Bennett, Hicks & Cullum (2000)
Evolution 54:1768-1773

Alistair J. Cullum
Department of Biology
Creighton University

An Experimental Test of the Thermoregulatory Hypothesis for the Evolution of Endothermy

Albert F. Bennett, James W. Hicks, and Alistair J. Cullum

Department of Ecology and Evolutionary Biology
321 Steinhaus
University of California, Irvine
Irvine, CA 92697-2525

Evolution 54(5):1768-1773 (2000)

Abstract - The thermoregulatory hypothesis proposes that endothermy in mammals and birds evolved as a thermoregulatory mechanism per se and that natural selection operated directly to increase body temperature and thermal stability through increments in resting metabolic rate. We experimentally tested this hypothesis by measuring the thermoregulatory consequences of increased metabolic rate in resting lizards (Varanus exanthematicus). A large metabolic increment was induced by feeding the animals and consequent changes in metabolic rate and body temperature were monitored. Although metabolic rate tripled at 32C and quadrupled at 35C, body temperature rose only about 0.5C. The rate of decline of body temperature in a colder environment did not decrease as metabolic rate increased. Thus, increasing the visceral metabolic rate of this ectothermic lizard established neither consequential endothermy nor homeothermy. These results are inconsistent with a thermoregulatory explanation for the evolution of endothermy.