J Comp Physiol B (1990) 160:537 542 Journal of Comparative ~"~" Systemic, and Environ- Physiology B mental Physiology 9 Springer-Verlag 1990 Circulatory water concentration in suckling and fasting northern elephant seal pups J. Margaret Castellini 1' *, Michael A. Castellini 1 and Maria B. Kretzmann 2 Institute of Marine Science, University of Alaska, Fairbanks, AK 99775, USA and 2 Long Marine Laboratory, University of California, Santa Cruz, CA 95060, USA Accepted June 8, 1990 Summary. This study examined circulatory water con- centrations in the neonatal northern elephant seal (Mi- rounga angustirostris) in order to determine how suckling and fasting would alter the percentage of water in the whole blood, plasma, and red blood cells (RBC). Plasma water concentration dropped by about 3 % during suck- ling and recovered about 1% during the fast (92.384-0.48% <1 week old, 90.154-0.36% weaning, 91.02 + 0.68 % end of fast). RBC water values during this time were more variable than plasma values: RBC water increased about 1% during the first 2 weeks of suckling (from 67.804-0.28% to 68.684-0.51%) but dropped to slightly below original neonatal values by weaning (67.154-0.63%). The first several weeks of fasting were marked by wide variability in RBC water, but by the end of the fast RBC water was comparable to that at wean- ing. These results indicate: 1) Northern elephant seal pups do not exhibit circulatory dehydration during 10 weeks of fasting; 2) Measurements of plasma or RBC metabolites (such as plasma glucose or RBC hemoglo- bin) may show variations or trends due not to metabolic regulation but rather to changes in circulatory water concentration. Key words: Marine mammals - Dehydration - Fasting - Blood chemistry Introduction Implicit in hematological studies of neonatal mammal development is that changes in circulatory metabolites are due to alterations in regulatory pathways and not to changes in circulatory water balance. For most neonates, the serious risks of dehydration (Guyton 1981) are offset by continual access to water through the mother's milk or by drinking. Northern elephant seal pups (Mirounga Abbreviations." Hb Hemoglobin; Het Hematocrit; MCHC Mean corpuscular; RBC Red blood cell; WB Whole blood * To whom offprint requests should be sent angustirostris), however, are weaned at about 25-30 days of age and then undergo an 8-11-week fast before leaving for sea. During this time on the beach they neither eat nor drink (Le Boeuf et al. 1972; Reiter et al. 1978). While it is generally assumed that the pups maintain an overall water balance as a result of their unusually high reliance on fat metabolism (Ortiz et al. 1978; Castellini et al. 1987), this has never been confirmed by actual measure- ments of circulatory water level during development. In fact, Wickham et al. (1989) have suggested that plasma osmolality may increase during the fast as a result of dehydration. It has been previously reported that hematocrit (Hct) alteration during apnea can significantly affect whole blood metabolite measurements in these seals (Castellini and Castellini 1989), but it is unknown if developmental changes in water concentration can also induce apparent alterations in metabolite concentrations. Northern ele- phant seal pups offer the unique opportunity to study circulatory water concentration during both suckling and post-weaning fasting under natural conditions. In this study the percentage of plasma, whole blood, and red blood cell (RBC) water was measured in a known popu- lation of northern elephant seal pups through suckling and 10 weeks of fasting. The observed perturbations in water concentration did not indicate circulatory de- hydration, but would affect metabolite measurements taken during the same period. The data suggest that analysis of blood metabolite concentrations in fasting species must also consider problems associated with changes in circulatory hydration state. Materials and methods Blood samples were collected from northern elephant seal pups using rear flipper venipuncture (Costa and Ortiz 1982) into heparin- ized Vacutainers (green top). Specificanimals were being studied at the seal rookery at Afio Nuevo, CA, USA, for developmental studies of body composition. Longitudinal samples were drawn approximately every 2 weeks starting when the pups were 2 days old and continuing until their departure to sea. The average weaning