"Metabolic rate is a fundamental life history trait. Yet it also shows remarkable intra-specific variation: some individuals consistently have 2-3 times the maintenance costs of conspecifics the same size, age and sex. The predicted consequences of this variation are central to our understanding of optimal life history strategies and resource allocation trade-offs. But how is this intra-specific variation maintained? And why have attempts to link variation in metabolic rate to fitness yielded very inconsistent results? I propose that this is due to the focus having been on resting metabolism, with other aspects – most notably maximal metabolic rate and aerobic scope - being overlooked. There is pronounced variation in these two neglected components of metabolic profile, uncorrelated with that in resting metabolism, and increasing evidence that they may influence species distributions. I point out that these different components of metabolic rate are inextricably linked to levels of mitochondrial uncoupling, and hence to energy efficiency, potential oxidative stress and senescence. I predict that different microhabitats will favour specific combinations of metabolic traits, leading to spatial and temporal structuring of metabolic phenotypes within and across populations. This project aims to test this new framework in a multidisciplinary project that uses experimental manipulations at levels ranging from the cellular to the population. The project combines cellular analyses and cutting edge technology with carefully designed experiments in an ecological setting. I will test whether variability in metabolic traits is maintained through context-dependent fitness. If successful, the work will reveal how metabolic physiology is a key determinant of the ability of organisms to cope with variable environments, so explaining the persistence of variation in physiological traits and increasing our understanding of the likely consequences for organisms of environmental change."