Mass Loss, Metabolic Rate, and Energy Utilization by Harp and Gray Seal Pups during the Postweaning Fast

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Abstract
Mass loss and energy requirements of harp (Phoca groenlandica) and gray seal (Halichoerus grypus) pups were examined in individuals that spent the postweaning fast either on land or in water. Water-fasted and land-fasted harp seals lost body mass at the same rate (day⁻¹) during an 8-wk fast. Approximately 50% of the mass loss (80% of energy utilized) was from blubber. The remainder was from the core (carcass and viscera). Specific metabolic rates did not differ between treatments. Gray seals fasting on land or in water lost mass at the same relative rate ( ) as harp seals. They differed from harp seals in that more of the mass loss (>70%), and hence energy utilized (>90%), was from blubber. As in harp seals, there was no significant difference in rate of total mass loss between treatments. However, water-fasted gray seal pups lost significantly more blubber and exhibited higher specific metabolic rates than those fasting on land. Differences in the relative amounts of blubber and core used by harp and gray seals during the postweaning fast are correlated with differences in breeding habitats. Phocid seals that normally fast on land deplete their blubber layer to a greater extent than do species that usually fast in water. Selection presumably favors preservation of the blubber layer for insulation in species that may be forced to cope with the higher thermal conductivity of water.