Shivering Thermogenesis in Birds and Mammals

Shivering is the only universal facultative thermogenic mechanism in endothermic vertebrates. Skeletal muscle constitutes a large proportion of body mass and a high scope of metabolic rate, and thus heat production, between resting level and active contraction. Its contractile activity is under rapid and accurate neural control. Together with the inherently low metabolic efficiency of muscle, these “preadaptive” factors have led to an independent evolution of shivering as the main facultative thermogenic effector in birds and mammals as well as other modes of contraction-related muscle thermogenesis in some heterothermic animals. Shivering fulfils the requirements of a facultative mechanism, since it accurately and rapidly tracks variations in thermal conditions and obligatory thermogenesis and is adjusted accordingly to maintain thermal balance. In contrast to the other facultative thermogenic tissue, mammalian brown fat, aerobic muscle has a high inherent capacity for ATP-splitting, which drives the combustion of cellular fuels. Thus, any uncoupling of mitochondrial respiration for thermogenic purposes would incur little selective advantage.