How do marine mammals control their salt intake?

02 August 2009

Question

If you drink sea water it dehydrates you because the salt in the water triggers osmosis at cellular level, et cetera. So you have to get the salt out of your body, lose some water to compensate. How do animals like marine mammals that are therefore eating a lot of salt in their diet quite naturally, how do they compensate? How do they get round that?

Answer

Helen - Well, actually they don't. As far as we can see, they don't drink that much seawater at all. In fact most whales and dolphins and things like that really don't drink seawater at all. They get most of the water they need in their diet from their food because lots of fish are about 60 or 80 percent water. And you can also get lots of water from metabolism, from oxidizing fat, and they have those blubbery layers and it can actually provide them with water. So half the time, they actually keep their mouths shut, but, of course, some does get in when they're eating their food and so on. And sea otters apparently do drink quite a bit of seawater and that could possibly be because they actually eat lots of invertebrates, sea urchins or things like that. They're quite salty and high in protein which means they create lots of urea. To process that salt and urea nitrogen does take a lot of water, So they do drink lots of water and one of the keys, really it comes down to their kidneys. They've got a very different type of kidney to most land animals in that if you look at the human kidney (or most mammals have kind of a kidney-shaped kidney - a sort of single lump with various things going inside) whereas, in fact, cetaceans and seals and things have lots of little bits to their kidneys. They're very complex structures with lobes - thousands of lobes and each one of those is an individual kidney, if you like. So they are actually able to concentrate the fluids in their urine to be stronger than seawater. So they do have that ability but they don't necessarily do it all the time, which is actually quite surprising that they don't have to do that. They actually just don't have that much salt in their system in the first place.Chris - Because one thing that is very often not apparent until perhaps you read a biochemistry book or something is that metabolism itself produces a lot of water. So when you burn sugars, you release enormous amounts of water anyway. So, therefore, some animals are very good at using all that water so their obligatory need to drink is quiet low.Helen - Absolutely and they think when certain seals and sea lions actually go through long periods when they don't eat and they're very much relying on their metabolism and their blubber at that point to provide them with enough water.

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