Our studies using American alligators, Alligator mississippiensis, green turtles, Chelonia mydas, and leatherback turtles, Dermochelys coriacea, have provided insights into the physiology of large extant and extinct reptiles. Respiratory and metabolic physiology studies indicate that many living large reptiles exhibit heat conservation adaptations and mechanisms which allow them to maintain constant warm body temperatures in cold environments with low “reptilian” metabolism. For example, leatherback turtles which are found in the oceans as far north as the Arctic Circle can maintain constant body temperatures above 25° C while water temperatures are below 7° C. This dramatic ability to maintain warm temperatures in cold, highly conductive water, that would quickly cause hypothermia and kill most endotherms, is made possible by a mechanism we describe as gigantothermy. Gigantothermy is the ability to maintain constant warm body temperatures with low energy consumption, control of peripheral circulation and extensive insulation due to large body size.
The muscles of leatherbacks show a fiber type unlike the specialized, endothermic heater organs of modern fish Scombroidei (tunas, billfish, bonitos, butterfly mackerel and relatives) that have evolved specialized “endotherm like” red muscle to maintain regional endothermy. The primitive large reptiles like leatherbacks do not show the specialization in muscle fiber type nor do the enzyme activities indicate the emergence or evolution of endothermy or a high rate of energy consumption. Yet leatherbacks can migrate over 70 km per day in the open ocean and dive to depths deeper than 1000 m on a regular basis.
Collectively these physiological studies on large living reptiles support the concept that dinosaurs, especially the larger more spectacular species, were able to maintain high body temperatures, be very active, move great distances quickly, as well as exhibit complex behavior such as communal nesting and nest building without evolving endothermy or high levels of energy consumption. The physiology of large living reptiles indicates that it is not necessary to attribute heretical ideas to explain the paleophysiology and capabilities of dinosaurs. As reptiles they were fully capable of complex and spectacular behaviors such as long and rapid migrations and surviving the cold. All evolutionary indications suggest that large size would provide a negative selection pressure for the development of endothermy especially in equable and or tropical climates.