KleiberGesetz

Kleibergesetz, commonly known as Kleiber's law, is a foundational principle in biology describing how the basal metabolic rate (BMR) of animals scales with body mass. Empirically, BMR scales with mass to the about 3/4 power, often written as B ∝ M^0.75 or B = aM^0.75, where a is a taxon-dependent constant.

The law originates from the comparative work of Max Kleiber in the early 20th century and has since become a central reference in comparative physiology. It is typically applied to basal metabolic rate measured under standardized conditions and is observed across a wide range of mammals and many other animal groups, both endothermic and ectothermic.

The exponent is not universal. While many studies find values near 0.75, observed exponents can vary, with reported ranges roughly from 0.67 to 0.80 depending on taxonomy, developmental stage, temperature, and measurement method. The law is most robust for mature individuals studied under thermoneutral and resting conditions; deviations occur in very small or very large species and under different physiological states.

Critiques and alternatives include the 2/3 power law based on surface-area arguments (B ∝ M^2/3) and theoretical approaches such as the network-based West–Brown–Enquist model, which derives a 3/4 scaling from constraints of transport networks. Empirical evidence remains mixed, leading some researchers to propose a variable exponent rather than a single universal constant.

Applications of Kleiber's law span energy budgeting, ecological modeling, growth and life-history analyses, and cross-species comparisons of energy requirements. It serves as a reference point for understanding how metabolic demand relates to body size across diverse taxa.