[Phylogeny of Mammalian metabolism]

Mammals are at the end of a gradual metabolic evolution in the course of which the step from anaerobic to aerobic cellular metabolism and the transition from water to land life formed the basis for an increase in metabolic rate (from brady- to tachymetabolism). The increased metabolic rate and the resulting endogenous heat production were the preconditions for enhanced long-term performance as well as for homeothermy which allowed mammals and birds to invade temperate zones. However, the underlying increase in membrane permeability also results in an increased energy demand (for membrane pump activity) which leads to the reduced hypoxia tolerance of mammals and requires a permanent substrate supply. As an adaptation to a seasonal discrepancy between increased thermoregulatory energy demand and decreased food supply, some small mammals apparently extended the newly evolved non-REM-sleep into hibernation. Mammalian hibernation is characterized by a profound metabolic reduction which is influenced by acidosis and limited to a tolerable degree by maintained thermoregulation. The lower limit of cooling seems to be determined by a critical minimal metabolic rate which is common to all mammals. The higher the normothermic metabolic rate, the lower is the temperature at which this minimal metabolic rate is reached. Since specific (i. e., weight-corrected) basal metabolic rate increases with decreasing body mass, small mammals exhibit a higher hypothermia tolerance than larger ones. On the other hand, the metabolic decrease to a uniform minimal level reflects an inactivation of the overall metabolic size relationship and, thus, forms a counterpart to the metabolic increase from a lower fetomaternal to the higher size-related level, occurring after birth. The postnatal metabolic increase which favours the onset of thermoregulation, parallels the increase in oxygen tension at the transition from fetal to adult circulation and, thus, probably enables mammalian neonates to readjust their metabolic needs in response to hypoxia. There is increasing evidence that, similar to the step from anaerobiosis to aerobiosis, the increase in metabolic rate resulting from any increase in oxygen supply is a general principle of evolution that, apart from its further adaptive benefits, protects tissues from oxygen excess and subsequent oxidative stress.