Endpoints; July 2001; Scientific American Magazine; by Staff Editors; 1 Page(s)

Although some marine mammals are known to drink seawater at least on occasion, it is not well established that they routinely do so. They have other options: sea-dwelling mammals can get water through their food, and they can produce it internally from the metabolic breakdown of food.

The salt content of the blood and other bodily fluids of marine mammals is not very different from that of terrestrial mammals or any other vertebrates: it is about one third as salty as seawater. Because a vertebrate that drinks seawater is imbibing something three times saltier than its blood, it must get rid of the excess salt by producing very salty urine. In the seal and sea lion species, for which measurements exist, the animals' urine contains up to two and a half times more salt than seawater does.