Grouped and Vibrating: The Influence of Group Size, Triggering Stimuli, and Leg Loss in Bobbing in an Arachnid

ABSTRACT Animals across taxa form aggregations, with increased defenses as one of the benefits. Some defensive behaviors of grouped animals involve body movements. However, how those behaviors are triggered varies, and how between‐individual morphological variation affects them remains unknown. In this project, we explored three drivers of bobbing—an up‐and‐down body vibration behavior assumed to be a defensive response—with field observations and a lab experiment in one species of the arachnid order Opiliones ( Prionostemma sp.2). First, we explored the variation in bobbing duration across groups of a variable number of individuals, as animals could modulate their engagement in this behavior to ensure a successful predator‐deterrent behavior. We found that the duration of bobbing increased with group size. Second, we explored the effect of different triggering stimuli that might represent natural scenarios of an approaching predator. We found that a touching stimulus (gentle stick touching) triggered longer bobbing than an airflow stimulus (gentle blow). Third, we explored if the individuals' leg condition (whether they had all eight or fewer legs when found) affects their engagement in this behavior. We found that variations in leg condition did not affect their defensive engagement, as bobbing duration was similar between intact individuals and those missing legs. Together, these data provide novel observational and experimental insights into the drivers of collective behavior in animals. While individual variation in morphological conditions does not affect bobbing in these understudied arachnids, variability in group size and the triggering stimuli impacted their engagement in movement behavioral defenses.