Size‐ and temperature‐independence of minimum life‐supporting metabolic rates

Abstract

Summary Mass‐specific metabolic rates of 173 animal species under various conditions of prolonged food deprivation (aestivation, hibernation, sit‐and‐wait existence) and/or living at temperatures near the freezing point of water were analysed. These minimum life‐supporting metabolic rates are independent of body mass over a nearly 80‐million‐fold body mass range and independent of temperature over a range of −1·7 to 30 °C, with a mean value of 0·1 W kg−1 and 95% CI from 0·02 to 0·67 W kg−1. Additionally, 66 measurements of anoxic metabolic rates in 32 species capable of surviving at least 1 h of anoxia were analysed. While similarly mass‐independent, anoxic metabolic rates are significantly more widely scattered (1200‐fold 95% CI); they are on average one order of magnitude lower than during normoxia and depend on temperature with Q10 = 2·8. Energy losses at the time of 50% mortality during anoxia are 30–300 times smaller than the energy losses tolerated by normoxic organisms in the various energy‐saving regimes studied. These principal differences form the basis for proposing two alternative strategies by which organisms survive environmental stress: the regime of abandoned metabolic control (‘slow death’), when, as in anoxic obligate aerobes, measured rates of energy dissipation can predominantly reflect chaotic processes of tissue degradation rather than meaningful biochemical reactions; and the regime of minimum metabolic control, when biochemical order is sustained at the expense of ordered metabolic reactions. Death or survival in the regime of abandoned metabolic control is dictated by the amount of accumulated biochemical damage and not by the available energy resources, as it is in the regime of minimum metabolic control.