Evolution of Body Size: Consequences of an Energetic Definition of Fitness

TLDR

An energetic definition of fitness could unify previously disparate ecological and evolutionary models that used different currencies. The study develops a general model linking body size to fitness. Fitness is defined as reproductive power, limited by a two‑step process where energy acquisition scales with mass^0.75 and conversion to offspring scales with mass^–0.25. The model predicts a right‑skewed distribution of body sizes, a taxon‑specific optimal size (~100 g for mammals) supported by island data, and trait relationships that shift slope and sign around this optimum.

Abstract

We develop a general model for the effect of body size on fitness. We define fitness as reproductive power, the rate of conversion of energy into offspring. Reproductive power is assumed to be limited by a two-step process: first, the rate of acquisition of energy from the environment, which scales allometrically as body mass raised to approximately the 0.75 power, and then the rate of conversion of energy into offspring, which scales as mass to approximately the -0.25 power. The model predicts (1) the distinctive right-skewed shape of the frequency distribution of logarithms of body sizes among species that is observed in a wide variety of organisms from bacteria to mammals; (2) a taxon-specific optimal body size, which for mammals is approximately 100 g and is supported by data on the body sizes of mammals on islands; and (3) that in each taxon the relationships between such life-history and ecological characteristics as longevity, clutch size, home range size, and population density will change both slope and sign on either side of the optimal size. An energetic definition of fitness has the potential to unify areas of ecology and evolutionary biology that have previously used models based on different currencies.