A unifying explanation for diverse metabolic scaling in animals and plants

Abstract

The scaling of metabolic rate with body mass has long been a controversial topic. Some workers have claimed that the slope of log-log metabolic scaling relationships typically obeys a universal 3/4-power law resulting from the geometry of resource-transport networks. Others have attempted to explain the broad diversity of metabolic scaling relationships. Although several potentially useful models have been proposed, at present none successfully predicts the entire range of scaling relationships seen among both physiological states and taxonomic groups of animals and plants. Here I argue that our understanding may be aided by three shifts in focus: from explaining average tendencies to explaining variation between extreme boundary limits, from explaining the slope and elevation (metabolic level) of scaling relationships separately to showing how and why they are interrelated, and from focusing primarily on internal factors (e.g. body design) to a more balanced consideration of both internal and external (ecological) factors. By incorporating all of these shifts in focus, the recently proposed metabolic-level boundaries hypothesis appears to provide a useful way of explaining both taxonomic and physiological variation in metabolic scaling relationships. This hypothesis correctly predicts that the scaling slope should vary mostly between 2/3 and 1 and that it should be related to metabolic (activity) level according to an approximately U-shaped function. It also implies that the scaling of other energy-dependent biological processes should be related to the metabolic level of the organisms being examined. Some data are presented that support this implication, but further research is needed.

Universal temperature and body-mass scaling of feeding rates

“…The handling times were expected to follow À 2 3 to 21 power-law relationships with consumer mass [38,45,46]. We found that only half of the groups tested fall into this range, whereas the others were systematically higher.…”

“…Besides the Metabolic Theory of Ecology, other frameworks invoke more flexible theories that predict exponents from 0.66 to 1 for metabolic rates [45,46], leading to a continuum of expectations for the handling time scaling in the range from 20.66 to 21.…”

Idiosyncratic species effects confound size-based predictions of responses to climate change

“…Glazier [37] and Killen et al [38] suggest that metabolic scalings not only are influenced by temperature and body mass, but that these scalings can be affected by the activity level and ecology of organisms. Glazier [37] has suggested that depending on the activity level of an animal, the scaling exponent can vary from 0.67 to 1; therefore, as metabolic (activity) level increases, the scaling exponent decreases from 1 to 0.67 and then starts to return to 1. This U-shaped relationship reflects an increase in metabolic rate from low resting rates to active rates.…”

Power laws in biology: Between fundamental regularities and useful interpolation rules

“…Temperature dependence of metabolic rates has been taken into account but the scaling exponents for fish, amphibians, reptiles, and mammals remained significantly heterogeneous after normalization to a temperature of 388C [17]. Eventually, Galzier [18] made an attempt to derive a unifying explanation for differences in metabolic rate scaling by broadening the focus of the analyses from considering average tendencies to understanding the variation between extreme boundary limits and from considering primarily internal factors like body design to explaining the influence of both, internal and external (ecological) factors. The inclusion of extreme situations, like sleep and in particular hibernation, suggests an interesting systematic extension of the ranges of scaling exponents at metabolic levels between the extremes of minimal and MMRs, where demand is much smaller than supply and demand is much larger than supply, respectively: The scaling exponent is supposed to be a 5 1 in the two limits and smaller in between.…”