The significant energy savings found in fasted king penguin chicks at the mitochondrial level was reflected at the whole-animal level with the reduction of resting metabolic rate. The reduction of mass-specific resting metabolic rate observed in fasted chicks is in line with the low level of resting metabolic rate previously reported in chick (Barré, 1984; Duchamp et al., 1989) and adult of king penguins (Fahlman et al., 2004; Rey et al., 2008) after several weeks of fasting. In the present study, the reduction in whole-animal metabolic rate was not ascribed to a difference in body temperature between fasted and re-fed birds. Indeed, all of the birds were resting at thermoneutrality during the indirect calorimetry experiments, an experimental condition for which the body temperature of winter-acclimatized chicks remains high and constant, independently of the nutritional status (Duchamp et al., 1989). In turn, the fact that resting metabolism is determined by the proportion of lean body mass raises the question of whether the observed variations could be due to differences in body composition between fasted and re-fed birds. It has been previously reported that lean body mass accounts for 46% of total body mass loss in fasted king penguin chicks during phase II of fasting, and the remaining 54% coming from lipids (Cherel et al., 1993b). From these respective contributions to body mass loss, we should expect a progressive increase in the proportion of lean body mass over the course of starvation. By using the pre-winter body composition of king penguin chicks (Cherel et al., 1993a), we can estimate the enrichment in the proportion of lean body mass to amount to 3.5% of body mass after 10 days of fasting, which might explain up to 20% of differences in mass-specific resting metabolism between fasted and re-fed chicks. However, it must be kept in mind that re-fed chicks were included in the present feeding protocol after an initial natural winter fast, once they had reached similar body weight, and therefore similar body composition, to fasted chicks. The fact that the feeding protocol was designed to maintain the initial body weight of chicks indicates that chicks were given enough food to cover daily energy expenditure but not to substantially change body mass composition. Therefore, the reported differences in mass-specific resting metabolism probably mainly reflect changes in cellular activity and mitochondrial energy efficiency, and may only moderately be influenced by variations in lean body mass proportion.