Max Kleiber (1893-1976), a Swiss-American animal scientist who was one of the first to make accurate measurements of the rate of energy use relative to body size among different species of mammals (1932). He claimed that the 3/4 power of body weight was the most reliable basis for predicting the basal metabolic rate of mammals and for comparing nutrient requirements among mammals of different sizes. Klieber’s Law: MR =aM0.75, where MR is metabolic rate (watts), M is body mass (kg), and a is a constant. However, although Kleiber's law has been widely used and thought to apply to all of life, significant variation in metabolic scaling has been observed among various taxonomic groups of animals and plants, and among various physiological states. Kleiber also provided the basis for the conclusion that total efficiency of energy utilization is independent of body size and pioneered the use of isotopes to study metabolic processes associated with lactation in cattle (1947).