Spatial positioning in the selfish herd

The antipredator benefits of grouping are relatively well understood; however, predation risk often differs for individuals that occupy different positions within a group. The selfish herd hypothesis describes how individuals can reduce risk of predation by moving to specific positions within the group. In existing theory, this movement occurs through the adoption of possible "movement rules" that differ in their cognitive complexity. Here, we investigate the effectiveness of different previously suggested rules in reducing risk for central and peripheral individuals within a group. We demonstrate that initial spatial position is important in determining the success of different risk-reducing movement rules, as initially centrally positioned individuals are likely to be more successful than peripheral ones at reducing their risk relative to other group members, regardless of the movement rules used. Simpler strategies are effective in low-density populations; but at high density, more complex rules are more effective. We also find that complex rules that consider the position of multiple neighbors are the only rules that successfully allow individuals to move from peripheral to central positions or maintain central positions, thus avoiding predators that attack from outside the group. Our results suggest that the attack strategy of a predator should be critically important in determining prey escape strategies in a selfish herd context and that prey should modify their behavioral responses to impending attack in response to their position within a group.