It remains that skeletal muscle is also the main tissue involved in thermogenesis, an energetically demanding process that remains operative during winter (Barré, 1984; Duchamp et al., 1989). Is there a cost of having more energy efficient mitochondria when these energetic processes are activated? Teulier et al. (Teulier et al., 2010) have shown that the thermogenic capacities of growing birds are associated with enhanced mitochondrial phosphorylation intensity but not with innate uncoupling or changes in the efficiency of ATP generation. In king penguin, the volume and content of mitochondria in skeletal muscle (Duchamp et al., 1991; Rey et al., 2008) and the rate of mitochondrial ATP synthesis (present study) are kept high during phase II of fasting in relation to the high capacity for thermogenesis that has been reported in winter fast chicks (Barré, 1984; Duchamp et al., 1989). Hence, changes in skeletal muscle mitochondrial efficiency would have no major impact on the intrinsic capacity of a chick's thermogenic mechanism during phase II of fasting. In conclusion, the regulation of the degree of mitochondrial coupling reported in fasted winter-acclimatized chicks triggers an economical management of resources at the level of mitochondria, which would, in association with short-term hypothermic stage, maximize the conservation of endogenous fuel stores and may contribute to the ability of king penguins to withstand fasting for several months (Cherel and Le Maho, 1985; Cherel et al., 1993a; Eichhorn et al., 2011).