The Evolution of Endothermy in Mammals and Birds: From Physiology to Fossils

Continuous, metabolically based maintenance of high and relatively stable body temperature in the face of greatly fluctuating ambient temperature is among the most remarkable attributes of mammals and birds. Such warm­ bloodedness or, more correctly, endothermic homeothermy, generally results from a combination of high resting, aerobically supported heat production rates (about fiveto tenfold that of reptiles) in virtually all soft tissues, and insulation sufficient to retard excessive heat loss. As a consequence, many temperature­ sensitive physiological processes in birds and mammals proceed at relatively stable rates over a wide range of ambient temperatures. In contrast, most organisms cannot thermoregulate in this way because endogenous heat production is insufficient to alter body temperature, and internal heat is necessarily derived primarily from the environment (ectother­ mic poikilothermy). These organisms tend to be thermoconformers whose body temperature frequently corresponds passively with environmental temperature. Usually such thermoconformity in thermally homogeneous environments (e.g. many marine or freshwater ecosystems) does not result in severe physiological disruption: genetic adaptation and/or phenotypic acclimation of numerous biochemical traits to ecologically relevant temperatures often ameliorate ef­ fects of temperature (e.g. enzyme function; G Somero, this volume) and mem­ brane structure (J Hazel, this volume; see also 63).