Changes in body mass, body temperature and plasma fuel levels during the natural breeding fast in male and female emperor penguinsAptenodytes forsteri

Summary

• 1. Changes in body temperature (T _b), body mass, plasma glucose, free fatty acid (FFA) and β-hydroxybutyrate levels were investigated in free-living male and female emperor penguins throughout the 115-and 45-day, respectively, natural breeding fast on the sea-ice. Moreover, changes during a prolongation of this fast that mimicked the supplementary energy expense for walking back to the open sea, were also studied in captive emperors.

• 2. A decrease of the daily weight loss (DWL),T _b and plasma fuel levels during the days following the arrival in the breeding area (Table 1) probably reflected recovery from the streneous exercise emperors had to perform walking towards the colony.

• 3. Plasma FFA levels were slightly and markedly increased during oesophageal ‘curd’ secretion in males, and egg-yolk formation in females, respectively. Marked hyperthermia and hyperglycemia was observed during egg laying in females.

• 4. Long-term natural fasting was characterized by homeothermia (37.1°C), maintenance of high plasma glucose (12.8 mmol/l) and moderate plasma FFA (0.7 mmol/l) levels, and progressive modest ketosis (1.5 mmol/l) (Figs. 1 and 2), which probably reflect continuous availability and preferential utilization of fat stores.

• 5. During prolongation of the fast, marked increases in DWL and decreases in plasma FFA and β-hydroxybutyrate were indicative of a metabolic shift from fat towards protein utilization after fat depletion. The decrease in plasma lipid fuels might serve to trigger the spontaneous interruption of the natural fast.