Survival advantage of sluggish individuals in aggregations of aposematic prey, during encounters with ambush predators

Movement is an important element of prey defense ensembles. The adaptive advantages of either remaining motionless or rapid escape are clear. In contrast, putative benefits are unclear for sluggish movement of aposematic prey that are neither fleeing nor avoiding detection of predators. Nonetheless, sluggish movement is common in aposematic insects. Our central hypothesis is that sluggish movement evolved in part by motion-oriented predators culling the fastest-moving insects from an aggregation. This would be particularly likely in chemically defended prey, which would deter continued predation. We test predictions of our hypothesis with feeding experiments using the sluggish, gregarious Eastern lubber grasshopper and a motion-oriented predator, the Northern leopard frog. Sluggish-moving (i.e., control) grasshoppers were significantly less likely to be eaten than fast-moving (i.e., motion-induced) grasshoppers (p = 0.0098). Next, non-moving grasshoppers were used as extreme sluggish-moving prey. Non-moving prey were significantly less likely to be eaten than sluggish-moving grasshoppers (p = 0.05). In addition, and most importantly, sluggish-moving grasshoppers in an aggregation were significantly less likely to be attacked than fast-moving grasshoppers in the aggregation (p = 0.0156). Finally, the survivorship of sluggish-moving grasshoppers in pairings vs. aggregations was not significantly different (p = 0.33). Our results demonstrate that the fastest-moving individual in an aggregation of aposematic insects is more likely than sluggish cohorts to be attacked by motion-oriented predators. This survival disadvantage for fast-moving, gregarious prey could create a selection pressure for the evolution of sluggish movement as a defense mechanism in aposematic, gregarious prey.