WATER-BALANCE AND THE ENERGETICS OF LACTATION IN GREY SEALS (HALICHOERUS-GRYPUS) AS STUDIED BY ISOTOPICALLY LABELED WATER METHODS

The energetics of lactation in grey seal females and their pups was in vestigated using labelled water methods. Average total energy output ( maternal maintenance energy expenditure plus milk energy output). from the estimated change in total body gross energy, of 15 females was 99 .8 MJ/d (S.E. 2.5), which was nearly 30% lower than previously publish ed estimates. In seven females, energy output was partitioned into mil k energy output plus maternal maintenance energy expenditure. Average investment in milk was 69.7 MJ/d (S.E. 5.0), and lactation costs accou nted for 70% of total maternal energy output. Average daily maternal m aintenance energy expenditure (heat output) of the same seven females was 30.0 MJ/d (S.E. 5.0), which was equivalent to 2.3 times the basal metabolic rate predicted from Kleiber's (1975) allometric equation. In seven pups, milk consumption rates were measured isotopically and the se averaged 3.0 kg/d (S.E. 0.2). In four pups energy expenditure, meas ured using doubly-labelled water, averaged 13.5 MJ/d (S.E. 1.2), equiv alent to 3.7 times the basal metabolic rare predicted from Kleiber's ( 1975) equation. All females were in negative water balance during lact ation which was associated with loss of tissue during fasting. Mean wa ter efflux was 39.66 ml/d (S.E. 286) and mean water influx was 2677 ml /d (S.E. 245), but there was no evidence that females are dehydrated d uring lactation.