The relationship of plasma indicators of lipid metabolism and muscle damage to overnight temperature in winter-acclimatized small birds David L. Swanson ⁎ , Nathan E. Thomas Department of Biology, University of South Dakota, 414 E. Clark Street, Vermillion, SD 57069, USA Received 25 May 2006; received in revised form 8 September 2006; accepted 9 September 2006 Available online 16 September 2006 Abstract Plasma glycerol and triglyceride levels and creatine kinase (CK) activity may increase during long-distance flights in migratory birds, but plasma profiles of these metabolites have not previously been reported for small birds during thermoregulation in cold climates. We measured early morning levels of plasma glycerol, triglycerides and CK activity in four species of small birds overwintering in South Dakota, Junco hyemalis, Spizella arborea, Passer domesticus, and Carduelis tristis. We hypothesized that metabolite levels and CK activity might vary with overnight temperature (measured as the temperature just prior to dawn), with higher levels during colder temperatures which require elevated thermogenesis. Triglyceride and glycerol levels were not significantly related to temperature for any of the four species. Triglyceride levels were significantly positively associated with time since sunrise in J. hyemalis and C. tristis, and the time–temperature interaction was significant for S. arborea, suggesting rapid replacement of fat stores. Plasma glycerol levels were also significantly positively related to time since sunrise in J. hyemalis and C. tristis, but not in other species. Plasma CK activity showed a significant negative relationship to overnight temperature only for S. arborea. These results suggest that triglycerides do not comprise a major contribution to lipid supply during intense shivering in small birds. Similarly, intense shivering does not generally appear to result in muscle damage in small birds. © 2006 Elsevier Inc. All rights reserved. Keywords: Lipid metabolism; Triglycerides; Glycerol; Birds; Winter; Cold tolerance; Thermogenesis; Creatine kinase 1. Introduction Winter can be an energetically expensive and potentially stressful period for small birds wintering in cold climates because of the combination of low temperatures, reduced food availability, short days for foraging and long nights of forced fasting. Indeed, for many small birds, daily energy expenditure in winter exceeds that during the breeding season (Weathers et al., 1999; Cooper, 2000; Doherty et al., 2001). Exposure to cold temperatures during winter requires high rates of thermogenesis for prolonged periods, particularly during cold episodes and during the nocturnal fast, at which time tem- peratures are usually coldest. Thermogenesis in birds is pri- marily accomplished by shivering, and both sustained shivering and endurance exercise (i.e., running and flying) are fueled primarily by lipid substrates in birds (Marsh and Dawson, 1989; Vaillancourt et al., 2005). During prolonged exercise, such as migratory flights and sustained shivering, birds are primarily reliant on extracellular lipid from adipose tissue stores (McWilliams et al., 2004). Such reliance on exogenous lipids contrasts with fuel supply during sustained activity in mammals, which rely to a large degree on intracellular stores within muscle (Weber et al., 1996a,b). In support of this conclusion, plasma free fatty acids (FFA) and glycerol generally increase during long-distance flight and sustained shivering in birds (Dawson et al., 1983; Marsh and Dawson, 1989; Jenni- Eiermann et al., 2002; McWilliams et al., 2004). Plasma triglycerides are generally elevated under conditions of fat deposition in wild and captive birds (Jenni-Eiermann and Jenni, 1994; Totzke and Bairlein, 1998; Williams et al., 1999; Gannes, 2001; Guglielmo et al., 2002a). Thus, a general inverse relationship exists between plasma levels of triglycerides and glycerol, with triglycerides increasing during fat deposition and glycerol increasing during fat catabolism (Guglielmo et al., 2005). However, in addition to elevated plasma levels of FFA Comparative Biochemistry and Physiology, Part A 146 (2007) 87 – 94 www.elsevier.com/locate/cbpa ⁎ Corresponding author. Tel.: +1 605 677 6175; fax: +1 605 677 6557. E-mail address: David.Swanson@usd.edu (D.L. Swanson). 1095-6433/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpa.2006.09.004