Body Size Matters in the Lives of Organisms

Abstract

Body size, arguably the most important attribute of all organisms in the living world, is a reliable predictor of their physiological rates and life-history attributes. Metabolic rates of all organisms (MR) are related directly to their body mass (M) as an allometric function $MR = \alpha M{\textstyle{3 \over 4}}$ (Kleiber’s rule). The high, mass-specific metabolic rates of small animals can be accounted for by their high surface area/volume ratios. The much-discussed ‘metabolic theory of ecology’ is essentially an expansion of Kleiber’s rule equation to include two additional metabolism-influencing factors—temperature and resource availability—with a more generalized quarter-power exponent. According to this theory, many physiological rates, life-history traits and ecological processes follow quarter-power scaling laws. Living organisms must also obey the laws of physics. The relative importance of different physical forces to the organisms living on land or in water is also dependent on their body size.