DOI: 10.5061/dryad.kh18932j5
title: Data from: To bob or not to bob: Context-dependence of an antipredator response in neotropical harvestmen

description: 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 strategy is across contexts. For neotropical
 harvestmen, aggregation behaviour is a group-level defence strategy, but
 can be supplemented by additional behaviour to enhance antipredator
 defence. A secondary behavioural 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
 behaviour 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 behaviour 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 behaviour that aligns with proposed costs and
 benefits of its expression.
description: Lab-based experiments were conducted to determine whether the
 social context of harvestmen (i.e., isolation or aggregation) affects the
 propensity of individuals to perform antipredator bobbing following a
 predator simulation. As control experiments for potential confounding
 factors from the isolation versus aggregation experiment, subsequent
 trials were conducted to determine whether the behaviour was affected by
 sex and density within an enclosure. In addition to the lab-based
 experiments, field observations were carried out to assess the propensity
 of harvestmen to exhibit bobbing as affected by group size.
 Analyses were performed in R v 4.3.2 (Team, 2020). Generalized
 linear and mixed effects models with the <em>lme4</em> (Bates
 et al., 2015) and <em>glmmTMB</em> (Brooks et al., 2017)
 packages were used to analyse the bobbing behaviour during experimental
 trials. The propensity of individuals to exhibit bobbing in experiment 1
 and experiment 2 was fitted using a generalized linear mixed effects model
 with a binomial distribution {bobbing ~ treatment + (1 | block:ID)}, where
 the response is explained by the treatment (i.e., aggregation or sex),
 with container ID nested within block included as a random effect. As
 individuals were observed alone in the density experiment, a generalized
 linear model with a binomial distribution was used for the analysis.
 Estimated marginal means, odds ratios, and associated 95% confidence
 intervals were extracted from all models using the package
 <em>emmeans</em> (Lenth et al., 2020). To
 analyse the field observations of the occurrence of bobbing behaviour, a
 generalized additive model with a binomial distribution was fit using the
 package <em>gam </em>(Hastie & Tibshirani, 1986). An
 initial model was fit including location, date, and the number of
 individual harvestmen observed at a tree. The number of harvestmen
 included a smooth function for non-linear fitting. Non-significant terms
 were removed from the final model. For the data used in the model, any
 aggregation with the number of individuals recorded as greater than 25,
 due to the inability to accurately determine the exact number, was given a
 value of 25.
description: # Data from: To bob or not to bob: Context-dependence of an antipredator
 response in neotropical harvestmen
 [https://doi.org/10.5061/dryad.kh18932j5](https://doi.org/10.5061/dryad.kh18932j5) ## Description of the data and file structure Lab-based experiments were conducted to determine whether the social context of harvestmen (i.e., isolation or aggregation) affects the propensity of individuals to perform antipredator bobbing following a predator simulation. As control experiments for potential confounding factors from the isolation versus aggregation experiment, subsequent trials were conducted to determine whether the behaviour was affected by sex and density within an enclosure. In addition to the lab-based experiments, field observations were carried out to assess the propensity of harvestmen to exhibit bobbing as affected by group size. ### Files and variables #### File: ALL_Calhoun_et_al_Harvestmen_Bobbing_data_2024.xlsx **Description:** Excel file containing three separate sheets of data: "Bobbing_harvestmen_2017", "Field_observations_harvestmen_2", and associated "Readme". ##### Variables **Sheet: "Bobbing_harvestmen_2017":** Data from experiment testing the propensity to bob when in aggregation or in isolation, and if it differs by sex ratio or density. * ID: sample identification * block: group of 6 trials run simultaneously * trial: “single_grp” = experiment 1 where individuals were placed in groups or isolation; “sex_effect” = experiment 2 where individuals were grouped by sex; “density_effect” = experiment 3 where individuals were allocated to restricted or full containers. * treatment: refers to the treatment of each trial according to the experiment conducted.  * sex: refers to sex of group or individuals according to the experiment conducted. * num_harvestmen: number of harvestmen within the trial. * bobbing: 1/0 indicating if individuals were bobbing or not. * num_bobbing: number of individuals bobbing within a trial. NA (not applicable) represents when the number could not be determined. **Sheet: "Field_observations_harvestmen_2022":** Data from field observations testing the propensity to bob according to group size. * Tree: specific tree being observed. * Location: Referring to the abbreviated name of the trail at La Selva Biological Station, Costa Rica. * Date: the date the observation was conducted. * Harvestmen: number of harvestmen observed at a tree. Numbers &gt;25 were written as “25+”.  * Harvestmen_cap: number of harvestmen observed at a tree with numbers &gt;25 written as “25”. * Bobbing: 1/0 indicating if individuals were bobbing or not. **Sheet: "Readme":** these are notes to self about the variables on each data sheet. They are the same as above.  ## Code/software Analyses were performed in R v 4.3.2 (Team, 2020).