High peripheral temperatures in king penguins while resting at sea: thermoregulation versus fat deposition

Acceleration predicts energy expenditure in a fat, flightless, diving bird

Energy drives behaviour and life history decisions, yet it can be hard to measure at fine scales in free-moving animals. Accelerometry has proven a powerful tool to estimate energy expenditure, but requires calibration in the wild. This can be difficult in some environments, or for particular behaviours, and validations have produced equivocal results in some species, particularly air-breathing divers. It is, therefore, important to calibrate accelerometry across different behaviours to understand the most parsimonious way to estimate energy expenditure in free-living conditions. Here, we combine data from miniaturised acceleration loggers on 58 free-living Adélie penguins with doubly labelled water (DLW) measurements of their energy expenditure over several days. Across different behaviours, both in water and on land, dynamic body acceleration was a good predictor of independently measured DLW-derived energy expenditure (R2 = 0.72). The most parsimonious model suggested different calibration coefficients are required to predict behaviours on land versus foraging behaviour in water (R2 = 0.75). Our results show that accelerometry can be used to reliably estimate energy expenditure in penguins, and we provide calibration equations for estimating metabolic rate across several behaviours in the wild.

Skeletal muscle metabolism in sea-acclimatized king penguins. I. Thermogenic mechanisms

At fledging, king penguin juveniles undergo a major energetic challenge to overcome the intense and prolonged energy demands for thermoregulation and locomotion imposed by life in cold seas. Among other responses, sea acclimatization triggers fuel selection in skeletal muscle metabolism towards lipid oxidation in vitro, which is reflected by a drastic increase in lipid-induced thermogenesis in vivo However, the exact nature of skeletal muscle thermogenic mechanisms (shivering and/or non-shivering thermogenesis) remains undefined. The aim of the present study was to determine in vivo whether the capacity for non-shivering thermogenesis was enhanced by sea acclimatization. We measured body temperature, metabolic rate, heart rate and shivering activity in fully immersed king penguins (Aptenodytes patagonicus) exposed to water temperatures ranging from 12 to 29°C. Results from terrestrial pre-fledging juveniles were compared with those from sea-acclimatized immature penguins (hereafter 'immatures'). The capacity for thermogenesis in water was as effective in juveniles as in immatures, while the capacity for non-shivering thermogenesis was not reinforced by sea acclimatization. This result suggests that king penguins mainly rely on skeletal muscle contraction (shivering or locomotor activity) to maintain endothermy at sea. Sea-acclimatized immature penguins also exhibited higher shivering efficiency and oxygen pulse (amount of oxygen consumed or energy expended per heartbeat) than pre-fledging juvenile birds. Such increase in shivering and cardiovascular efficiency may favor a more efficient activity-thermoregulatory heat substitution providing penguins with the aptitude to survive the tremendous energetic challenge imposed by marine life in cold circumpolar oceans.

Skeletal muscle metabolism in sea-acclimatized king penguins. II. Improved efficiency of mitochondrial bioenergetics

At fledging, juvenile king penguins (Aptenodytes patagonicus) must overcome the tremendous energetic constraints imposed by their marine habitat, including during sustained extensive swimming activity and deep dives in cold seawater. Both endurance swimming and skeletal muscle thermogenesis require high mitochondrial respiratory capacity while the submerged part of dive cycles repeatedly and greatly reduces oxygen availability, imposing a need for solutions to conserve oxygen. The aim of the present study was to determine in vitro whether skeletal muscle mitochondria become more 'thermogenic' to sustain heat production or more 'economical' to conserve oxygen in sea-acclimatized immature penguins (hereafter 'immatures') compared with terrestrial juveniles. Rates of mitochondrial oxidative phosphorylation were measured in permeabilized fibers and mitochondria from the pectoralis muscle. Mitochondrial ATP synthesis and coupling efficiency were measured in isolated muscle mitochondria. The mitochondrial activities of respiratory chain complexes and citrate synthase were also assessed. The results showed that respiration, ATP synthesis and respiratory chain complex activities in pectoralis muscles were increased by sea acclimatization. Furthermore, muscle mitochondria were on average 30-45% more energy efficient in sea-acclimatized immatures than in pre-fledging juveniles, depending on the respiratory substrate used (pyruvate, palmitoylcarnitine). Hence sea acclimatization favors the development of economical management of oxygen, decreasing the oxygen needed to produce a given amount of ATP. This mitochondrial phenotype may improve dive performance during the early marine life of king penguins, by extending their aerobic dive limit.

