Scaling functions to body size: theories and facts.

Editorial Scaling functions to body size: theories and facts 'Why do small animals live faster and shorter?' 'What sets the pace of life?' These are questions that have interested biologists for more than 150 years, leading to debates between theorists and experimentalists that continue today. As early as 1839, Sarrus and Rameaux (1839) realized that metabolic power cannot increase with the third power of the linear dimension or body mass, but is limited by the capacity to get rid of heat; hence, for organisms to stay in energy balance, metabolism can only vary in proportion to their surface area. Rubner (1883) found in fact that metabolic rate in dogs was in proportion to body surface area and proposed that it should scale with body mass raised to the power of 2/3. Obtaining estimates of basal metabolic rate (BMR) on a large number of species small and large, Kleiber (1932) experimentally found a (close to) 3/4 exponent to describe the relationship between BMR and body mass rather than the 2/3 exponent predicted from Rubner's 'law'. Kleiber's 'law' has been confirmed by many studies since (Schmidt-Nielsen, 1984), even though it continues to be contested. In contrast to Rubner's surface law, Kleiber's 3/4 exponent is enigmatic, with no obvious relation to body design. A quest for explaining such an important allometric relationship fostered new theories. McMahon (1975) developed the concept of elastic similarity, stating that the likelihood of elastic failure of support structures should be kept similar in animals of all sizes. The result of this analysis indicates that legs of smaller animals can be more slender than legs of large animals. Considering elastic similarity and that the power costs of muscle work are proportional to muscle cross-sectional area, the 3/4 scaling of MR is obtained, and thus scaling theory and the experimental evidence can be brought in line. Using an entirely different intellectual approach, West et al. (1997) have more recently invoked the fractal nature of the (energy) distributing vascular network in animals, to arrive at the 3/4 scaling exponent from first principles. A similar approach, also yielding a 3/4 scaling exponent but using fewer assumptions, has been proposed by Banvar et al. (1999). Bejan's 'constructal theory' (Bejan, 2000, and p. 1677) also explains a 3/4 scaling exponent by considering that flow architectures can be deduced by a single law of maximization of access for currents. So where do we stand today? …