Enquist et al. reply — Energy equivalence, as originally defined1, was an empirical relationship observed in animals: species differing in body mass, M, by many orders of magnitude tend to have almost equal rates of energy use per unit area by the population, because of an inverse allometric scaling relationship between energy use by the individual, or its metabolic rate, B, and the maximal population density, Nmax. Because B ∝M3/4 and Nmax∝M−3/4, energy use is proportional to BNmax∝M3/4M−3/4∝M.