Measuring the body composition of Antarctic fur seals (Arctocephalus gazella): validation of hydrogen isotope dilution

We examined the accuracy of hydrogen isotope dilution (tritium and deuterium) as a means of determining total body water (TBW), and hence body composition, by comparison with whole-carcass chemical analysis in five adult female and four pup Antarctic fur seals. On average, tritium isotope (HTO) dilution overestimated TBW by 1.92% ± 1.00% (n = 9). Deuterium isotope (D₂O) dilution also overestimated TBW (1.71% ± 1.74%, n = 5), but this trend was not significant. The following equations were developed for predicting TBW from hydrogen isotope dilution estimates: TBW (kg) = 0.11 + 0.97 · HTO space (kg) (r² = 0.999, n = 9, P < 0.0001) and TBW (kg) = 2.10 + 0.89 · D₂O space (kg) (r² = 0.980, n = 5, P < 0.001). Whole-carcass chemical analysis showed adult females were composed of 14.5% ± 1.0% total body lipid (TBL), whereas pups were composed of only 9.4% ± 1.0% lipid. Adult females had lower TBW contents (60.1% ± 0.7%) than pups (67.6% ± 1.2%) and, consequently, higher energy contents (adults, 10.59 ± 0.30 MJ · kg⁻¹; pups, 8.24 ± 0.41 MJ · kg⁻¹). Pups had higher ratios of TBW to lean body mass (LBM) (0.747 ± 0.009) than adults (0.707 ± 0.004), which indicates they had not yet reached chemical maturity. The following best-fit predictive equations based on TBW and body mass (BM) were developed for TBL, total body protein (TBP), total body ash (TBA), and total body gross energy (TBGE): TBL (kg) = 0.875 · BM (kg) - 1.187 · TBW (kg) - 0.633 (adults); TBL (%) = 66562 - 0.845 · TBW (%) (pups); TBP (kg) = 0.058 · BM (kg) + 0.209 · TBW (kg) + 0.996 (adults); TBP (kg) = 0.267 · BM (kg) - 0.130 · STBW (kg) + 0.087 (pups); TBA (kg) = 0.071 · TBW (kg) - 0.088 (adults and pups); TBGE (MJ · kg⁻¹) = 34.1 - 0.391 · TBW (%) (adults); TBGE (MJ · kg⁻¹) = 31. 7 - 0.347 · TBW (%) (pups). With the use of these predictive equations, hydrogen isotope dilution is an accurate means of determining body composition in live Antarctic fur seals. Differences in these relationships between adults and pups, and in comparison to those in phocids, emphasize the need for the use of equations derived from appropriate data sets when predicting body composition in pinnipeds.