The effect of heat waves on pill bug Armadillidium vulgare risk-taking strategies

Global climate change is expected to increase both across- and within-day temperature fluctuations, likely affecting animal behaviour, particularly in small-bodied ectotherms. However, their impact on individual behavioural strategies remains understudied. Here, we investigated how heat waves influence risk-taking behaviour in the common pill bug Armadillidium vulgare, and whether prior experience with fluctuating temperatures alters this effect. Individuals were habituated to either fluctuating (alternating optimal 25ºC with stressful 31ºC daily) or stable (25ºC) thermal regimes. Risk-taking behaviour (conglobation time) was then repeatedly assessed at both 25ºC and 31ºC. Under thermal stress, A. vulgare exhibited reduced risk-taking, with 54-108% increase in conglobation time. This pattern was absent in females habituated to fluctuating thermal regime, showing only a minor decrease in risk-taking (~ 11%). Behavioural predictability (environment-independent within-individual behavioural variation) increased by 41% from the smallest to the largest individuals. Behavioural predictability decreased at higher temperatures in a sex-dependent fashion (females: 27%; males: 7%). Male repeatability was higher at 31ºC than at 25ºC in the fluctuating thermal treatment, while female repeatability was higher in the fluctuating than in the stable thermal treatment at 31ºC. Overall, our findings indicate that thermal stress alters risk-taking strategies and may promote between-individual behavioural differences, although prior thermal experience can buffer its effects. We conclude that climate change, even in the absence of other ecological changes, can reshape ectotherm antipredator strategies. We emphasize the need for long-term studies using the animal personality framework to understand how thermal landscape stability influences behavioural variation across ectotherm species.