The relation between the size of southern elephant seal mothers, the growth of their pups, and the use of maternal energy, fat and protein during lactation

Pregnant female southern elephant seals vary in size by more than a factor of three when they come ashore to give birth and nurse their pups. Pups are fed exclusively from the mother's body reserves, which vary in proportion to her mass at parturition. We measured the use of body materials and energy over the course of lactation using a combination of isotope dilution and mass change during four breeding seasons on South Georgia. On average, mothers lost 35% of their mass at parturition during lactation. This included approximately 52% of the energy, 61% of the fat, and 24% of the protein in the mother's body. The relative amount that mothers expend on their pups is highly variable and shows little consistent trend with the mother's mass. Some large mothers used approximately 30% of their stored energy (comprising around 40% of stored fat and 20% of body protein) to produce medium- or large-sized pups. Whereas some smaller mothers produced only small pups, others used all of or more than the reserves estimated to be available without incurring deleterious effects (68% of energy, 80% of fat, and 27% of protein). These small animals may be at risk of compromising their future reproduction. The production of small pups by these smaller females ma3' reflect a compromise between the survival of the pup and the future success of the mother. While we expected that the largest females might show a reduced efficiency, of mass transfer during lactation (because of high metabolic overheads), their ability, to reduce the duration of lactation seems to compensate for this, and no such reduction could be shown.