[The origin of homoiothermy--unsolved problem].

The analysis of allometric dependence of energy expenditure on body mass among reptiles, birds and mammals has shown that standard metabolic rate of reptiles when they are warmed up to the temperature of homoiothermic animals is an order of magnitude lower than that of birds and mammals. Basal metabolism is originated as special feature historically related to the metabolism during active behavior, rather than thermal regulation. Facultative endothermy was not advantageous for large animals because of long time needed to warm up the body. The ancestors of birds and animals escaped negative consequences of van't-Hoff equation by choosing constant body temperature. Heat conductivity of reptile's covers is so great, that it cannot keep endogenous warm of resting animal at any temperature of the body. Reptile "dressed" in covers of bird or mammal would be able to keep warm under conditions of maximal aerobic muscular activity and body temperature similar to that of homoiothermic animals. The base of chemical thermoregulation in birds and mammals is a thermoregulatory muscle tonus which remains unknown. One can suppose that during evolution of birds and mammals the saltation-liked origin of endothermy "fixed" the level of metabolism typical for running reptile and transformed in into the basal metabolism. This event took place at the cell and tissue level. The absence of palaeontological evidences and intermediate forms among recent species does not allow easy understanding of homoiothermy origin.