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The energy expenditure of breeding male harbour seals, Phoca vitulina, on Sable Island, Nova Scotia, was investigated by measuring changes in body mass, body composition, and water flux using isotope dilution. Seals lost 0.47% +/- 0.04% (n = 34) of their initial mass per day during the breeding season (4 wk), and fat, water, and protein accounted for 64.3% +/- 4.8%, 27.8% +/- 3.3%, and 6.9% +/- 1.4% of this mass loss, respectively (n = 31). Total energy expenditure was estimated as 33.3 +/- 1.9 MJ d(-1), or 3.9 +/- 0.2 W kg(-1) (n = 17), similar to rates measured in terrestrially mating pinniped species. However, unlike terrestrially mating pinnipeds, male harbour seals did not fast during the breeding season, and energy intake from foraging accounted for 61.8% +/- 4.0% of the total energy expended. Males derived most of their expended energy from food intake early in the breeding season. However, as oestrus females became increasingly available, reduced rates of food intake in males were coupled with increased rates of total energy expenditure. Larger males expended significantly more energy from body stores and more total energy than smaller males. Male harbour seals appeared to balance the energetic costs of reproduction against the constraints of small body size by foraging during deep-diving trips before the appearance of oestrus females and by opportunistic feeding throughout the breeding season while at sea. We suggest that size dimorphism may be less pronounced in aquatically mating pinnipeds partly because the temporal and spatial separation of foraging and reproduction is less distinct than it is for terrestrially breeding pinnipeds.

Resting metabolic rate and body temperature of California sea lion pups (Zalophus californianus: Otariidae) were measured for 39 animals (12,000 g, mean body mass) captured from breeding areas on San Nicolas Island, California. These 3-5-wk-old pups had a mean specific resting metabolic rate of 0.780 ± .010 ml O₂ g⁻¹ h⁻¹, which is 2.380 times the value predicted by Kleiber for adult mammals with similar body mass; mean body temperature was 37.8 ± .08 C. This level of metabolism is similar to those reported for young seals (Phocidae). The heat-transfer coefficient (h) was 24.6 times that predicted from body mass, and it appeared that some individuals were on the verge of hypothermia at ambient air temperatures below 19 C. Although high metabolic rates are typically associated with high growth rates in terrestrial mammals, high heat loss associated with relatively poor insulation in sea lion pups may require allocation of relatively large amounts of energy to thermoregulation (=maintenance), thus limiting t...

Isotope dilution was used to measure milk intake of 43 California sea lion pups on San Nicolas Island, California, from 1982 to 1984. Deuterium concentration was monitored in serial blood samples collected at about 5-day intervals for 3 wk after isotope administration. Estimated milk intakes differed significantly between male (723 ± 31.0 g ․ day⁻¹) and female (609 ± 24.0 g ․ day⁻¹) pups but did not increase from the first to second month postpartum. On a metabolic size basis (weight0.83), gross energy intakes did not differ between male and female pups but did differ between years, apparently as a consequence of the 1983 El Nino. The mean energy intakes (kcal ․ day⁻¹) of sea lion pups in the first (61 · W0.83) and second (308 · W0.83) months were high relative to terrestrial species. Regression of energy intake on growth rate predicted that maintenance energy requirements (kcal ․ day⁻¹) were also high (168 and 252 · W0.83 in the first and second months). The fact that California sea lion pups devote a la...