The underlying physiological mechanisms responsible for winter fast-induced metabolic depression in chicks have not been thoroughly investigated so far. Because mitochondria provide most of the useable energy in the form of ATP, which chicks can ultimately allocate into several vital energy consuming functions (maintenance, growth, tissue maturation, locomotion, thermoregulation), mitochondrial metabolism may represent an important proximate factor that is responsible for the remarkable resistance of king penguin chicks to starvation. Therefore, the aim of this study was to determine whether the high fasting endurance of king penguin chicks was associated with an adjustment of mitochondrial ATP efficiency in skeletal muscle, which is directly involved in cold-induced thermogenesis and predator avoidance. Indeed, mitochondrial ATP synthesis is well known to have a variable degree of coupling to oxygen consumption, which mainly depends on mitochondrial maintenance cost, i.e. the oxygen consumed to compensate for energy wastage pathways, such as membrane permeability and/or slip reactions of the proton-pumping machinery (Brand, 2005). From the low level of circulating thyroid hormones reported in winter acclimatized chicks (Le Ninan et al., 1988; Cherel et al., 2004), one should expect a lower mitochondrial maintenance cost and thus an optimal production of ATP (Harper and Brand, 1993; Nogueira et al., 2002). To test this hypothesis, we measured muscle mitochondrial oxygen consumption and ATP synthesis at maximal and submaximal oxidative phosphorylation rates in vitro. Winter-acclimatized chicks were used and results from naturally fasted chicks were compared to experimentally re-fed chicks. The resting metabolic rate of birds was also measured in vivo. Furthermore, we also investigated the impact of heterothermy on the mitochondrial energy transduction system, by measuring in vitro muscle mitochondrial bioenergetics at 30°C (hypothermia) and 38°C (normothermia) in naturally fasted winter-acclimatized chicks.