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How do sea mammals stay hydrated if sea water is not drinkable due to its high salt content?

One of our readers asked us the other day, How do sea mammals stay hydrated if sea water is not drinkable due to its high salt content?

The Answer:

Most sea mammals get their water from the food they eat. Their bodies are designed to absorb water using the metabolic breakdown of carbohydrates and fat from digestion. Sea mammals are almost always carnivores and avoid food with a high salt level. Despite their habitat, the salt level of fish that sea mammals eat are two thirds less than the salt levels of the ocean. Scientists hypothesize that animals such as squid, octopus and fish supply sea mammals with most – if not all – the water they need.

Most seafaring mammals are known to consume at least some salt water. Their kidneys pick up the extra work to process the salt from their bodies. Too much salt would lead to dehydration and death, so their bodies have to expel excess and filter it out of the body. Salt water is managed via the mammal’s kidneys in two steps. The glomerulus is a filtration part of the kidneys that catches excess salt in the blood. Large blood cells and molecules are separated from plasma, water, and salts. This filtered mix then passes through a long tube called the loop of Henle. The walls of the Henle absorb the water back into the blood while the remaining fluid and salt are urinated out of the body. Research has shown that this process in manatees and porpoises results in a highly concentrated salty urine.

There is much that scientists don’t know about how sea mammals process salt. Dolphins and whales are the creatures most shrouded in mystery. Because they are usually found in remote areas of the ocean, they are notoriously difficult to document. Many scientists have hypothesized that dolphins have an advanced filtration system. Just like other mammals, they would suffer from a myriad of liver and kidney problems without fresh water. Seals, on the other hand, have been documented eating snow. Snow contains no salt and is their primary source of freshwater. There are a few ocean mammals that survive by only hunting in brackish water. Manatees will search out fresh water and drink from diluted sea water. Meanwhile, it has been proven in California, that Sea Lions can survive on a diet consisting only of fish.

Although Albatrosses don’t live in the water, they spend a large amount of time drifting around in the ocean. Far from freshwater, their bodies have developed a unique way to deal with drinking salt water. Albatrosses have dedicated glands near their eye sockets that process the extra salt. They produce a highly concentrated salty liquid and it leaks out through the albatross’ beak.

The salt content of the ocean poses an interesting conundrum for the animals that live there. They have evolved to filter the salt with special glands, elevated kidney functions and a selective carnivorous diet that is low in sodium.

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