Background. Kleiber’s law describes the quantitative association between whole-body resting energy expenditure (REE, in kcal/d) and body mass (M, in kg) across mature mammals as REE . The basis of this empirical function is uncertain. Objectives. The study objective was to establish an organ-tissue level REE model across mammals and to explore the body composition and physiologic basis of Kleiber’s law. Design. We evaluated the hypothesis that REE in mature mammals can be predicted by a combination of two variables: the mass of individual organs/tissues and their corresponding specific resting metabolic rates. Data on the mass of organs with high metabolic rate (i.e., liver, brain, heart, and kidneys) for 111 species ranging in body mass from 0.0075 (shrew) to 6650？kg (elephant) were obtained from a literature review. Results. predicted by the organ-tissue level model was correlated with body mass (correlation ) and resulted in the function , with a coefficient and scaling exponent, respectively, close to 70.0 and 0.75 ( ) as observed by Kleiber. There were no differences between and calculated by Kleiber’s law; was correlated ( ) with . The mass-specific , that is, , was correlated with body mass ( ) with a scaling exponent ？0.246, close to ？0.25 as observed with Kleiber’s law. Conclusion. Our findings provide new insights into the organ/tissue energetic components of Kleiber’s law. The observed large rise in REE and lowering of REE/M from shrew to elephant can be explained by corresponding changes in organ/tissue mass and associated specific metabolic rate. 1. Introduction Resting energy expenditure (REE), defined as the whole-body energy expenditure under standard conditions, is the largest fraction of total energy expenditure. Body mass was applied early in exploring the quantitative association between REE and body composition. The best empirical fit between REE (in kcal/d) and body mass (M, in kg) from mouse to elephant with a ~330,000-fold difference in body size was derived by Kleiber [1, 2] and Brody [3], Equation (1) is the well-known Kleiber’s law or power law, one of the most widely discussed rules in bioenergetics [2, 4]. Based on (1), Kleiber’s law can also be expressed in terms of mass-specific REE, According to Kleiber’s law, small mammals (e.g., shrew) have lower REE but higher REE/M than do large mammals (e.g., elephant). Although many investigators have attempted to clarify plausible mechanisms, a full understanding of Kleiber’s law is still uncertain and represents a knowledge gap in the studies of bioenergetics [12]. Primary
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