Temporal responses in energy expenditure and respiratory quotient following feeding in the muskox: influence of season on energy costs of eating and standing and an endogenous heat increment

Seasonal energy metabolism was investigated in young (2- to 3-year-old) muskoxen (Ovibos moschatus) during the winters of 1994 (January-April) and 1996 (January) and summer of 1995 (July and August). Energy expenditure (EE) increased 35%-42% following a meal of chopped brome hay (Bromus inermis) and declined as a double-exponential process over 8 h. The mean energy cost of eating (321 and 361 J.g dry matter(-1)) was lower in winter than in summer, and declined with body mass (BM) (r(2) = 0.58). The mean energy cost of standing was 21% (SE = 2.7%) higher than that of bedding. Prefeeding energy expenditure (EE(p)) was 26% higher in summer than in winter. An endogenous heat increment, measured as EE(p) - EE, at 7-8 h post feeding was lower (P < 0.001) in winter than in summer (39 and 58 kJ center dot kg BM(-0.75)center dot d(-1), respectively). Mean cumulative EE (minus activity costs) for 8 h post feeding was 124 (SE = 4) and 148 (SE = 4) kJ center dot kg BM(-0.75) (P < 0.001) in winter and summer, respectively. Respiratory quotients (RQs) >1 were recorded during feeding in winter and a mean RQ of 0.9 was recorded in summer. Seasonal EE(p), postfeeding EE, and RQ are consistent with a low cost of maintenance metabolism in winter and an increased requirement for productivity in summer.