Physiological response to extreme fasting in subantarctic fur seal (Arctocephalus tropicalis) pups: metabolic rates, energy reserve utilization and water fluxes.
Résumé
Surviving prolonged fasting requires various metabolic adaptations such as energy and protein sparing, notably when animals are simultaneously engaged in energy-demanding processes such as growth. Due to the intermittent pattern of maternal attendance, subantarctic fur seal pups have to repeatedly endure exceptionally long fasting episodes throughout the 10-month rearing period while preparing for nutritional independence. Their metabolic responses to natural prolonged fasting (33.4 +/- 3.3 d) were investigated at 7 months of age. Within 4-6 fasting days, pups shifted into a stage of metabolic economy characterized by a minimal rate of body mass loss (0.7 %*d(-1)) and decreased resting metabolic rate (5.9 +/- 0.1 mL O2*kg(-1)*d(-1)) that was only 10 % above the level predicted for adult terrestrial mammals. Field metabolic rate (289 +/- 10 kJ*kg(-1)*d(-1)) and water influx (7.9 +/- 0.9 mL*kg(-1)*d(-1)) were also amongst the lowest reported for any young otariid, suggesting minimized energy allocation to behavioral activity and thermoregulation. Furthermore, lean tissue degradation was dramatically reduced. High initial adiposity (>48 %) and predominant reliance on lipid catabolism likely contributed to the exceptional degree of protein sparing attained. Blood chemistry supported these findings and suggested utilization of alternative fuels such as ss-hydroxybutyrate and de novo synthesized glucose from fat-released glycerol. Regardless of sex and body condition, pups tended to adopt a convergent strategy of extreme energy and lean BM conservation that appears highly adaptive because allowing some tissue growth during the repeated episodes of prolonged fasting they experience throughout their development. Key words: starvation, metabolic rate reduction, protein sparing, lipid metabolism.