WATER CONSERVATION AND PROTEIN-METABOLISM IN NORTHERN ELEPHANT SEAL PUPS DURING THE POSTWEANING FAST

Urine production and N output were monitored in northern elephant seal (Mirounga angustirostris) pups progressing through 10 weeks of a natural postweaning fast. Urine output declind by 84% (to 69 +/- 12 ml . day-1) at 10 weeks (P < 0.05). Glomerular filtration rate at 10 weeks was 51 % of the 67 +/- 3 ml serum . min-1 observed during week 1 (P < 0.05). Urine N excretion fell by 69 % to 1.2 +/- 0.17 g . day-1, while urinary concentration increased (P < 0.05). Serum urea declined from an initial 11 mmol . l-1 to 5-7 mmol . l-1 by 5 weeks. The fall in urinary N loss (and thus amino acid oxidation) was concomitant with depressed metabolic rate. Therefore, protein contributed little toward meeting energy demands (i.e., < 4% of average metabolic rate) throughout fasting. These data indicate that fasting pups improve water conservation and minimize protein catabolism during prolonged natural fasts without an exogenous source of water.