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

Abstract

The balance of protein versus lipid reserves utilization in fasting animals depends on their initial adiposity, a high prefasting adiposity being associated with an efficient protein sparing during fasting. Yet it remains unclear if the level of energy expenditure influences the efficiency of body protein sparing. We examined the effect of a high energy demand on body reserve mobilization in barn owls (Tyto alba) fasting in the cold (5 °C). Changes in body composition of captive birds were determined during the three characteristic phases of body fuel utilization of a long-term but reversible fast. Although showing a low prefasting adiposity (12%), barn owls spared body protein in phase II as efficiently as the fattest species (contribution to energy expenditure of <9%). This low protein utilization most probably results from an increased lipid mobilization in the cold. This argues for an influence of a high energy demand on the relative efficiency of protein 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 protein 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 the interaction between high energy demand in the cold and fasting. Indeed, high cold-induced thermoregulatory needs could result in selective preservation of organs involved in the thermoregulatory process (muscles) to the detriment of lesser solicited organs such as those involved in digestion.