Physiological ecology is concerned with the way that physiological traits fit organisms for the ecological circumstances in which they live, so there is always an implicit evolutionary component to it. This book is concerned with physiological studies that make the evolutionary considerations explicit. The first part explores physiological models that predict how, under different ecological pressures, resources should be invested in such metabolic processes as costs of maintenance, growth patterns and allometries, ageing and physiological adaptability. In the context of the integrated metabolism of whole organisms, the second part of the volume considers aspects of the physiological ecology of specific organisms. The underlying theme of these chapters is the link between genotype and physiological phenotype.