated behaviours. This research project aims to investigate both the ultimate function and the proximate mechanisms underlying the occurrence of repeated, startle-like collective behaviours. As a study system, I selected sulphur-adapted live-bearing fishes (genera Poecilia and Gambusia) that respond to bird predation by performing collective diving behaviour. Inhabiting sulphur-rich, almost anoxic waters in Mexico, these fish are naturally constrained to the water’s well-oxygenated surface layer. Here, they occur at greatdensities and are exposed to high rates of bird predation. Upon attack, these fish exhibit a unique collective escape response, whereby individuals quickly dive and resurface near their initial location, before diving again. This motion results in a repeated, wave-like pattern spreading over the water surface. By combining field observations of natural predation events with highly-controlled experiments and individual-based computer models, I aim to investigate (A) whether and if, how wave-like diving behaviour functions to mitigate bird predation, (B) how fish adapt their diving behavior in the face of changing environmental demands, and (C) how the latter is affected by species interactions in mixed-species fish shoals. My innovative approach to combine theory alongside empirical analyses will serve todevelop a robust and holistic characterization of collective behaviour.