The present work was carried out at the French Alfred Faure station on Possession Island (Crozet Archipelago 46°25′S, 51°45′N) from July to August during two austral winter campaigns (2010 and 2011). According to the Agreed Measures for the Conservation of Antarctic and Sub-Antarctic Fauna, the project and all of the present experimental procedures received the ethical approval of the French Polar Research Institute (IPEV, program no. 131). Two groups of king penguin chicks of either sex, 6–8 months old, captured at the breeding colony of Baie du Marin and kept outside in an open-top enclosure, were established. In the first group, fourteen chicks were left to fast for 10 days. The mean body mass was 8.9±0.2 kg at capture and 7.8±0.2 kg after 10 days of fasting (Fig. 1). In the second group, twelve naturally fasted chicks were force fed defrosted fishes (Scomber vernalis) up to 200 g day−1 for 10 days. The mean body mass was 7.7±0.3 kg at the beginning and 7.8±0.3 kg after 10 days of feeding (Fig. 1). The day before experimentation, chicks were fasted. After stabilization within the thermostatic chamber at thermoneutrality, the resting metabolic rate was recorded in vivo by indirect calorimetry as already described in king penguin chicks (Duchamp et al., 1989; Teulier et al., 2013). The day after the indirect calorimetry experiments, ~3 g of the superficial pectoralis muscle was surgically biopsied under isoflurane-induced general anaesthesia and immediately used for mitochondrial extraction (Rey et al., 2008). On completion of the study, all penguin chicks were fed and kept until full recovery and then released at the site of their capture.