Down feather morphology reflects adaptation to habitat and thermal conditions across the avian phylogeny

Down feathers are the first feather types that appear in both the phylogenetic and the ontogenetic history of birds. Although it is widely acknowledged that the primary function of downy elements is insulation, little is known about the interspecific variability in the structural morphology of these feathers, and the environmental factors that have influenced their evolution. Here, we collected samples of down and afterfeathers from 156 bird species and measured key morphological characters that define the insulatory properties of the downy layer. We then tested if habitat and climatic conditions could explain the observed between-species variation in down feather structure. We show that habitat has a very strong and clearly defined effect on down feather morphology. Feather size, barbule length and nodus density all decreased from terrestrial toward aquatic birds, with riparian species exhibiting intermediate characters. Wintering climate, expressed as windchill (a combined measure of the ambient temperature and wind speed) had limited effects on down morphology, colder climate only being associated with higher nodus density in dorsal down feathers. Overall, an aquatic lifestyle selects for a denser plumulaceous layer, while the effect of harsh wintering conditions on downy structures appear limited. These results provide key evidence of adaptations to habitat at the level of the downy layer, both on the scale of macro- and micro-elements of the plumage. Moreover, they reveal characters of convergent evolution in the avian plumage and mammalian fur, that match the varying needs of insulation in terrestrial and aquatic modes of life.

Body surface rewarming in fully and partially hypothermic king penguins

Penguins face a major thermal transition when returning to land in a hypothermic state after a foraging trip. Uninsulated appendages (flippers and feet) could provide flexible heat exchange during subsequent rewarming. Here, we tested the hypothesis that peripheral vasodilation could be delayed during this recovery stage. To this end, we designed an experiment to examine patterns of surface rewarming in fully hypothermic (the cloaca and peripheral regions (here; flippers, feet and the breast) < 37 °C) and partially hypothermic (cloaca at normothermia ≥ 37 °C, but periphery at hypothermia) king penguins (Aptenodytes patagonicus) when they rewarmed in the laboratory. Both groups rewarmed during the 21 min observation period, but the temperature changes were larger in fully than in partially hypothermic birds. Moreover, we observed a 5 min delay of peripheral temperature in fully compared to partially hypothermic birds, suggesting that this process was impacted by low internal temperature. To investigate whether our laboratory data were applicable to field conditions, we also recorded surface temperatures of free-ranging penguins after they came ashore to the colony. Initial surface temperatures were lower in these birds compared to in those that rewarmed in the laboratory, and changed less over a comparable period of time on land. This could be explained both by environmental conditions and possible handling-induced thermogenesis in the laboratory. Nevertheless, this study demonstrated that appendage vasodilation is flexibly used during rewarming and that recovery may be influenced by both internal temperature and environmental conditions when penguins transition from sea to land.

The dive performance of immature king penguins following their annual molt suggests physiological constraints

Like all birds, penguins undergo periodic molt, during which they replace old feathers. However, unlike other birds, penguins replace their entire plumage within a short period while fasting ashore. During molt, king penguins (Aptenodytes patagonicus) lose half of their initial body mass, most importantly their insulating subcutaneous fat and half of their pectoral muscle mass. The latter might challenge their capacity to generate and sustain a sufficient mechanical power output to swim to distant food sources and propel themselves to great depth for successful prey capture. To investigate the effects of the annual molt fast on their dive/foraging performance, we studied various dive/foraging parameters and peripheral temperature patterns in immature king penguins across two molt cycles, after birds had spent their first and second year at sea, using implanted data-loggers. We found that the dive/foraging performance of immature king penguins was significantly reduced during post-molt foraging trips. Dive and bottom duration for a given depth were shorter during post-molt and post-dive surface interval duration was longer, reducing overall dive efficiency and underwater foraging time. We attribute this decline to the severe physiological changes that birds undergo during their annual molt. Peripheral temperature patterns differed greatly between pre- and post-molt trips, indicating the loss of the insulating subcutaneous fat layer during molt. Peripheral perfusion, as inferred from peripheral temperature, was restricted to short periods at night during pre-molt but occurred throughout extended periods during post-molt, reflecting the need to rapidly deposit an insulating fat layer during the latter period.

Thermal strategies of king penguins during prolonged fasting in water

Most animals experience periods of unfavourable conditions, challenging their daily energy balance. During breeding, king penguins fast voluntarily for up to 1.5 months in the colony, after which they replenish their energy stores at sea. However, at sea, birds might encounter periods of low foraging profitability, forcing them to draw from previously stored energy (e.g. subcutaneous fat). Accessing peripheral fat stores requires perfusion, increasing heat loss and thermoregulatory costs. Hence, how these birds balance the conflicting demands of nutritional needs and thermoregulation is unclear. We investigated the physiological responses of king penguins to fasting in cold water by: (1) monitoring tissue temperatures, as a proxy of tissue perfusion, at four distinct sites (deep and peripheral); and (2) recording their oxygen consumption rate while birds floated inside a water tank. Despite frequent oscillations, temperatures of all tissues often reached near-normothermic levels, indicating that birds maintained perfusion to peripheral tissues throughout their fasting period in water. The oxygen consumption rate of birds increased with fasting duration in water, while it was also higher when the flank tissue was warmer, indicating greater perfusion. Hence, fasting king penguins in water maintained peripheral perfusion, despite the associated greater heat loss and, therefore, thermoregulatory costs, probably to access subcutaneous fat stores. Hence, the observed normothermia in peripheral tissues of king penguins at sea, upon completion of a foraging bout, is likely explained by their nutritional needs: depositing free fatty acids (FFA) in subcutaneous tissues after profitable foraging or mobilizing FFA to fuel metabolism when foraging success was insufficient.