Juvenile king penguins develop adaptive thermogenesis after repeated immersion in cold water. However, the mechanisms of such metabolic adaptation in birds are unknown, as they lack brown adipose tissue and uncoupling protein-1 (UCP1), which mediate adaptive non-shivering thermogenesis in mammals. We used three different groups of juvenile king penguins to investigate the mitochondrial basis of avian adaptive thermogenesis in vitro. Skeletal muscle mitochondria isolated from penguins that had never been immersed in cold water showed no superoxide-stimulated proton conductance, indicating no functional avian UCP. Skeletal muscle mitochondria from penguins that had been either experimentally immersed or naturally adapted to cold water did possess functional avian UCP, demonstrated by a superoxide-stimulated, GDP-inhibitable proton conductance across their inner membrane. This was associated with a markedly greater abundance of avian UCP mRNA. In the presence (but not the absence) of fatty acids, these mitochondria also showed a greater adenine nucleotide translocase-catalysed proton conductance than those from never-immersed penguins. This was due to an increase in the amount of adenine nucleotide translocase. Therefore, adaptive thermogenesis in juvenile king penguins is linked to two separate mechanisms of uncoupling of oxidative phosphorylation in skeletal muscle mitochondria: increased proton transport activity of avian UCP (dependent on superoxide and inhibited by GDP) and increased proton transport activity of the adenine nucleotide translocase (dependent on fatty acids and inhibited by carboxyatractylate).

) were obtained from a commercial stockbreeder (Eclosion Grimaud-lacorbière, Roussay, France). Birds were fed ad libitum with commercial mash (Moulin Guenard, 645000MI, Vonnas, France) and had free access to water. From the age of 10 days, ducklings were caged for Accepted 7 April 2010 SUMMARY Despite their lack of brown adipose tissue, some bird species develop regulatory non-shivering thermogenesis (NST) of skeletal muscle origin in response to cold acclimation. Mechanisms involved in avian NST are still unclear but may involve reduced energetic coupling in skeletal muscle mitochondria through the expression of an avian homologue of mammalian uncoupling proteins. The aim of this work was to investigate whether the expression of avian uncoupling protein (avUCP) would correlate with the capacity for cold-induced muscle NST. Various levels of cold acclimation were obtained by rearing 1-week-old ducklings (Cairina moschata) for 4 weeks at three different ambient temperatures (25°C, 11°C or 4°C). Muscle NST was measured by simultaneous recordings of metabolic rate and electromyographic activity (gastrocnemius muscle) at ambient temperatures (T a ) ranging from 27°C to -5°C. The expression of avUCP gene and mitochondrial bioenergetics were also determined in gastrocnemius muscle. Results showed that muscle NST capacity depends on the T a at which ducklings were acclimated, i.e. the lower the rearing temperature, the higher the capacity for NST. This increased metabolic heat production occurred in parallel with an upregulation of avUCP, which was not associated with a change in mitochondrial membrane conductance. The intensity of mitochondrial oxidative phosphorylation also increased in proportion with the harshness of cold, while the efficiency of ATP generation was equally effective in all three acclimation temperatures. In the absence of mitochondrial uncoupling, these data indicate a clear link between avUCP expression and the capacity of ducklings to adjust their muscular aerobic activity to cold exposure.

Energy conservation is a key priority for organisms that live in environments with seasonal shortages in resource supplies or that spontaneously fast during their annual cycle. The aim of this study was to determine whether the high fasting endurance of winteracclimatized king penguin chicks (Aptenodytes patagonicus) is associated with an adjustment of mitochondrial bioenergetics in pectoralis muscle, the largest skeletal muscle in penguins. The rates of mitochondrial oxygen consumption, and ATP synthesis and mitochondrial efficiency (ATP/O ratio) were measured in winteracclimatized chicks. We used pyruvate/malate and palmitoyl-Lcarnitine/malate as respiratory substrates and results from naturally fasted chicks were compared to experimentally re-fed chicks. Bioenergetics analysis of pectoralis muscle revealed that mitochondria are on average 15% more energy efficient in naturally fasted than in experimentally fed chicks, indicating that fasted birds consume less nutrients to sustain their energy-demanding processes. We also found that moderate reductions in temperature from 38°C to 30°C further increase by 23% the energy coupling efficiency at the level of mitochondria, suggesting that king penguin chicks realize additional energy savings while becoming hypothermic during winter. It has been calculated that this adjustment of mitochondrial efficiency in skeletal muscle may contribute to nearly 25% of fasting-induced reduction in mass-specific metabolic rate measured in vivo. The present study shows that the regulation of mitochondrial efficiency triggers the development of an economical management of resources, which would maximize the conservation of endogenous fuel stores by decreasing the cost of living in fasted winteracclimatized king penguin chicks. KEY WORDS: Bird, Skeletal muscle, Mitochondria, Energy efficiency, Starvation INTRODUCTIONSpontaneous fast is a major characteristic of the annual cycle of sea birds when breeding or moulting on land, whereas they feed exclusively at sea. In situations where food is lacking or not used, the allocation of limited endogenous resources causes trade-offs between competing traits, such as reproduction, somatic growth and maintenance. During food deprivation, animals cannot maximize all of their life-history traits and must exhibit adaptive behavioural, physiological and biochemical responses to reduce metabolism and/or the cost of current activities in order to maintain biological value (Wang et al., 2006). Among sea birds, penguins, especially the Aptenodytes genus, which inhabits cold environments, have evolved to tolerate some of the greatest relative body mass losses and to survive several months of starvation, which can be up to 5 months in winter-acclimatized king penguin chicks (Cherel and Le Maho, 1985;Groscolas, 1990;McCue, 2010). RESEARCH ARTICLEKing penguin chicks (Aptenodytes patagonicus, Miller J. F. 1778) experience a severe nutritional shortage when left alone for a long period of time, and they may be fed infrequently or not at all by their p...