Metabolic Scaling in Complex Living Systems

Abstract

In this review I show that four major kinds of theoretical approaches have been used to explain the scaling of metabolic rate in cells, organisms and groups of organisms in relation to system size. They include models focusing on surface-area related fluxes of resources and wastes (including heat), internal resource transport, system composition, and various processes affecting resource demand, all of which have been discussed extensively for nearly a century or more. I argue that, although each of these theoretical approaches has been applied to multiple levels of biological organization, none of them alone can fully explain the rich diversity of metabolic scaling relationships, including scaling exponents (log-log slopes) that vary from ~0 to >1. Furthermore, I demonstrate how a synthetic theory of metabolic scaling can be constructed by including the context-dependent action of each of the above modal effects. This "contextual multimodal theory" (CMT) posits that various modulating factors (including metabolic level, surface permeability, body shape, modes of thermoregulation and resource-transport, and other internal and external influences) affect the mechanistic expression of each theoretical module. By involving the contingent operation of several mechanisms, the "meta-mechanistic" CMT differs from most metabolic scaling theories that are deterministically mechanistic. The CMT embraces a systems view of life, and as such recognizes the open, dynamic nature and complex hierarchical and interactive organization of biological systems, and the importance of multiple (upward, downward and reciprocal) causation, biological regulation of resource supply and demand and their interaction, and contingent internal (system) and external (environmental) influences on metabolic scaling, all of which are discussed. I hope that my heuristic attempt at building a unifying theory of metabolic scaling will not only stimulate further testing of all of the various subtheories composing it, but also foster an appreciation that many current models are, at least in part, complementary or even synergistic, rather than antagonistic. Further exploration about how the scaling of the rates of metabolism

Effects of Contingency versus Constraints on the Body-Mass Scaling of Metabolic Rate

“…Others claim that heterogeneous contingent factors are importantly involved in determining both the slopes and elevations of metabolic scaling relationships [10,16,23,32,[50][51][52]. As a point of departure for my discussion about the contingency versus constraints debate, I focus on published data showing that an extrinsic factor, namely ambient temperature, not only significantly affects the scaling of metabolic rate, but also does so in fundamentally different ways in ectothermic and endothermic animals.…”

“…Therefore, like the MTE, cell-size theory predicts that changing temperature should affect the elevation of metabolic scaling relationships, but not their slopes (but see Section 3.2.2), which again is contradicted by the results described in this study. Most other models focused on the differential scaling of various resource-demanding processes and tissues or organs with different metabolic rates (see e.g., [10,38,52]) have yet to consider how ambient temperature may affect metabolic scaling slopes (but see discussion of dynamic energy budget theory below). Furthermore, a recent study has shown that temperature acclimation does not affect the size and metabolic rate of various organs of the prawn Macrobrachium tolmerum [78].…”

“…How biological traits vary with body size is the subject of an exciting and controversial field called "allometry" or "biological scaling" [1][2][3][4][5][6][7][8]. This field of study is important because it has great potential for contributing to a unifying theory of biological systems [9,10]. All living systems have size and most of their structural and functional features scale to size.…”

“…However, in the 1930s, animal scientists such as Kleiber and Brody discovered that the interspecific scaling of basal (resting) metabolic rate in mammals more closely matched a 3/4-power relationship than a 2/3-power relationship [17,18]. In 1960, Hemmingsen [19] claim that heterogeneous contingent factors are importantly involved in determining both the slopes and elevations of metabolic scaling relationships [10,16,23,32,[50][51][52].…”

The vast unknown microbial biosphere

“…It would be of interest to explore whether there are alternative fits to the power-law N vs. N max or S vs. N functions. Even in cases where the functional relationship seemed incontestable in the past, such as the 3/4 power-law of metabolism, alternative fits suggested alternative theories to explain the origin of the pattern, not always suited for all organisms (9). An early explanation for log-normal distributions of abundance was that they arose from random demographic processes.…”