METABOLIC RESPONSES OF HOMING PIGEONS TO FLIGHT AND SUBSEQUENT RECOVERY

This study examines metabolic changes occurring during short to endura nce flights and during subsequent recovery in free-flying pigeons, in particular the change towards lipid utilization with increasing flight duration, lipid supply to the flight muscles, protein utilization and the time needed to metabolically recover. Eight plasma metabolite con centrations were primarily on carbohydrates to a lipid-based endurance measured in homing pigeons released from sites 20-200 km from the lof t (0.3-4.8 h flight duration) just after landing and after keeping bir ds fasting at rest for 30 and 60 min, respectively, after their return . Birds kept in the loft fasting at rest were used as controls. Plasma free fatty acid and glycerol concentrations increased rapidly with fl ight duration and leveled off after about 1.5 h. This indicates a mark ed change towards a high and stable lipid utilization from adipose tis sues within 1-2 h of flight. Plasma triglyceride levels and very-low-d ensity lipoproteins were decreased after short flights, but subsequent ly regained or surpassed fasting levels at rest. This indicates that r e-esterification of free fatty acids and delivery as very-low-density lipoproteins to the flight muscles to circumvent constraints of fatty acid supply, as described previously for small passerines, is not as s ignificant in the pigeon which has a much lower mass-specific energy r ate. An initial increase in plasma glucose levels and a transient decr ease to fasting levels at rest was observed and may reflect the initia l use and subsequent exhaustion of glycogen stores. Contrary to other birds and mammals, beta-hydroxy-butyrate levels increased markedly wit h flight duration. This may suggest a more important sparing of carboh ydrates and protein as gluconeogenic precursors in the pigeon than in other species. Plasma uric acid levels increased linearly up to about 4 h flight duration. This indicates an accelerated protein breakdown d uring flight which may primarily serve to deliver amino acids as gluco genic precursors and citrate cycle intermediates. With increasing flig ht duration, the energy sources change from an initial phase based pri marily on carbohydrates to a lipid-based endurance phase. It is discus sed whether this metabolic change depends on the level of power output or the performed work (energy spent) since the start of flight. Durin g the first hour of recovery, most metabolites reached or approached f asting levels at rest, indicating a marked reduction in lipolysis and protein breakdown. beta-hydroxy-butyrate levels remained at flight lev els and glucose levels increased slightly, indicating a restoration of glycogen stores.