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Summary
Body size matters. It sets the energetic demands of organisms, regulates the rates of physiological processes, and influences population densities and other key characteristics of animal communities and populations (1). Thus, it is no surprise that most animals have a characteristic (2) and highly heritable size (3). Yet the body sizes of mammal lineages have varied greatly over evolutionary history. The demise of dinosaurs ∼65.5 million years ago marked the onset of rapid morphological and ecological diversification in terrestrial mammals that ultimately led to size increases of more than four orders of magnitude. On page 959 of this issue, Secord et al. (4) illustrate the critical role of temperature in driving body size evolution between ∼55.5 and 54.5 million years ago.
