Annu. Rev. Physiol. 2000. 62:207–35 Copyright  by Annual Reviews. All rights reserved 0066–4278/00/0315–0207$12.00 207 MECHANISMS UNDERLYING THE COST OF LIVING IN ANIMALS A. J. Hulbert 1 and Paul Lewis Else 2 1,2 Metabolic Research Centre, 1 Departments of Biological Science, and 2 Biomedical Science, University of Wollongong, Wollongong, NSW, Australia 2522; e-mail: hulbert@uow.edu.au, paul_else@uow.edu.au Key Words metabolism, membranes, polyunsaturation, active transport, allometry Abstract The cost of living can be measured as an animal’s metabolic rate. Basal metabolic rate (BMR) is factorially related to other metabolic rates. Analysis of BMR variation suggests that metabolism is a series of linked processes varying in unison. Membrane processes, such as maintenance of ion gradients, are important costs and components of BMR. Membrane bilayers in metabolically active systems are more polyunsaturated and less monounsaturated than metabolically less-active systems. Such polyunsaturated membranes have been proposed to result in an increased molec- ular activity of membrane proteins, and in this manner the amount of membrane and its composition can act as a pacemaker for metabolism. The potential importance of membrane acyl composition in metabolic depression, hormonal control of metabo- lism, the evolution of endothermy, as well as its implications for lifespan and human health, are briefly discussed. INTRODUCTION To measure the metabolic rate (MR) of an animal is to quantify its cost of living. The MR most often measured is basal/standard MR (BMR/SMR), which is the MR of a fasted adult at rest in a thermo-neutral environment. In endotherms, BMR is the energy turnover at normal body temperature, whereas in ectotherms, the temperature at which SMR is measured should be specified. BMR is to some extent an artificial construct and does not include the cost of growing, feeding, processing food, nor the cost of activity, all of which are normal processes of free-living animals. In endotherms it also does not include the costs associated with thermoregulation. Over several decades, the SMRs of many animal species have been measured and catalogued. Although it can vary extensively between different species, in general, the SMR of a vertebrate species is surprisingly predictable, requiring only knowledge of the body mass and the phylogenetic group to which the species belongs. Despite this mathematical predictability, the mechanistic determinants