1254394707

113

113

Fali

Midwinter

Digestiyę

Muscle

Muscle

»^vi> EŁłSBi gggnssifcz?) I

End wlnter

Muscle

□ Msum ■ BMR

Year

Muscle, Cardiopulmonary (Excrotorył

Mus.ctenExcretory

U

0,0 0.1 0,2

0.3    0.4

R* adjusted

0.5    0.6    0,7

Figurę 4.5 Contribution of body components to metabolic performance throughout the year. Bars represent the r² adjusted from the best model explaining size-independent BMR or whole Msum for each period (see methods). Organs contributing to the models are given in bars. Organs between () were retained in the models but were not significant. Summer is not represented sińce BMR and Msum were not found to relate to body components during this period.

4.6 Discussion

In this study, we predicted that chickadees would express higher metabolic performance in midwinter, at the peak of cold, and that this would be mainly explained by enlargements in exercise, digestive and excretory organs. Morę specifically, we expected a significant contribution of the digestive and excretory organs in BMR variations while changes in Msum would mainly result from variations in the size of exercise organs such as the muscles. Our results only partially supported these predictions. As expected, thermogenic capacity was higher and muscles were larger at midwinter than at any other periods and muscles explained most of the variation in Msum at that time. However, seasonal changes in BMR were minimal (4-6%) and non-significant when considering the effect of Iean dry body mass. Visceral organs showed either no seasonal change (e.g. excretory organs) or a decline from fali to summer (e.g. digestive organs) and although excretory organs were found to relate