I know that as a human being, I need to drink a fair amount of fresh water every day.  I'm curious about what marine mammals (whales, dolphins, seals) do for water.  Do they have a way of processing salt water?  Or do they need any kind of water at all?  If they do need fresh water, where do they get it?  Come to that, why do I need so much water?

Hi Jennifer,

Right, a few questions there!
Starting at question two, yes, they do need water!
Question four: why do you need so much water? Water is a vital part of the make up of living things, for lots of very important reasons. Suffice to say that water is a large component of protoplasm (the 'living' bits of chemicals inside all cells), it is an almost universal solvent (lots of things dissolve in it) and lots of other things including its density, thermal properties etc. Many of these can be found in standard biological texbooks.

On to your marine mammals...
The Cetacea and Pinnipedia (whales & dolphins and seals & sea lions) are secondarily marine, that is, they have moved from the land back into the water. As you know, life began in the sea and as such, some creatures evolved their bodily functions to be isotonic (having the same concentration of salt in the blood as there is in sea water), these include hagfish and the cartilaginous fish (sharks and skates etc). Bony fish (teleosts) like makerel are hypotonic, that is, their blood salt level is lower than that of seawater. Teleosts drink seawater but also gain the salts contained within in it. They deal with the excess salt by excreting it back into the water by using chloride cells in their gills. The control of water within living systems is called osmoregulation.

Marine mammals are also hypotonic; the main problem in these animals is gaining salts from their food and any seawater accidentally ingested whilst feeding. These animals are able to deal with this by having kidneys that can produce urine that has a higher salt concentration than the blood, which allows them to excrete excess salt in the urine. Whilst the kidneys of marine mammals are extremely efficient, they also minimize their salt gain by not drinking seawater. Instead, they get all their water from the food they eat. The efficiency of their kidney is reserved for producing highly concentrated urine, which allows them to conserve their water.

So, marine mammals have evolved a way to obtain water from the foods they eat and they have an extremely efficient way of concentrating the salts into the urine they expel.

I hope that this helps you understand how marine mammals survive without drinking fresh water.