Individual and collective behaviour of fish subject to differing risk-level treatments with a sympatric predator

The ability to adapt behaviour according to context is vital to the success of many organisms, particularly when it comes to mitigating risk, across time and space, in unpredictable environments. While many studies have examined the role of predation threat in shaping behaviour, very few have quantitatively investigated prey responses across different elements of perceived risk or ecological contexts. Here, we present a detailed analysis of how mosquitofish (Gambusia holbrooki) adapt their individual and collective behaviour in five experimental treatments designed to replicate different core elements of predator threat. As risk increased, within and across treatments, fish decreased movements across key regions of their environment, swimming speed and nearest neighbour distances, reflecting a threat-sensitive approach. Furthermore, movement predictability also decreased when fish occupied higher risk areas. Within and between treatments, increased risk shaped the local interactions among fish. Local density, group polarisation, and alignment of near neighbours increased, while patterns of local density differed according to whether the fish were in a preferred (region of low perceived risk) or non-preferred (region of high perceived risk) area. Groups in non-preferred areas became elongated, suggesting more rapid locomotion. The rate of turning response and changes in speed relative to the positions of near neighbours increased as a function of risk in safer areas but became less coordinated in areas and treatments associated with higher risk. Importantly, the interactions between fish under the greatest risk also showed a novel qualitative shift in patterns, with fish prioritising group cohesion over group order. Taken together, these results demonstrate the considerable flexibility of fish to adapt according to risk, producing both quantitative and qualitative changes in behaviour as a function of proximate threat levels

A statistical method for identifying different rules of interaction between individuals in moving animal groups.

The emergent patterns of collective motion are thought to arise from application of individual-level rules that govern how individuals adjust their velocity as a function of the relative position and behaviours of their neighbours. Empirical studies have sought to determine such rules of interaction applied by 'average' individuals by aggregating data from multiple individuals across multiple trajectory sets. In reality, some individuals within a group may interact differently from others, and such individual differences can have an effect on overall group movement. However, comparisons of rules of interaction used by individuals in different contexts have been largely qualitative. Here we introduce a set of randomization methods designed to determine statistical differences in the rules of interaction between individuals. We apply these methods to a case study of leaders and followers in pairs of freely exploring eastern mosquitofish (Gambusia holbrooki). We find that each of the randomization methods is reliable in terms of: repeatability of p-values, consistency in identification of significant differences and similarity between distributions of randomization-based test statistics. We observe convergence of the distributions of randomization-based test statistics across repeat calculations, and resolution of any ambiguities regarding significant differences as the number of randomization iterations increases