Bobbing behavior duration in aggregtions of Neotropical Opiliones

Animals perform antipredator behaviors when grouped, like bee shimmering. Individual phenotypic variations can affect the quality and success of defensive behavior. We tested three observational and experimental hypotheses regarding the bobbing behavior in one species of the arachnid group of Opiliones. We first tested if bobbing times varied with group size by recording the bobbing in field and lab groups. We found that the mean bobbing time increased with group size. Second, we tested if bobbing time differed between the types of stimuli eliciting it. We found that bobbing time lasted longer after a tactile stimulus (gentle stick touching) than a chemical stimulus (gentle blow). Third, we tested if bobbing time differs based on variation in the leg condition of these opiliones. We found no difference in the bobbing time between autotomized and intact individuals. Our findings on Neotropical Opiliones provide insights into the drivers of decision-making processes of movement defensive behavior by animals of variable body condition and how the between-individual phenotypic variation affects their engagement in defensive behavior. This work expands our knowledge of the sources of selection on collective behavior.

Methods
We video-recorded aggregations of Opiliones of the genus Prionostemma in the tropical rainforest of La Selva Research Station, Sarapiquí, Costa Rica. In the field, we recorded and subsequently quantified the bobbing time of individuals in the groups. We used a GoPro Hero 8 camera, with 120 fps and a narrow field of view (FOV) setting (to avoid fisheye lens’ distortion). At the beginning of each video, we presented a tactile stimulus (stick) or a chemical stimulus (gently blowing on them). Each group was recorded twice: once with tactile stimulus and once with chemical stimulus. We randomized the order of presenting the stimuli. We stopped each recording when all opiliones in the FOV stopped bobbing. We analyzed all videos in the QuickTime software by visually following ten randomly chosen opiliones for each stimulus type (chemical and tactile). We noted the frame that marked the start of each individual's bobbing and the last frame in which we could observe the animal bobbing. With these measures, we calculated each individual's bobbing duration in seconds. Then, to represent the group bobbing duration, we averaged the time of all the bobbing individuals per group (33 ± 18 opiliones/group).

We then performed a lab experiment to test if the bobbing time differed as a function of group size, the stimuli presented, and if Opiliones missing legs bobbed for longer than intact individuals. We collected individuals in the forest at night. Then, we placed them in plastic shoe boxes (33 × 20 × 12 cm). We placed a wooden surface or bark fragment (20 × 10 cm) in the bottom of each box to serve as a substrate. The following morning (8:00 - 10:00 h), we repeated the field procedures described above (presented two stimuli). Then, we quantified the bobbing duration of ten randomly selected individuals. Lastly, we compared the bobbing duration of opiliones given their leg condition. When setting up the lab groups at night, we counted the number of legs of each individual and sorted them into two classes: autotomized (5 - 7 legs) or intact (8 legs).