Body reserve dynamics and energetics of barn owls during fasting in the cold

The balance of protein versus lipid reserves utilization in fasting animals depends on their initial adiposity, a high prefasting adiposity being asso ciated with an efficient protein sparing during fasting. Yet it remains unc lear if the level of energy expenditure influences the efficiency of body p rotein sparing. We examined the effect of a high energy demand on body rese rve mobilization in barn owls (Tyto alba) fasting in the cold (5 degrees C) . Changes in body composition of captive birds were determined during the t hree characteristic phases of body fuel utilization of a long-term but reve rsible fast. Although showing a low prefasting adiposity (12%), barn owls s pared body protein in phase II as efficiently as the fattest species (contr ibution to energy expenditure of <9%). This low protein utilization most pr obably results from an increased lipid mobilization in the cold. This argue s for an influence of a high energy demand on the relative efficiency of pr otein sparing. For lipids, the pattern of mobilization from tissue sources is similar in barn owls to that of species fasting at thermoneutrality. For proteins, in contrast, and despite a low decrease of the total body protei n mass (16%), digestive tract and liver were affected most, with respective losses of 43% and 62% at phase III. This could be another consequence of t he interaction between high energy demand in the cold and fasting. Indeed, high cold-induced thermoregulatory needs could result in selective preserva tion of organs involved in the thermoregulatory process (muscles) to the de triment of lesser solicited organs such as those involved in digestion.