Kleiber's Law - Current Debate

Current Debate

Current discussion and debate in the literature has refrained from consideration of the theoretical foundations for Kleiber's law. The role of thermogenesis in metabolism remains unexplained, in part because Kleiber's law, as originally formulated, was based upon the idea that metabolic energy was entirely related to measurements of heat generation and loss. This appears in the unit of metabolic rate most favored, calories/s rather than watts (as in the version of Kleiber's law that includes ME, where ME is a ratio of redox amperes), and in the limitation of disputation as to whether ME is 89 or 100%. Others have criticized West, Brown, and Enquist on the point that the size-invariance of capillaries, which is the same from leaves to mammalian blood flow, dooms attempts to account for motor activity as part of metabolism. This is why metabolic rates are almost always associated with the organism at rest, where metabolic rate is figured by the basal rate for the cell rather than for the rate for the organism in its day-to-day life in the field. West et al. claim that Kleiber's law refers to the basal metabolic rate of an organism's cells, not the field metabolic rate of the organism itself, and regard field metabolic rate to be the product of the average basal metabolic rate and number of cells in the organism. Biologists point out that BMR cannot possibly account for motor activity even by this reckoning, and the equation is therefore of limited value either way.

That metabolic efficiency should deviate from the favored high values is not part of the current debate, even though it creates inconsistencies within standard models, especially with regard to the nature of aging and the nature of the metabolic relation between the cell and the organism's mass. The standard versions of the equation's exponent (those that do not consider ME) cannot account for the wide variation in the lifetimes between rodents and birds of similar mass. This inconsistency could be explained simply if rodents had an ME less than 25% whereas birds didn't. Nor can the standard exponent explain why primates live so long when mammals of far greater mass do not live correspondingly longer, e.g., humans vs. whales, or chimpanzees vs. buffaloes. This is a simple matter if ME's are 31% vs. 28% in the first case, and 30% vs. 27% in the second. The relation of cell metabolic rate to organism mass, a contentious subject for proponents of the standard exponent, is modeled as non-existent when ME is considered, appearing instead as the relation of the cell's metabolic rate to the organism's ME, where the ME of the organism is the same as for its cells.

Major proponents of the equation, in the form of 'quarter power scaling', always limit themselves to mass specific metabolic rates, where the mass is one gram. The equation shows that at one gram mass, metabolic rate is the same for all MEs. This is why one gram is favored. It eliminates the role of ME in the equation, and makes the exponent ¾ or ⅔ at least plausible to the initiate concerned with laboratory rather than mathematical study of metabolism. Gram specific masses limit the understanding of metabolism to the in vitro level, a limitation perpetuated by the unavailability of in vivo metabolic measuring equipment aside from oxygen-use and temperature monitoring. Attention to fundamental principles of the electrochemical nature and dependence of biomass, is deflected in favor of continuing disputation about the equation's relevance, the appropriateness of Euclidean considerations in a fractal world of capillary fluid dynamics, and the whispered depths to its secrets with regard to aging and to cancer, secrets unattainable so far. The inclusion of the term ME in the exponent allows for the energetic basis of biological organization to be modeled, where replication is biomass response to fluctuations in energy availability, and can be seen from bacterial multiplication and quorum sensing, to the relation between mating strategies and food supply in large mammals.