Energetics and the evolution of body size in small terrestrial mammals

Many species of very small terrestrial mammals exhibit clinal patterns in their body size, often with larger individuals predominating in populations from higher latitudes (Bergmann's law). Classically, this pattern has been explained as reflecting the advantageous surface-to-volume ratio of larger animals, which provides them with a thermoregulatory advantage in colder regions, because it reduces their energy demands. Attempts to expand on this energetics explanation are, however, fraught with difficulty. A major problem relates to the fact that it is not possible to speculate on the direction of selection on body size by considering costs alone. However, when both costs and gains are considered there is a range of potential effects, which depend critically on the interspecific scaling relationships of energy gains and energy costs. Since these traits may vary in time, in space, and across species, general predictions from the energetics hypothesis of the direction of selection on body size are therefore not possible, and a wide range of directional effects of latitude on size is therefore predicted (and observed). It is generally the case that, within a species, mass explains less than 50% of the individual variation in metabolic energy expenditure. This low explained variation raises the question of why some individuals sustain high rates of metabolism, when this would be likely to make them more susceptible to failure in achieving an energy balance. In wood mice, Apodemus sylvaticus, we have found a correlation between basal metabolic rate (BMR) and the thermogenic capacity (noradrenaline-induced metabolic rate). These latter data suggest that animals which might be at a disadvantage in terms of achieving an energy balance might be at an advantage in terms of achieving a heat balance under severe temperature conditions. This indicates a more complex involvement of energetics in the phenomenon of overwinter survivorship than is addressed by models of energy balance alone.