Changes in Milk Composition during Lactation in the Northern Elephant Seal

Abstract

Elephant seals, Mirounga angustirostris, are exceptional mammals in that females fast entirely during nursing while their pups may quadruple in weight over the 28-day suckling period. Obviously, all water and nutrients for both pup and mother must be derived from the females' body reserves. Thus one might anticipate changes in milk composition during nursing which reflect the physiological requirements of both parent and offspring. Milk samples were taken from 20 females in the field at known times in their respective nursing cycles during the 1976-77 breeding season on Año Nuevo Island, California. The milk was analyzed for: percent H₂O, percent protein, percent fat, fatty acid composition, and the presence of reducing sugars. During the first 21 days of lactation, fat and water content of milk changed with mirror-image kinetics; fat rising from approximately 15% to a plateau of approximately 55%, while water content fell from approximately 75% to a constant level of approximately 35%. Plateau values persisted until weaning. Protein content remained fairly constant during lactation, comprising 5%-12% of milk by weight. Fatty acid composition of milk lipids varied little throughout nursing; 16:0, 18: 1, and 20:1 species accounting for ≥70% of total lipid. In all samples reducing sugars were below the detectable limit (0.25%). These results must reflect strong selection pressure for short, intense, and highly efficient nursing regimes in species which fast during lactation, as compared with conventional mammalian nursing strategies.