Biochemical universality of living matter and its metabolic implications

1. Recent discussions of metabolic scaling laws focus on the model of West, Brown & Enquist (WBE). The core assumptions of the WBE model are the size-invariance of terminal units at which energy is consumed by living matter and the size-invariance of the rate of energy supply to these units. Both assumptions are direct consequences of the biochemical universality of living matter. However, the second assumption contradicts the central prediction of the WBE model that mass-specific metabolic rate q should decrease with body mass with a scaling exponent mu = -(1)/(4), thus making the model logically inconsistent. 2. Examination of evidence interpreted by WBE and colleagues in favour of a universal mu = -(1)/(4) across 15 and more orders of magnitude range in body mass reveals that this value resulted from methodological errors in data assortment and analysis. 3. Instead, the available evidence is shown to be consistent with the existence of a size-independent mean value of mass-specific metabolic rate common to most taxa. Plotted together, q-values of non-growing unicells, insects and mammals in the basal state yield mu approximate to 0. Estimated field metabolic rates of bacteria and vertebrates are also size-independent. 4. Standard mass-specific metabolic rates of most unicells, insects and mammals studied are confined between 1 and 10 W kg(-1). Plant leaves respire at similar rates. This suggests the existence of a metabolic optimum for living matter. With growing body size and diminishing surface-to-volume ratio organisms have to change their physiology and perfect their distribution networks to keep their q in the vicinity of the optimum.