Design, limitations and sustained metabolic rate: lessons from small mammals

Physiological limitations affect an organism's capacity to acquire and expend energy over long periods of activity. These limitations could be related to the central machinery used for acquiring, processing and allocating energy, or by the energy-consuming machinery. Another possibility is that the capacities of central and peripheral organs and tissues are co-adjusted, implying an optimized design. Given the important consequences that rates of energy expenditure have on many ecological aspects of animal life, we need to understand which factors impose ceilings on sustained metabolic rate. Ceilings on sustainable energy expenditure represent the limit below which all the activities performed by an individual must occur. There have been many studies of design constraints on energy budgets, but the different procedures used to identify the type of physiological limitation do not necessarily resolve which factors actually impose metabolic ceilings in small mammals, which precludes a clear understanding of the ecological and evolutionary consequences of design constraints on energy budgets. We propose that the following steps are necessary to identify the physiological limits on sustained metabolic rate: (1) combining peak energy demands to differentiate a central limitation from a peripheral limitation; (2) pushing the animals to their physiological limits (e.g. asymptotic food intake); (3) testing for a central excess capacity (if the limit is set peripherally), or a peripheral excess capacity (if there is a central limitation); (4) utilizing different levels of energy demand to test for symmorphosis.