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Estimates of the water and sodium fluxes of unrestrained Weddell seal pups were derived from the turnover of tritiated water and 22sodium chloride throughout the nursing period. Milk intake was calculated from water influx. There was a strong correlation between daily body weight gain in pups (x̄= 1·98 ± 0·44 kg/d) and daily weight loss by dams (x̄= 4·55±1·24 kg/d; r= 0·85, p <0·001). The maximum weight loss recorded was 249 kg over 38 days. This considerable drain on the dams' energy reserves amounts to a loss of almost 60 % of the female's body weight after parturition. Mean total body water of pups decreased from 72·5 % at birth to 48·6 % at day 40 of lactation.

Total milk consumption was positively correlated with body weight gain by pups (r= 0·87, p < 0.001), with daily milk intake ranging from 2·76 to 5·46 1/day (x̄= 3.54 ± 0·87 1/d). The pups gained 0·51 ± 0·08 kg body weight for each litre of milk consumed, and the efficiency of conversion of milk energy to stored energy was 52·1 ± 7·7 %.

Sodium influx rates averaged 2±24 · 0±41 mmol/kg. day. Approximately 70% of sodium intake was derived from milk, with the remainder coming from the ingestion of small amounts of sea water.

The high energy content of the milk (20·8 MJ/kg) and reduced milk yield (relative to body weight) means that pups can consume sufficient energy for rapid growth with comparatively brief nursing periods and short lactational duration. The relatively longer duration of lactation in the Weddell seal, during which the dams and pups engage in extensive swimming activities, has resulted in: (a) reduced daily energy intake by pups, and (b) increased percentage weight loss by dams compared to other phocids.