  The origin and shape of metabolic scaling has been controversial since
Kleiber found that basal metabolic rate of animals seemed to vary as a power
law of their body mass with exponent 3/4, instead of 2/3, as a
surface-to-volume argument predicts. The universality of exponent 3/4 -claimed
in terms of the fractal properties of the nutrient network- has recently been
challenged according to empirical evidence that observed a wealth of robust
exponents deviating from 3/4. Here we present a conceptually simple
thermodynamic framework, where the dependence of metabolic rate with body mass
emerges from a trade-off between the energy dissipated as heat and the energy
efficiently used by the organism to maintain its metabolism. This balance tunes
the shape of an additive model from which different effective scalings can be
recovered as particular cases, thereby reconciling previously inconsistent
empirical evidence in mammals, birds, insects and even plants under a unified
framework. This model is biologically motivated, fits remarkably well the data,
and also explains additional features such as the relation between energy lost
as heat and mass, the role and influence of different climatic environments or
the difference found between endotherms and ectotherms.
