The energetics of lactation in Antarctic fur seals (Artocephalus gazella)
The use of hydrogen isotope dilution to measure total body water (TBW) and body composition in fur seals was validated by comparison with whole-carcass chemical analysis. Adult females had proportionately lower TBW and higher gross energy contents than pups. Pups had higher TBW to lean body mass ratios than adults indicating they had not yet reached chemical maturity. At all ages, there were significant differences in body composition between pup sexes. Whereas female pups were generally lighter than males, for any given mass they had significantly higher body lipid reserves. The amount of milk energy consumed by pups during the 1-2 day maternal attendance periods ranged from 49 - 68 MJ and increased steadily with age before decreasing significantly in the last 30 - 40 days before weaning. There were no significant differences in milk consumption between male or female pups. The amount consumed during these attendance periods was positively related to the duration of maternal absence such that daily energy consumption (on average 8.3 MJ) was independent of maternal foraging trip duration. Because pups must fast during the long maternal absences, the majority of the resources transferred to the pup are used for maintenance instead of growth. Analysis of the relationship between foraging behaviour and energy expenditure indicated that females undertaking short foraging trips had higher dive rates but lower metabolic rates than females making longer trips. However, females making longer trips gained more mass such that the energetic efficiency with which they gained mass was independent of foraging trip duration or diving behaviour. The total energetic cost of milk production and metabolism during the lactation period are estimated at 940 MJ and 2161 MJ, respectively. The total food energy consumed by individual adult female Antarctic fur seals at South Georgia during the lactation period is estimated at 3710 MJ.