Selective Factors Associated with the Origin of Fur and Feathers1

Abstract Conventional wisdom notwithstanding, fur and feathers are unlikely to have arisen in direct association with elevated metabolic rates in early mammals, birds, or their ancestors. A complete insulative fur coat probably appeared first in the earliest mammals long after mammalian ancestors (therapsids) had attained mammalian, or near-mammalian, metabolic rates. The evolution of feathers was unlinked to the evolution of modern avian metabolic rates since early, fully flighted birds (i.e., Archaeopteryx) retained an ectothermic metabolic status. Recent claims of “feathered dinosaurs” should be regarded with caution.