Regardless of the respiratory substrate used, ADP was less effective at stimulating oxidative activities at 30°C, whereas no significant differences were found in the rates of ATP synthesis (supplementary material Fig. S2). The apparent affinity constants of mitochondrial fluxes (oxygen consumption or ATP synthesis) to added ADP were not significantly different between the two thermal conditions (supplementary material Fig. S2, insets). Fig. 3 shows the effect of temperature on the efficiency of oxidative phosphorylation in pectoral muscle mitochondria respiring on pyruvate (Fig. 3A) or palmitoyl-l-carnitine (Fig. 3B). Regardless of respiratory substrate used, the slopes were not different between the two thermal conditions, whereas the non-phosphorylating respiration rates (the intercepts with the x-axis) were significantly lowered by an average of 58% in mitochondria functioning at 30°C in comparison with those at 38°C. Hence, the linear relations between the rates of ATP synthesis and oxygen consumption were parallel and significantly shifted to the left at 30°C. This implies that at any given rate of oxygen consumption, the rate of ATP synthesis was higher at 30°C, indicating that the effective efficiency of oxidative phosphorylation (effective ATP/O) increases in skeletal muscle mitochondria when temperature decreases (Fig. 3C,D). It follows that, to sustain energy demanding processes, the mitochondrial energy transduction system becomes intrinsically more ‘economic’, consuming less oxygen (−23% on average, Fig. 4) and thus oxidizing less fuel at 30°C (hypothermia) than at 38°C (normothermia).