If such predictions play out in the real world, individuals should try and surround themselves with others to reduce their domain of danger, known as the ‘selfish herd’ effect ( Hamilton, 1971). Some of the most long-standing theoretical work suggests that, when predators appear at random and attack the nearest prey, predation risk should be highest on the edge (‘marginal predation’) and front of mobile groups ( Bumann et al., 1997 Hamilton, 1971 Morrell and Romey, 2008 Vine, 1971). Previous work has identified many different, and sometimes contradictory, factors that predict prey vulnerability in groups. Hence, a better understanding of the factors that influence predation risk within animal groups is of fundamental importance. Where and whom predators attack within groups not only has major implications for the selection of individual phenotypes, and thereby the emergence of collective behaviour and the functioning of animal groups ( Farine et al., 2015 Jolles et al., 2020 Ward and Webster, 2016), but also shapes the social behaviour of prey and the properties and structure of prey groups. However, the costs and benefits of grouping are not shared equally among individuals within groups, and besides differential food intake and costs of locomotion, group members themselves may experience widely varying risks of predation ( Handegard et al., 2012 Krause, 1994 Krause and Ruxton, 2002). This helps explain why strong predation pressure is known to drive the formation of larger and more cohesive groups ( Beauchamp, 2004 Krause and Ruxton, 2002 Seghers, 1974). Via effects such as enhanced predator detection ( Lima, 1995 Magurran et al., 1985), predator confusion ( Landeau and Terborgh, 1986), and risk dilution effects ( Foster and Treherne, 1981 Turner and Pitcher, 1986), individuals living and moving in groups can reduce their risk of predation ( Ioannou et al., 2012 Krause and Ruxton, 2002 Pitcher, 1993 Ward and Webster, 2016). Editor's evaluationĪ key challenge in the life of most animals is to avoid being eaten. Our results highlight the crucial interplay between predators’ attack strategy and response of prey underlying the predation risk within mobile animal groups. While the majority of attacks were successful (70%), targeted individuals that did manage to avoid being captured exhibited a higher maximum acceleration response just before the attack and were further away from the pike‘s head. From the prey’s perspective, those fish in central locations, but relatively far from, and less aligned with, neighbours, were most likely to be targeted. We found that pike tended to stealthily approach the largest groups, and were often already inside the school when launching their attack, making prey in this frontal ‘strike zone’ the most vulnerable to be targeted. Here, using high-resolution tracking of solitary predators (Northern pike) hunting schooling fish (golden shiners), we not only provide insights into predator decision-making, but show which key spatial and kinematic features of predator and prey predict the risk of individuals to be targeted and to survive attacks. While it is well appreciated that predation risk is likely not shared equally among individuals within groups, its detailed quantification has remained difficult due to the speed of attacks and the highly dynamic nature of collective prey response. Predation is one of the main evolutionary drivers of social grouping.
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