The relationship between ectothermy and endothermy in evolution of vertebrates
Authors: Cherlin V. A. 
Published: October 2024

A new version of the description of thermobiological statuses in vertebrates is proposed: primary and secondary ectotherms, primary and secondary endotherms. Primary ectothermal animals are the first amphibian-like tetrapods (among modern animals – fish and amphibians). They had a low level of metabolism, and most of the body temperature for a number of physiological reasons could not rise above 30°C and almost did not differ from the ambient temperatures. Then they developed a complex of biochemical and physiological aromorphoses, which increased their levels of mitochondrial oxidation and basal metabolism, and began to force them to raise their body temperature. This significantly improved the quality of their activity and other functional characteristics, allowed them to go on land and begin to master it. Already the first terrestrial tetrapods (stegocephalians, seymourians) had an increased metabolism about 330 million years ago. These were basic primary endotherms – mesometabolic animals whose body temperature could hardly rise noticeably more than 30°C; they still had insufficiently developed mechanisms of regulation and control over the levels of metabolism and heat production. In the synapsid line, metabolism gradually increased along with body temperature, and through theriodonts led to the appearance of secondary endothermic animals with constantly high, controlled and regulated tachymetabolism and thermometabolism – mammals. Sauropsids also had an increase in metabolism, and in some archosaurs (dinosaurs, etc.) it sometimes rose to the level of modern birds, and body temperature reached 39–44°C. Some of them developed into secondary endothermic tachymetabolic birds, and some other – into secondary ectothermic bradymetabolic modern reptiles with a periodic increase in body temperature to 30–45°C due to external heat. But secondary ectotherms (mainly modern reptiles) are not a “return” to the state of primary ectothermy, but a powerful evolutionary step forward. Having passed through the mesothermic stage of ancient reptiles in their evolution, they acquired the ability, unlike primary ectotherms, to withstand and use high body temperature (30°C) for their functional and evolutionary benefit. It was by raising their body temperature that vertebrates increased the level of basal metabolism, improved the quality of activity, etc. Thus, the evolutionary function of reptiles is to “teach” primary ectothermic vertebrates to use high body temperature and in this regard become an “elevator” for further evolution of vertebrates. The vast majority of reptiles during their existence were meso- and tachymetabolic endothermic animals, i. e. warm-blooded to varying degrees, and bradymetabolic ectotherms, i. e., classical cold-blooded, turned out to be evolutionarily advanced modern reptiles. In general, ectothermal animals tend in their evolution to “align” with the temperature conditions of the external environment, “fit in” with them, use them. They periodically raise their body temperature due to external heat during periods when it is naturally available, thereby increasing the level of metabolism, the quality of activity and vital activity in the most energetically cheap way. Endothermic animals, on the contrary, try to reliably autonomize themselves from external conditions, raising body temperature mainly due to the endogenous thermogenesis, as a result of which their metabolism reliably and constantly increases, the quality of activity and vital activity improves. This approach is much more energy-intensive, but more reliable, and significantly less dependent on changeable environmental conditions, improving environmental valence and competitiveness. Thus, ectothermy and endothermy are two independent directions of the evolutionary development of vertebrates, each with its own strategy and ways of its implementation. At the same time, ectothermy is not a stage in the development of endothermy, but an independent evolutionary direction of the development of vertebrates, parallel to endothermy.