Protein catabolism in suckling and fasting northern elephant seal pups (Mirounga angustirostris)

Nursing elephant seal pups are hypothesized to be preadapted to the postwea ning fast, yet no comparison of lipid or protein use for meeting metabolic costs has been made between these contrasting nutritional periods. To addre ss this, protein catabolism was estimated in five elephant seal pups from m easurements of urea turnover made twice during nursing and twice during the postweaning fast. Changes in body composition were measured in tell separa te weaned pups via tritiated water dilution and matched to fasting urea tur nover measurements in order to assess errors in protein catabolism derived from urea turnover rates. Estimates of lean mass loss based upon urea turno ver and tritiated water dilution were in general agreement, supporting esti mates of protein catabolism derived from urea turnover measurements. Protei n catabolism was estimated to contribute less than 4% to the average metabo lic rate of suckling and fasting pups implying strict protein conservation during both periods and supporting the shypothesis that suckling pups are p readapted to Fasting. It is proposed that strict protein conservation acros s suckling and fasting compensates for relative reductions in maternal inve stment associated with the abbreviated lactation period of the elephant sea l.