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aggregation, antipredator, bobbing, context-dependent, harvestmen, plasticity


Inducible defence strategies evolve in response to temporally and spatially variable predation risk. Selection should favour the expression of these strategies when risk crosses a threshold, but expression may also depend on how effective a given strategy is across different contexts (e.g., social environment). For neotropical harvestmen, aggregation behaviour has been shown to be a group-level defence strategy but can be supplemented by additional behaviour to enhance antipredator defence. A secondary behavioural strategy termed “bobbing” has been documented as being associated with gregarious behaviour, in which individuals rapidly move their bodies up and down. This strategy has been proposed to confuse or deter predation as the group moves en masse. However, the benefits of this behaviour may depend on the presence of conspecifics and group size, both in terms of an effective predator deterrent signal and because the behaviour may attract predators when performed by only one or a small number of harvestmen. Here we test the hypothesis that the aggregation size of neotropical harvestmen affects the propensity of individuals to engage in the antipredator response of bobbing, due to the benefits of plasticity in a behaviour with an apparent context dependent pay-off. We predicted that individuals would be more likely to perform bobbing behaviour in aggregations as opposed to in isolation. We collected harvestmen (Prionostemma spp.) at La Selva Biological Research Station, Costa Rica, and exposed them to a simulated predator cue when housed within experimental chambers. Individual harvestmen were more likely to engage in bobbing behaviour when in a group than alone. Moreover, we performed field observations that found bobbing behaviour to be a more frequent response as group size increased. Our results show social context dependence of an inducible antipredator defence behaviour that aligns with proposed costs and benefits of its expression.