The question of the adaptive basis for the origin of mammalian endothermy remains unresolved despite a great deal of research effort. Controversy continues over which physiological adaptations were of greatest importance in starting ectothermic nonmammalian synapsids of the Late Paleozoic on the path that culminated in modern endothermic mammals. Models of the selective basis for the origin of endothermy fall into two main categories: “thermoregulation first” and “aerobic capacity first.” Studies of lizards show a dichotomy between a low-energy “sit-and-wait” (SW) foraging mode in Iguania and a more energy-intensive “widely foraging” (WF) mode in Autarchoglossa. It is proposed that in the transition from basal synapsids (“pelycosaurs”) to therapsids, a shift from the primitive SW mode to the WF mode put the ancestors of mammals on the path to increased aerobic capacity and the ability to sustain high levels of foraging activity. Selection for increased energy expenditure disproportionately increased the amount of food energy consumed, thus improving foraging efficiency. A shift from reliance on anaerobic muscle metabolism for short but rapid dashes to capture prey to a reliance on aerobic metabolism for active searching for prey necessitated improvements of the cardiovascular system and lungs for increased aerobic capacity and greater stamina. Over time, therapsids became locked into high food requirements, which selected for improvements in aerobic metabolism, locomotor and food-processing ability, and neurosensory/behavioral specializations. Evidence of a link between maximum activity metabolism and resting (basal) metabolism in anurans and rodents suggests that further increases in aerobic activity metabolism required an increased basal metabolic rate, which led to high body temperatures and, ultimately, homeothermy. Therapsids show adaptations for increased activity, greater food-getting and food-processing ability, and higher metabolic rates than basal synapsids (“pelycosaurs”). It is argued that the “foraging mode” model is preferable to the “parental care” model of Farmer and the “correlated progression” model of Kemp for understanding the origin of mammalian endothermy.
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