Estimation of total body water in pinnipeds using hydrogen-isotope dilution.

gen-isotope dilution space in pinnipeds and to develop a single Hydrogen-isotope dilution methods have been used widely to predictive equation to estimate TBW in species for which data study the body composition, milk intake, solid-food intake, are lacking. Adopting such an approach will provide a consisand energy metabolism of pinnipeds (Costa 1987; Oftedal and tent basis for comparative studies not only within pinnipeds Iverson 1987) and other vertebrates (Nagy 1987). Indeed, much but in relation to other mammalian taxa. of the current understanding of water balance and of the reproIn the Otariidae (i.e., fur seals and sea lions), only one study ductive and foraging energetics of large mammals comes from has compared hydrogen-isotope dilution space with TBW destudies of this group of species. An important assumption in rived from carcass analysis of the same individuals. Arnould the use of isotope dilution for these purposes is that the tracer et al. (1996) examined the accuracy of both HTO and D2O as mixes completely, and only, with total body water (TBW) of a means of estimating TBW in four pups and five adult female an animal that has been given a known quantity of deuterium Antarctic fur seals (Arctocephalus gazella). HTO dilution space oxide (D2O) or tritiated water (HTO). It has been known for significantly overestimated TBW of pup and adult females, by some time that a small fraction of hydrogen isotope is lost to an average of 1.9% { 1.00% (n A 9). D2O dilution space also rapidly exchangeable hydrogen atoms in organic constituents overestimated TBW of adult females by an average of 1.7% of the body (see, e.g., Ussing 1935). Hevesy and Jacobsen { 1.74% (n A 5), but this difference was not significantly (1940) estimated that 0.5%–2% of body mass is accounted for different than zero. Using these data, Arnould et al. (1996) by rapidly exchanging hydrogen atoms of isotopes in organic derived predictive regression equations to estimate TBW from compounds. For this reason, hydrogen-isotope dilution space either HTO or D2O dilution space. However, in the case of will usually overestimate TBW, and it may be necessary to apply HTO, the slope of the regression did not differ significantly an appropriate correction factor to dilution space. Although from 1.0, and the intercept did not differ from zero. It is relationships between TBW and hydrogen-isotope dilution therefore not clear that the use of this relationship would imspaces have been studied in other mammalian groups (see, prove the estimation of TBW compared to the use of estimates e.g., Nagy and Costa 1980), these relationships in pinnipeds of HTO dilution space. However, given the small sample size, are not well understood. failure to reject the null hypothesis that the slope equals 1.0 The only true validation of isotope-dilution space estimates may have simply reflected the low statistical power of the test. of TBW depends on measurement by isotope dilution followed In contrast to the study in otariids, hydrogen-isotope diluby carcass desiccation of the same individual. This has been tion space has been compared with carcass desiccation in four done in a number of taxa, and the generally close corresponspecies of the Phocidae (i.e., hair seals). Reilly and Fedak (1990) dence between estimated dilution space and TBW measured by administered both D2O and HTO to four grey seals (Halidesiccation indicates that isotope dilution is a valid technique choerus grypus; two pups and two adults) that were subse(Pinson 1952; Nagy and Costa 1980). However, recent practice quently desiccated. Isotope dilution overestimated TBW in all