To bob or not to bob: context-dependence of an antipredator response in neotropical harvestmen

Inducible defence strategies evolve in response to temporally and spatially variable predation risk. Selection should favor the expression of these strategies when risk crosses a threshold, but expression may also depend on how effective a strategy is across contexts. For neotropical harvestmen, aggregation behavior is a group-level defence strategy but can be supplemented by additional behavior to enhance antipredator defence. A secondary behavioral strategy termed “bobbing,” in which individuals rapidly move their bodies up and down, has been proposed to confuse or deter predation as the group moves en masse. While this behavior may be performed in isolation, we propose that its net pay-off may depend on the presence of conspecifics and group size. Therefore, due to the benefits of plasticity in a behavior with such an apparent context dependent pay-off, we hypothesize that the aggregation size of neotropical harvestmen affects the propensity of individuals to engage in the antipredator bobbing defence. We predicted that individuals would be more likely to perform bobbing in aggregations as opposed to in isolation. We collected harvestmen (Prionostemma sp.) at La Selva Biological Research Station, Costa Rica, and exposed them to a simulated predator cue when housed within experimental chambers. Individuals were more likely to engage in bobbing when in a group than alone. Moreover, we performed field observations that found bobbing to be a more frequent response as group size increased. Our results show social context-dependence of an inducible antipredator defence behavior that aligns with proposed costs and benefits of its expression.