Science Questions

How do marine mammals control their salt intake

Sun, 2nd Aug 2009

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Question

Miguel Navarro asked:

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. 

seaSo 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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Miguel Navarro asked the Naked Scientists: Hi Dr. Chris and everyone at The Naked Scientists!! Got another little question for you, this one about marine biology. I was wondering how do marine mammals (and all sea animals for that matter) like whales and dolphins manage to get rid of the extra salt in the water that they drink, if they actually drink any. I heard that if you drink sea water it actually dehydrates you because the salt in the water will trigger osmosis at the cellular level or something like that. So how do they manage to survive this? By the way, things are looking better now here in Mexico with the H1N1 flu virus, probably the economic effects will be the worse part besides from the loss of lives of course. That's all from your fans here in Mexico, keep up the good work! Miguel Navarro Gamboa What do you think? Miguel Navarro , Fri, 22nd May 2009

Whales don't drink the salt water (for the most part), instead they get their supply from the organisms they eat. Some of their food (eg the plankton favourite dish of baleen whales) has a very high salt content though the whales kidneys are many times more effective than our own and they can produce urine with a much higher concentration than sea water itself. The more concentrated the urine the greater the amount of freshwater the animal can gain.
The amount of water needed for a marine mammal is probably less than one imagines since they have very little to no 'sweat' loss. There is some respiratory evaporative loss, but whales employ what is called apneustic breathing. Which basically means they breathe periodically. This allows for a greater O2 exchange and lower respiratory evaporation.

Most fish drink the salt water and eliminate the salt through their gills.

HTH JnA, Mon, 8th Jun 2009

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