NOREN, D.P.; University of California, Santa Cruz and National Marine Mammal Laboratory: Differential Energy Reserve Utilization During the Postweaning Fast of Northern Elephant Seals: Implications for the First Year of Life

During fasting, lipid reserves are preferentially utilized for energy while proteins are spared. For marine mammals, however, lipids are not only important energy stores, but are also major components of blubber, the primary thermoregulatory structure. Consequently, rates of protein and lipid utilization and duration of the postweaning fast in northern elephant seals should depend on body condition at weaning. To test this, body mass and composition of 60 pups were measured at 2.3±0.2 days and 55.9±0.3 days postweaning. Thermoregulatory costs for 17 pups in air and water were also measured at the end of the fast. Lipid content increased with body mass and fueled 85-98% of total energy expended, with fatter pups having higher lipid utilization rates during the fast. Metabolic rates at the end of the fast were highest in air and lowest in 3.8°C water, but were not influenced by body composition. Duration of the postweaning fast (32-78 days) and the first foraging trip (114-266 days) were positively correlated with body mass and lipid content at weaning. Furthermore, pups with shorter foraging trips returned to land more frequently during the first year of life. These results suggest that body condition influences fasting physiology and foraging behavior in northern elephant seal pups. Variation in energy reserve utilization and unequal fasting durations ensure that all pups have relatively high lipid content at the end of the fast and initiate the first foraging trip with adequate insulation for thermal balance in the ocean. Small pups appear to forage locally while large pups may rely on their greater energy stores to exploit prey that are farther away from the natal rookery.