Many small animals routinely regulate resting body temperature several degrees below active-phase levels (rest-phase hypothermia), which reduces heat transfer rate and tissue oxygen demand and so may confer energy savings. Small birds that winter at high latitude show limited capacity for rest-phase hypothermia, meaning they cannot avoid upregulating heat production when facing winter cold. Substrates for upholding body temperature are provided by mitochondria in the form of adenosine triphosphate (ATP), but mitochondrial respiration is probably lower during hypothermia because of the temperature-dependence of biological processes (i.e., Q10 effect). Thus, there could be a conflict between increased organismal fuel demand and a lower mitochondrial capacity to provide it during rest-phase hypothermia. We studied this enigma by assessing mitochondrial function in the blood cells of wintering great tits (Parus major) that had spent the preceding night in warm, mild, or cold temperatures, at each of a hypothermic and a normothermic thermal state in vitro. Hypothermia reduced mitochondrial respiration by 13 %. However, this did not affect respiration allocated to oxidative phosphorylation, because hypothermia was also associated with a reduction in non-phosphorylating respiration, from 17 % in normothermia to 4 % in hypothermia. The proportion of non-phosphorylating respiration also decreased in response to colder night temperatures Thus, the coupling efficiency between electron transport and ATP production in maximally stimulated cells increased from 91% in normothermia to 98% in hypothermia. Our study shows that mitochondrial function can be highly plastic on short temporal scales. Thus, functional changes in the electron transport system might safeguard ATP production at lower tissue temperatures and when organismal demand for fuel increases in cold winter temperatures.

Many animals downregulate body temperature to save energy when resting (rest‐phase hypothermia). Small birds that winter at high latitudes have comparatively limited capacity for hypothermia and so pay large energy costs for thermoregulation during cold nights. Available evidence suggests this process is fueled by adenosine triphosphate (ATP)‐dependent mechanisms. Most ATP is produced by oxidative phosphorylation in the mitochondria, but mitochondrial respiration may be lower during hypothermia because of the temperature dependence of biological processes. This can create conflict between increased organismal ATP demand and a lower mitochondrial capacity to provide it. We studied this in blood cell mitochondria of wild great tits (Parus major) by simulating rest‐phase hypothermia via a 6°C reduction in assay temperature in vitro. The birds had spent the night preceding the experiment in thermoneutrality or in temperatures representing mild or very cold winter nights, but night temperatures never affected mitochondrial respiration. However, across temperature groups, endogenous respiration was 14% lower in hypothermia. This did not reflect general thermal suppression of mitochondrial function because phosphorylating respiration was unaffected by thermal state. Instead, hypothermia was associated with a threefold reduction of leak respiration, from 17% in normothermia to 4% in hypothermia. Thus, the coupling of total respiration to ATP production was 96% in hypothermia, compared to 83% in normothermia. Our study shows that the thermal insensitivity of phosphorylation combined with short‐term plasticity of leak respiration may safeguard ATP production when endogenous respiration is suppressed. This casts new light on the process by which small birds endure harsh winter cold and warrants future tests across tissues in vivo.

Although mitochondrial respiration is believed to explain a substantial part of the variation in whole-animal basal (BMR) or resting metabolic rate (RMR), few studies have addressed the relationship between organismal and cellular metabolism and how this may vary in environments where individual demands for energy differ. We investigated the relationship between whole-individual metabolic rate, measured in temperatures ranging thermoneutrality to far below thermoneutrality, and mitochondrial respiration of intact or permeabilized blood cells in two separate studies on wild great tits (Parus major L.). Our results show that, in permeabilized cells, there are significant positive relationships between BMR or RMR and several mitochondrial traits, including phosphorylating respiration rate through both complexes I and II (i.e., OXPHOS respiration). However, surprisingly, the LEAK respiration (i.e., basal respiration that mainly counteract for proton leakage) was not related to BMR or RMR. When measurements were performed using intact blood cells, BMR was positively related to ROUTINE respiration (i.e., mitochondrial respiration on endogenous substrates) in one of the two studies, but no other mitochondrial traits could explain variation in BMR or RMR in any thermal environment. These studies seem to show that the level of activation of mitochondrial metabolism as well as the permeabilization status of blood cells play a primary role on the extent to which blood metabolism might explain variations in the whole-individual metabolic rate.