Two captive female common dolphins (59 kg and 55 kg) were given intravenous injections of tritiated water (HTO) and sodium-22 (Na-22) after the first day of a 5-day fast. Blood samples were collected each of the next 4 days. Total body water was 37% of total body weight, and the mean rate of water turnover was 77 ml∙kg⁻¹day⁻¹. This low value for total body water of a mammal is due to the high blubber content of the body. Exchangeable sodium levels were 37 and 40 meq∙kg⁻¹, with turnover rate of 6 meq∙kg⁻¹ day⁻¹. Skin was shown to be impermeable to sodium, so the only sodium source for these animals was seawater. Seawater drunk at a rate of 12 and 13 ml∙ kg⁻¹day⁻¹ accounted for the observed sodium turnover and about 1/6 of the total water turnover. Estimated water turnover from food and seawater consumption is the same as total water turnover predicted for terrestrial mammals of the same weight. However, the low body water content of these dolphins indicates that they have a high fractional rate of water turnover. Most of the body water turnover is due to the permeability of the skin. The rapid fractional water turnover, seawater consumption, and dilute urine indicate that conservation is not a major aspect of the fasting dolphin's water budget. Feeding dolphins, however, may benefit from the urinary osmotic space that seawater provides for the urea produced by protein catabolism.
