Behavioural plasticity adjusts rapidly to repeated temperature increases: mean, among- and within-individual responses

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Plasticity is a flexible response to environmental change. Although many studies show plastic responses to sustained increases in temperature, few investigate temporal changes in plasticity, especially under variable temperatures. We show that increasing temperature by 5 °C for just 2 days per week dramatically increased boldness in the ectotherm Armadillidium vulgare during the first exposure. Plasticity was reduced over the next 2 weeks, primarily because of large increases in boldness at the cooler reference temperature. After 2 weeks of reprieve from protocols some deacclimation occurred, as boldness was significantly reduced at the reference temperature in week 6. Individual variation in plasticity was evident in weeks 1 and 2, but not in the remaining weeks of exposure, suggesting rapid individual acclimation towards an assumed adaptive mean. Another form of behavioural flexibility, residual intraindividual variability, did not vary with time or temperature. The repeatability of individual behaviour was high at the cooler reference temperature, as was the repeatability of thermal plasticity and predictability, indicating that each aspect of their behavioural reaction norm is potentially heritable. Our study highlighted the complex interplay between inflexible (repeatable) individual differences in behaviour and flexible temporal changes in plasticity, highlighting their considerable adaptive capacity to thermal change and the importance of studying temporal dimensions of plasticity.

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