A long-standing assumption in social behavior is that leadership incurs costs as well as benefits, and this tradeoff can result in diversified social roles in groups. The major cost of leadership in moving animal groups is assumed to be predation, with individuals leading from the front of groups being targeted more often by predators. Nevertheless, empirical evidence for this is limited, and experimental tests are entirely lacking. To avoid confounding effects associated with observational studies, we presented a simulation of virtual prey to real fish predators to directly assess the predation cost of leadership. Prey leading others are at greater risk than those in the middle of groups, confirming that any benefits of leading may be offset by predation costs. Importantly, however, followers confer a net safety benefit to leaders, as prey leading others were less likely to be attacked compared with solitary prey. We also find that the predators preferentially attacked when solitary individuals were more frequent, but this effect was relatively weak compared with the preference for attacking solitary prey during an attack. Using virtual prey, where the appearance and behavior of the prey can be manipulated and controlled exactly, we reveal a hierarchy of risk from solitary to leading to following social strategies. Our results suggest that goal-orientated individuals (i.e., potential leaders) are under selective pressure to maintain group cohesion, favoring effective leadership rather than group fragmentation. Our results have significant implications for understanding the evolution and maintenance of different social roles in groups.

Lay SummaryLiving in groups has been shown to have antipredator benefits for prey animals but risk still differs for individuals within a group. Using fish as predators presented with simulations of virtual prey, we show that prey on the edge of groups are more likely to be attacked than others, even at relatively small group sizes. We also show that encountering prey on the edge is likely to explain this effect rather than predatory confusion.

Consistent inter-individual differences in behaviour (i.e. animal personality variation) can influence a range of ecological and evolutionary processes, including predation. Variation between individual predators in commonly measured personality traits, such as boldness and activity, has previously been linked to encounter rates with their prey. Given the strong selection on predators to respond to prey, individual predators may also vary consistently in their response to prey in a manner that is specific to the context of predation. By studying wild piscivorous fish (pike cichlids, Crenicichla frenata) in their natural environment using experimental presentations of prey and control stimuli, we show that individual predators differ consistently in the amount of time spent near prey. Crucially, these differences were not explained by the behaviour of the same individuals in control presentations (the same apparatus lacking prey), suggesting that variation in the response to prey reflects a 'predator personality trait' which is independent from other individual traits (body size, boldness and/or neophobia) and environmental factors. Pike cichlids which spent more time near prey also attacked prey at a higher rate. These findings imply that the risk posed by individual predators cannot always be adequately predicted from typically studied axes of personality variation.

Innovative behaviour may allow animals to cope with changes in their environment. Innovative propensities are known to vary widely both between and within species, and a growing body of research has begun to examine the factors that drive individuals to innovate. Evidence suggests that individuals are commonly driven to innovate by necessity; for instance by hunger or because they are physically unable to outcompete others for access to resources. However, it is not known whether the factors that drive individuals to innovate are stable across contexts. We examined contextual variation in the drivers of innovation in rock pool prawns (Palaemon spp), invertebrates that face widely fluctuating environments and may, through the actions of tides and waves, find themselves isolated or in groups. Using two novel foraging tasks, we examined the effects of body size and hunger in prawns tested in solitary and group contexts. When tested alone, small prawns were significantly more likely to succeed in a spatial task, and faster to reach the food in a manipulation task, while hunger state had no effect. In contrast, size had no effect when prawns were tested in groups, but food-deprived individuals were disproportionately likely to innovate in both tasks. We suggest that contextual variation in the drivers of innovation is likely to be common in animals living in variable environments, and may best be understood by considering variation in the perception of relative risks and rewards under different conditions.

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