Roll with the fear: environment and state dependence of pill bug (Armadillidium vulgare) personalities

Abstract

Most studies on animal personality evaluate individual mean behaviour to describe individual behavioural strategy, while often neglecting behavioural variability on the within-individual level. However, within-individual behavioural plasticity (variation induced by environment) and within-individual residual variation (regulatory behavioural precision) are recognized as biologically valid components of individual behaviour, but the evolutionary ecology of these components is still less understood. Here, we tested whether behaviour of common pill bugs (Armadillidium vulgare) differs on the among-and within-individual level and whether it is affected by various individual specific state-related traits (sex, size and Wolbachia infection). To this aim, we assayed risk-taking in familiar vs. unfamiliar environments 30 times along 38 days and applied double modelling statistical technique to handle the complex hierarchical structure for both individual-specific trait means and variances. We found that there are significant among-individual differences not only in mean risk-taking behaviour but also in environment-and time-induced behavioural plasticity and residual variation. Wolbachia-infected individuals took less risk than healthy conspecifics; in addition, individuals became more risk-averse with time. Residual variation decreased with time, and individuals expressed higher residual variation in the unfamiliar environment. Further, sensitization was stronger in females and in larger individuals in general. Our results suggest that among-individual variation, behavioural plasticity and residual variation are all (i) biologically relevant components of an individual's behavioural strategy and (ii) responsive to changes in environment or labile state variables. We propose pill bugs as promising models for personality research due to the relative ease of getting repeated behavioural measurements.

Understanding the unexplained: The magnitude and correlates of individual differences in residual variance

“…We did not pick up clear signs of publication bias in the studies considered, with no effect observed of whether the original publication considered variation in rIIV (see Mitchell, 2021, for full output). However, care should still be taken in interpreting these results Contrary to the prediction that field studies would show greater variance in rIIV due to reduced control over conditions (Stamps et al, 2012;Westneat et al, 2015), field studies typically appeared to have lower estimates than laboratory studies (differ- 2020; Highcock & Carter, 2014) rather than latency to emerge (e.g., Horváth et al, 2019;Stamps et al, 2012), and measures of activity were based on tracking over large spatial scales (e.g., Hertel, Niemelä, et al, 2020;Hertel, Royauté, et al, 2020) rather than tracking within an arena (e.g., Mitchell & Biro, 2017;Prentice et al, 2020).…”

Ain’t going down without a fight: state-and environment-dependence of antipredator defensive aggressive personalities in Carpetan rock lizard

“…Therefore, the absence of defensive behaviour in some of the lizards in our experiment might be considered as a sign of tonic immobility. Tonic immobility strategy has been previously considered in personality studies (Edelaar et al 2012;Santos et al 2015;Horváth et al 2019). Our present findings hint that between-individual differences in antipredatory last resort behavioural traits might be interpreted along a tonic immobility-antipredator aggression continuum.…”

“…Hence, antipredator aggression could be seen as a 'last resort' against predation, similarly to another, yet completely opposite antipredator behavioural trait, tonic immobility (i.e. the lack of movement; see Gallup and Rager 1996;Edelaar et al 2012;Horváth et al 2019).…”

State-dependent mortality can enhance behavioral unpredictability

“…When an animal is repeatedly observed, its behavioral expressions can be highly variable. This within-individual variability in behavior can be categorized into phenotypic plasticity and behavioral unpredictability [1][2][3][4]. Phenotypic plasticity explains within-individual variability observed along some environmental gradients such as wind velocity, predation risk and temperature [5].…”

“…Phenotypic plasticity explains within-individual variability observed along some environmental gradients such as wind velocity, predation risk and temperature [5]. Behavioral unpredictability (also known as "residual behavioral variance") is the remaining (residual) variability after the effects of environmental and explanatory variables are accounted for [1][2][3][4]. Although behavioral unpredictability may appear as mere noise in behavioral expressions, there are some patterns in expressions of behavioral unpredictability.…”