317 The metabolic consequences of cold exposure and exercise have been well characterized in mammals (Himms-Hagen, 1996; Rowell and Shepherd, 1996), but little information is available for birds (Bicudo et al., 2001; Butler, 1991; Hohtola et al., 1998; St-Laurent and Larochelle, 1994), and the physiological changes caused by shivering and exercise are even less well understood for migrants (Jenni-Eiermann et al., 2002; Klaassen, 1996; Kvist et al., 2001; Piersma et al., 2003; Ramenofsky, 1990; Ward et al., 2002). Ruff sandpipers Philomachus pugnax are long-distance migrant shorebirds that fly from wintering areas in Africa to nesting grounds in North- eastern Siberia; this annual round trip can reach 30·000·km (Cramp and Simmons, 1983). These remarkable athletes have been able to extend the physiological limits of endurance capacity, but nothing is known of their pattern of metabolic fuel selection. Quantifying the relative contributions of the different fuels to total metabolism has proved very difficult, mainly because methods routinely used in mammalian research are not easily adaptable to birds. Migration flights are energetically very demanding, but other activities than flying also offer significant physiological challenges. For instance, metabolic rate must be increased when low temperatures are encountered (during the night or at high altitude), or when the birds are rapidly building fat reserves and spend a lot of time running while feeding. In some species, leg muscles are even known to hypertrophy during stopovers (Piersma et al., 1999). Therefore, shivering thermogenesis and terrestrial locomotion are two ecologically relevant activities regularly performed by ruff sandpipers. Acclimation to cold environments has been the subject of many bird studies (Ballantyne and George, 1978; Block, 1994). For acute cold exposure, however, most of the work has been carried out on juvenile birds, and it has been shown that they cannot thermoregulate (Østnes et al., 2001). As they age, shivering thermogenesis develops in leg and pectoral muscle, and the latter becomes a major site of heat production (Marjoniemi and Hohtola, 1999), although few studies have determined which oxidative fuels are being used. One study provides indirect information on metabolic fuels during cold exposure, and reports respiratory exchange ratios (RER) of The Journal of Experimental Biology 208, 317-325 Published by The Company of Biologists 2005 doi:10.1242/jeb.01397 The metabolic consequences of cold exposure and exercise are not well characterized in birds. Ruff sandpipers Philomachus pugnax are migrant shorebirds traveling between Africa and Siberia for up to 30·000·km annually. Our goal was to quantify the fuel selection pattern of these remarkable athletes during shivering and terrestrial locomotion. We used indirect calorimetry and nitrogen excretion analysis to measure their rates of lipid, carbohydrate and protein oxidation at different temperatures (22, 15, 10 or 5°C) and different treadmill speeds (15, 20, 25, 30, 35 or 40·m·min –1 ). Results show that lipid oxidation supplies nearly all the energy necessary to support shivering and running, and that the pattern of oxidative fuel selection is independent of shivering or running intensity. During shivering, total ATP production is unequally shared between lipids (82%), carbohydrates (12%) and proteins (6%). During running, lipids remain the dominant substrate (66%), with carbohydrates (29%) and proteins (5%) playing more minor roles. The prevailing use of lipids during intense shivering and high- speed running is not consistent with the fuel selection pattern observed in exercising and cold-exposed mammals. The exact mechanisms allowing birds to use lipids at extremely high rates are still largely unexplored, and quantifying the relative importance of different fuels during long-distance flight remains a major challenge for future research. Key words: oxidative fuel utilization, fuel selection, lipid, carbohydrate, animal energetics, indirect calorimetry, energy expenditure, shivering, exercise, ruff sandpiper, Philomachus pugnax. Summary Introduction Energetics of a long-distance migrant shorebird (Philomachus pugnax) during cold exposure and running Eric Vaillancourt 1 , Sophie Prud’Homme 1 , François Haman 1 , Christopher G. Guglielmo 2 and Jean-Michel Weber 1, * 1 Biology Department, University of Ottawa, Ottawa, Ontario, Canada and 2 Biological Sciences, University of Montana, Missoula, Montana, USA *Author for correspondence (e-mail: jmweber@science.uottawa.ca) Accepted 15 November 2004 THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY