ABSTRACT
Despite centuries of thoughtful contemplation, the true meaning of “time” remains elusive. Lacking insights into its nature, humans have resorted to defining the passage of time by its measurement. Chronological time so determined is based on repetitive and predictable astronomical phenomena. Intriguingly, however, physiological time—the rate that bodily processes occur—is not governed by chronological time but rather by body mass. That is, physiological time runs faster in smaller animals than in larger ones and in children compared to adults. The relationship between physiological time and mass follows a power law, P ~ aMb, where P is the rate of the physiological function and b is a scaling exponent relating body mass M to P. While such scaling exponents have been empirically obtained for numerous functions and in animals of various sizes, the explanations for the values obtained remain controversial. A mass exponent of 0.25 is common among disparate physiological functions, which is expected to serve as a clue to some underlying fundamental aspect of living matter. What this essential principle is, however, remains obscure. Maximal exercise alters the scaling exponent for mass for metabolic rate, which may reflect a shift in the responsible tissues at rest versus that (i.e., skeletal muscle) during vigorous exercise.
