Sea Water Drinking and Water Flux in Starved and in Fed Harbor Seals, Phoca vitulina

Abstract

Harbor seals maintained in sea water were either starved or fed graded amounts of herring of known water and total chloride contents. Total body water and exchangeable chloride in the seals were measured by initial dilution of injected tritiated water and Na36Cl. The decline in specific activities of plasma water and chloride was then followed for 12 to 14 days. Average daily fluxes of water and Cl− were calculated. Sea water ingestion was calculated from the daily chloride flux and the chloride contents of the food and sea water. In five animals fed from 0 to 1500 g herring/day, total water flux was linearly related to food intake and ranged from 500 to 1590 ml water per day. In the same animals ingested sea water and metabolic plus inspired water were also linearly related to food intake, and ranged respectively from 35 to 140 ml and 470 to 625 ml water per day. The data provide definitive evidence that starving Harbor seals derive sufficient oxidative water to satisfy the major part of their needs and that they drink very little sea water. In feeding seals the sum of preformed water and metabolic plus inspired water accounts for about 90% of the total water flux, which is comparable in magnitude to that of most terrestrial mammals. The small volume of sea water ingested by Harbor seals, and its linear relationship to food intake, show compellingly that sea water ingestion is coincident to swallowing food under water rather than due to deliberate drinking.