Energy Transfer by Lactating Hooded Seals and Nutrient Deposition in Their Pups during the Four Days from Birth to Weaning

Hooded seals (Cystophora cristata) have a lactation period of 4 d. The rate of milk energy transfer must be very high to ensure that pups are weaned with sufficient energy reserves to withstand the lengthy postweaning fast. Milk energy transfer was measured by isotope dilution (n = 6), and energy deposition was assayed by chemical analysis of pups at birth (n = 4) and weaning (n = 5). Estimates of body water content obtained by isotope dilution and direct analysis were not significantly different. Water constituted only 71.7% ± 0.50% (SE) of lean body mass at birth, indicating developmental maturity. Newborn pups were also high in fat (14.0% ± 0.78%) and energy (10.1 ± 0.25 MJ/kg). Pups consumed 7.52 ± 0.469 kg/d milk containing 187 ± 11.6 MJ/d. Total milk and energy yields were estimated as 30.1 kg and 746 MJ. Pups deposited 88% of ingested fat, 41% of protein, and 84% of energy. Of the weight gain, 82% was fat. The energy content of pups increased nearly fourfold from birth to weaning, but protein content increased by only 11%. At weaning, the energy content of the pup was similar, relative to maternal metabolic size (17. 7 MJ/kg0.75), to that in other species with longer lactation periods. We conclude that extreme abbreviation of lactation is feasible as a reproductive strategy in the hooded seal because (1) most of the growth of the lean body and 27% of the energy deposition in pups occur in utero, (2) females produce large amounts of high-fat milk, and (3) the pups are very eficient at converting milk fat to stored lipids.