Temperature-dependence of metabolism and fuel selection from cells to whole organisms.

Temperature affects nearly every aspect of how organisms interact with and are constrained by their environment. Measures of organismal energetics, such as metabolic rate, are highly temperature-dependent and governed through temperature effects on rates of biochemical reactions. Characterizing the relationships among levels of biological organization can lend insight into how temperature affects whole-organism function. We tested the temperature dependence of cellular oxygen consumption and its relationship to whole-animal metabolic rate in garter snakes (Thamnophis elegans). Additionally, we tested whether thermal responses were linked to shifts in the fuel source oxidized to support metabolism with the use of carbon stable isotopes. Our results demonstrate temperature dependence of metabolic rates across levels of biological organization. Cellular (basal, adenosine triphosphate-linked) and whole-animal rates of respiration increased with temperature but were not correlated within or among individuals, suggesting that variation in whole-animal metabolic rates is not due simply to variation at the cellular level, but rather other interacting factors across scales of biological organization. Counter to trends observed during fasting, elevated temperature did not alter fuel selection (i.e., natural-abundance stable carbon isotope composition in breath, δ13 Cbreath ). This consistency suggests the maintenance and oxidation of a single fuel source supporting metabolism across a broad range of metabolic demands.