Optimization of short-term animal behaviour and the currency of time

Modern ageing theory is based on the observation that oxidative metabolism causes damage that results in a gradual loss of vitality, leading to senescence and death. If animals can oxidize only a given amount of substrate in a lifetime (i.e. the 'metabolic time' is allocated from a fixed budget), then behaviour may be aimed at maximizing benefits per unit of 'metabolic time' expended. We analyse the consequences of this view for two types of behaviour that are commonly expressed as rates with respect to clock time. Examples are given of locomotory behaviour in which the animals' preferred speeds are generally interpreted as a result of maximization of energetic efficiency. The same behaviour could be expected if animals were 'speed maximizers' with respect to metabolic time. Examples are given of foraging behaviour that is also best predicted on the basis of maximization of energetic efficiency. This makes sense only if energy is allocated from a fixed budget. However, foraging animals not only expend energy but also consume it so energy cannot be considered as being allocated from a fixed budget. The same behaviour could be expected if animals were 'energy intake rate maximizers' with respect to metabolic time. This makes sense if metabolic time is allocated from a fixed budget as suggested by ageing theory. The metabolic time concept can provide a crucial link between the optimum intensity of short-term behaviour and its long-term fitness consequences. We discuss the implications of this approach for the modelling of foraging behaviour. (C) 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.