Within-individual variation in glucocorticoid and metabolic rate plasticity: a mechanism of phenotypic fine-tuning to complex environmental challenges

Complex environmental variation underlying phenomena like climate change can be represented as a continuum between gradual, predictable (i.e. global warming) and sudden, unpredictable components (i.e. extreme weather events). Reversible phenotypic plasticity in key physiological traits like glucocorticoids and metabolic rates, as measured by the reaction norm slope, represents the most informative metric of animal capacity to respond at any point along this continuum. In this article, I first describe which patterns of variation in physiological plasticity within and among individuals may favour populations' persistence to climate change. In doing so, I argue that within-individual variation in plasticity, i.e. the capacity of single individuals to change the steepness of their physiological reaction norm depending on the context, represents an overlooked, yet potentially crucial mechanism of phenotypic fine-tuning of animal responses to sudden and unpredictable environmental challenges. Second, I review currently published physiological reaction norm work in search of evidence for the above patterns of variation in glucocorticoid and metabolic rate plasticity. Finally, I remark that for understanding populations' coping capacity to ongoing climate change, future reaction norm research should investigate multilevel, multidimensional and multivariate physiological plasticity, its causes, consequences, temporal dynamics and covariation patterns. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.