Changes in Standard Metabolism during Long-Term Fasting in Northern Elephant Seal Pups (Mirounga angustirostris)

Measurements of body mass, body composition, and O₂ consumption rate V̇o2 were used to quantify some of the factors that influence metabolism during the first 4 mo of development in the northern elephant seal pup. During the 4-wk nursing period, average body mass of the pups increased from 42.0 ± 3.5 kg to 127.3 ± 15.9 kg. By the end of the 10-wk postweaning fast mean body mass declined to 83.1 ± 1.5 kg. Body fat increased from 4% of body mass near birth to approximately 48% at weaning but remained relatively constant throughout the fast. Dramatic changes in body mass and body composition such as these would be expected to influence metabolism significantly. To test this theory, O₂ consumption was measured in pups from birth to 18 wk of age through open-circuit respirometry. Changes in metabolism were correlated most strongly with increases in lean body mass (r² = .51, P < 0.01). Together, changes in lean body mass and age accounted for 75% of the variability seen in V̇o2 (P < 0.01). In addition, a 19% increase in lean mass-specific V̇o2 was documented with the onset of feeding in two laboratory animals, after a 10-wk postweaning fast. This implies a depression of metabolism, possibly as an adaptation for energy conservation during long-term fasting.