The energetics of male reproduction in an aquatically mating pinniped, the harbour seal

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.