Female mass loss and metabolic rate were measured along with changes in pup mass during the perinatal period in Antarctic and Galápagos fur seals. Antarctic fur seal females lost 3.15%, or 1.231 kg per day, of their initial body mass of roughly 39 kg. Heavier females gave birth to heavier young (r = .644, P < .05; n = 12) and lost significantly more mass per day than did smaller females (r = .796, P < .01; n = 12). In contrast, Galápagos fur seal females (37.4 kg) lost half as much mass per day (1.68% day⁻¹ 0.639 kg day⁻¹). Metabolic rates, determined by the doubly labeled water method, were considerably higher in Antarctic fur seals (4.56 W kg⁻¹) than Galápagos fur seals ( 1.48 W kg⁻¹). We suggest that the lower metabolic rate of Galápagos fur seal females is an adaptation to reduce thermal stress on land in the warm equatorial habitat. Measurements of metabolic rate from water influx overestimated metabolism measured with the doubly labeled water method by only 3.2% in Antarctic fur seals, while seawater drinking precluded its use in Galápagos fur seals. Metabolic rates of nonlactating Antarctic fur seals were not different from those of lactating individuals. This is consistent with other studies, which indicates that the cost of lactation is primarily associated with the energy content of milk, with little or no measurable increase in metabolism owing to the cost of milk synthesis. Total body water determinations (57.2% Galápagos; 59.7% Antarctic) were used to estimate a total fat content of 26% and 22% at parturition, respectively. Because this is only about half that reported for phocid females, we suggest that the greater fat stores of phocid seals are a major factor enabling them to fast throughout lactation.
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Costa et al. (1988) studied this question.