A review of the empirical evidence for costs of plasticity in ectothermic animals.

Phenotypic plasticity can represent a vital adaptive response to environmental stressors, including those associated with climate change. Despite its evolutionary advantages, the expression of plasticity varies significantly within and among species, and is likely to be influenced by local environmental conditions. This variability in plasticity has important implications for evolutionary biology and conservation physiology. Theoretical models suggest that plasticity might incur intrinsic fitness costs, although the empirical evidence is inconsistent and there is ambiguity in the term 'cost of plasticity'. Here, we systematically review the literature to investigate the prevalence of costs associated with phenotypic plasticity in ectothermic animals. We categorized studies into those assessing 'costs of phenotype' (trade-offs between different plastic trait values) and 'costs of plasticity' (intrinsic costs of the capacity for plasticity). Importantly, the experimental designs required to detect costs of plasticity are inherently more complex and onerous than those required to detect costs of phenotype. Accordingly, our findings reveal a significant focus on costs of phenotype over costs of plasticity, with the former more frequently detecting costs. Contrary to theoretical expectations, our analysis suggests that costs of plasticity are neither universal nor widespread. This raises questions about the evolutionary dynamics of plasticity, particularly in stable environments. Our analysis underscores the need for precise terminology and methodology in researching costs of plasticity, to avoid conflating costs associated with plastic traits with costs more intrinsic to plasticity. Understanding these nuances is crucial for predicting how species might adapt to rapidly changing environments.