Energy metabolism of Weddell seals (Leptonychotes weddellii) during the lactation period

Abstract

Weddell seals (Leptonychotes weddellii) give birth on antarctic fast ice in the austral spring and nurse their pups for six to seven weeks. Lactating Weddell seals lose body mass rapidly and may lose almost half of their postpartum body mass yet show no signs of hyperketonaemia. Food intake by lactating females is suspected but available evidence of the incidence, onset and importance of foraging by lactating Weddell seals is inconclusive.

The aim of this study was to investigate the energy metabolism of lactating Weddell seals, with particular emphasis on the importance of food intake during the lactation period. To this aim, mass loss, energy expenditure, ketone body concentrations and the incidence and onset of foraging were studied in lactating Weddell seals at Hutton Cliffs, McMurdo Sound, Antarctica, from October to December 1998.

A new technique for the detection of food intake in free-living marine mammals, the biomarker method, was applied to Weddell seals and its usefulness was evaluated. The biomarker method correlated well with indicators of nutritional status. It was also suitable for field use and provided the possibility of monitoring onset of feeding in individual animals. Results indicate that the majority (≥67 %) of lactating Weddell seals studied were feeding in late lactation (≥27 days postpartum), with estimated onset of feeding at three to four weeks postpartum. Individual lactating females appeared to start foraging as early as 9 days postpartum.

Mean mass loss in lactating females (4.5 kg.day⁻¹) measured in this study was consistent with earlier reports. Between mid - and late lactation, seals had a mean daily energy deficit of 114 MJ as calculated from changes in body composition measured with isotope dilution. The observed daily energy deficit is comparable to that of lactating females in other phocid species but is likely to underestimate total energy expenditure in lactating Weddell seals since the majority of females started feeding.

Plasma concentrations of the ketone body β-hydroxybutyrate were low (<400 µM) in both feeding and fasting lactating Weddell seals and highly correlated to plasma concentrations of acetylcarnitine and free carnitine. The absence of ketone body accumulation in Weddell seals is compared with findings in fasting seals in general, and possible explanations for this phenomenon are discussed.