A groundbreaking argument on how endothermy-arguably the most important innovation in vertebrate evolution-developed in birds and mammals This pioneering work investigates why endothermy, or "warm-bloodedness," evolved in birds and mammals, despite its enormous energetic costs. Arguing that single-cause hypotheses to explain the origins of endothermy have stalled research since the 1970s, Barry Gordon Lovegrove advances a novel conceptual framework that considers multiple potential causes and integrates data from the southern as well as the northern hemisphere. Drawing on paleontological data; research on extant species in places like the Karoo, Namaqualand, Madagascar, and Borneo; and novel physiological models, Lovegrove builds a compelling new explanation for the evolution of endothermy. Vividly narrated and illustrated, this book stages a groundbreaking argument that should prove provocative and fascinating for specialists and lay readers alike.