The skate may provide that answer. The marine creature is a living fossil that provides a snapshot of what animals were like a very long time ago. It's also a fish that's isotonic with its environment, meaning concentrations of things like salt inside its body are in exact balance with concentrations outside its body (in the ocean it lives in). As a result, the skate is never thirsty or in danger of not getting enough to drink. But -- intriguingly -- it does have a rudimentary colon that soaks up water, as Theodosiou and her colleagues learned.