1. Evolutionary ecologists increasingly study reaction norms that are expressed repeatedly within the same individual's lifetime. For example, foragers continuously alter anti-predator vigilance in response to momentto-moment changes in predation risk. Variation in this form of plasticity occurs both among and within individuals. Among-individual variation in plasticity (individual by environment interaction or I 9 E) is commonly studied; by contrast, despite increasing interest in its evolution and ecology, within-individual variation in phenotypic plasticity is not. 2. We outline a study design based on repeated measures and a multilevel extension of random regression models that enables quantification of variation in reaction norms at different hierarchical levels (such as among and within individuals). The approach enables the calculation of repeatability of reaction norm intercepts (average phenotype) and slopes (level of phenotypic plasticity); these indices are not specific to measurement or scaling and are readily comparable across data sets. 3. The proposed study design also enables calculation of repeatability at different temporal scales (such as shortand long-term repeatability), thereby answering calls for the development of approaches enabling scale-dependent repeatability calculations. 4. We introduce a simulation package in the R statistical language to assess power, imprecision and bias for multilevel random regression that may be utilised for realistic data sets (unequal sample sizes across individuals, missing data, etc). 5. We apply the idea to a worked example to illustrate its utility. We conclude that consideration of multilevel variation in reaction norms deepens our understanding of the hierarchical structuring of labile characters and helps reveal the biology in heterogeneous patterns of within-individual variance that would otherwise remain 'unexplained' residual variance.
There is growing evidence that individuals within populations show consistent differences in their behaviour across contexts (personality), and that personality is associated with the extent to which individuals adjust their behaviour as function of changing conditions (behavioural plasticity). We propose an evolutionary explanation for a link between personality and plasticity based upon how individuals manage uncertainty. Individuals can employ three categories of tactics to manage uncertainty. They can 1) gather information (sample) to reduce uncertainty, 2) show strategic (state‐dependent) preferences for options that differ in their associated variances in rewards (i.e. variance‐sensitivity), or 3) invest in insurance to mitigate the consequences of uncertainty. We explicitly outline how individual differences in the use of any of these tactics can generate personality‐related differences in behavioural plasticity. For example, sampling effort is likely to co‐vary with individual activity and exploration behaviours, while simultaneously creating population variation in reactions to changes in environmental conditions. Individual differences in the use of insurance may be associated with differences in risk‐taking behaviours, such as boldness in the face of predation, thereby influencing the degree of adaptive plasticity across individuals. Population variation in responsiveness to environmental changes may also reflect individual differences in variance‐sensitivity, because stochastic change in the environment increases variances in rewards, which may both attract and benefit variance‐prone individuals, but not variance‐averse individuals. We review the existing evidence that individual variation in strategies for managing uncertainty exist, and describe how positive‐feedbacks between sampling, variance‐sensitivity and insurance can maintain and exaggerate even small initial differences between individuals in the relative use of these tactics. Given the pervasiveness of the problem of uncertainty, alternative strategies for managing uncertainty may provide a powerful explanation for consistent differences in behaviour and behavioural plasticity for a wide range of traits.
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