Flock response to sustained asynchronous predator attacks

Collective behaviour is a ubiquitous emergent phenomenon where organisms share information and conduct complicated manoeuvres as a group. Dilution of predation risk is presumed to be a major proponent contributing towards the emergence of such fascinating behaviour. However, the role of multiple sources of predation risk in determining the characteristics of the escape manoeuvres remains largely unexplored. The current work aims to address this paucity by examining the response of a flock to multiple persistently pursuing predators, using an agent-based approach employing a force-based model. Collective features such as herding, avoiding and split-and-join are observed across a wide spectrum of systemic conditions. The transition from one response state to another is examined as a function of the relative angle of predator attack, a parameter exclusive to multi-predator systems. Other concomitant parameters, such as the frequency of attacks and compatibility of target selection tactics of the predators, have a significant effect on the escape probability of the prey (i.e., the success rate of escape manoeuvres). A quantitative analysis has been carried out to determine the most successful combination of target selection while also focusing on beneficial ancillary effects such as flock splitting. The long-term dynamics of the system indicate a faster decay of prey numbers (higher prey mortality) at higher coordination strength due to a monotonically decreasing relation between coordination strength and prey speed supplanted by coincidental synchrony of predator attacks. The work highlights the non-additive nature of the effects of predation in a multi-predator system and urges further scrutiny of group hunting dynamics in such systems. Author summaryCollective motion is a natural phenomenon observed across a wide range of length and time scales. One purported reason for the development of such behaviour is to reduce the individual risk of predation through the many-eyes effect and group manoeuvring in case of attacks. However, the behaviour of the prey flock can turn out to be starkly different when there are multiple predators involved. We examine the response of the flock in the presence of multiple predators and find the circumstances leading to the occurrence of different escape manoeuvres. We observe the stricter penalty warranted on the flock due to certain manoeuvres, such as split-and-join, due to the asynchronous and persistent nature of the predator attacks. We also identify the issues with superfluous coordination among prey and its ramifications in terms of prey mortality. The combined effect of the predators is found to outpace the sum of individual predator prowess. The current work emphasises the distinct dynamics of a multi-predator system and puts forth pertinent queries regarding synchronisation among predators and group hunting tactics.