1254394844

46

1.6.4 Is metabolic expansibility a meaningful yariable?

In this study, we considered metabolic expansibility as an indicator of the capacity of an organism to produce heat (because it was based on Msum) for a given size of metabolic machinery (which would be reflected by BMR) (Arens & Cooper, 2005; Cooper & Swanson, 1994; Hinds et al., 1993; Swanson, 2010; Swanson et al., 2012). However, the lack of correlation between BMR and Msum, as observed by others (Swanson, 2010; Swanson et al., 2012; Vćzina et al., 2006; Wiersma et al., 2007), as well as the temporal mismatch in these parameters during seasonal acclimatization suggest a lack of a functional relationship between BMR and Msum. Although BMR includes the resting energy consumption of physiolgical components involved in active thermogenesis (/.«. muscles), evidences suggest that, in the context of cold acclimatization, its variations are mainly influenced by activity and size of organ systems involved in energy acąuisition and digestion (Liknes Sc Swanson, 201 lb; Liu & Li, 2006; Zheng et al., 2008) while Msum would mostly reflect active energy consumption of shivering muscles (Cooper, 2002; Saarela & Hohtola, 2003; Vezina et al., 2007). It therefore becomes apparent to us that metabolic expansibility, as we defined it earlier, may lead to misleading conclusions about machinery adjustments. We therefore suggest caution with the interpretation of this variable and recommend interpreting variations in BMR and Msum separately to infer on animals metabolic capacity.

Acknowledgraents

We are grateful to the Corporation de la Foret d’Enseignement et Recherche Macpćs who granted us access to the field facilities. We especially thank Clóment Mandin for his help and enlightened discussions during these two years of fieldwork. We also thank Josee-Anne Otis, Philip Bertrand, Jean-Franęois Lamarre, Myriam Milbergue and Stephane Ono for their help and advices in catching birds as well as Jonathan Coudć, Danie Masse and Pauline Dubć for technical and administrative support. We are deeply grateful to Yves Turcotte for training M.P. and A.L. in bird catching and banding, to Alain Caron for statistical advices and to Sabrina Clavijo Baąuet, Grisel Cavieres, Deborah Buehler and Quinn Fletcher for their comments on an earlier version of this paper.