Mass changes and metabolism in territorial male Antarctic fur seals (Arctocephalus gazella)

Energy expenditure of fasting territorial male Antarctic fur seals at South Georgia was measured by mass loss and analysis of carcass composition using tritium dilution in 1987 and 1988. Mean mass loss was 1.52 ± 0.04 kg • d⁻¹, mean length of territory tenure was 30.7 ± 2.21 d, mean mass at the start of tenure was 188.0 ± 1.92 kg; there were no differences between the two years (P > 0.1). Mass loss was 53.8% fat (80% of which was from blubber), 36% water, and 10.2% protein. Fat accounted for 91.6% of energy expenditure, the remainder being from protein. Energy expenditure was 42.1 ± 2.34 MJ • d⁻¹, or 3.16 ± 0.34 W • kg⁻¹, which is 3.3 times the predicted basal metabolic rate. Large body size in male compared to female fur seals may be related to the energy requirements of territoriality, but only in 1988 was there a significant correlation between starting mass and duration of tenure. This suggests that energy reserves are only one of several factors influencing tenure duration. Male fur seals are composed of 24% fat when they arrive at the breeding grounds, which is lower than for most other pinnipeds that fast through an extended period ashore