Water flux, body composition, and metabolic rate during molt in female southern elephant seals (Mirounga leonina)

Tritiated water dilution was used to measure changes in the proximate body composition of adult female southern elephant seals at the end of lactation and at the beginning and end of molt. During the 72 ± 0.9 d foraging phase between lactation and molt, seals gained 1.50 ± 0.16 kg·d⁻¹. Of the total mass gain (1065 ± 10.5 kg) 49% was water, 39% was fat, and 11% was protein. This represented an increase in total body gross energy of 2,111 ± 283 MJ throughout the foraging period. The rate of mass lost during molt was 4.70 ± 0.21 kg· d⁻¹ comprising 49% water, 33% fat and 16% protein. Although it was impossible to measure accurately the duration of fasting during the molt, the minimum cost of molt was 1,631 ± 146 MJ, which was not significantly different from the energy gained between lactation and molt. Females invested half as much in molt as in the growth of their pups. The metabolic rate during molt was 2.15 ± 17 W·kg1⁻¹, which was 2.8 ± 0.2 times the predicted resting metabolic rate. Water influx was greater than expected from metabolic water production, and seals had an additional water influx of 1. 75 ± 0.31 L·d⁻¹. This additional influx was negatively related to metabolic water production. There was some evidence from measurements of water influx that seals fed during molt, but this accounted for only 11.5% of the daily energy expenditure