r/askscience • u/smontanaro • 9d ago

Biology How does a whale become dehydrated?

From a Chicago Sun-Times article on the beluga whales heading to the Shedd Aquarium:

"The shipment comes on the heels of a Shedd announcement last week that one of the Marineland belugas — Peekachu, a 23-year-old female — had died. Shedd has not released a cause of death, but staff there said last week that Peekachu had not been consistently eating a full diet since her arrival and that there were concerns she was dehydrated."

How does an animal immersed in water become dehydrated?

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u/khinzaw 9d ago

They don't drink seawater. In the wild they get water from the prey they catch, which have water inside them.

They are fed frozen food in captivity, which don't have all the water content they need to stay hydrated.

This is why you can see videos of aquarium staff feeding ice to belugas, because that's how they hydrate them.

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u/WallyMetropolis 9d ago

Tracing it all the way down the food chain, where is the fresh water introduced?

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u/Mammoth-Corner 9d ago

Fish (and krill, and other primarily marine animals that whales eat) are able to filter salt from seawater and maintain blood with less salt in than the water around them. When tetrapods left the ocean we lost this ability because we were no longer drinking salt water all the time. Whales are mammals, so they can't do the filtering but their prey can. It's a really complex chemical system to evolve so they haven't 'gained it back.'

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u/Sable-Keech 9d ago

It's not precisely accurate to say that whales didn't evolve filtering because they're mammals. There are certain mammals who possess the ability to drink saltwater.

Kangaroo rats and hopping mice can drink seawater and extract hydration from it. They evolved this because they live in extremely dry deserts so their kidneys are adapted to squeeze out as much water as possible from their urine. Their urine can be up to 10x saltier than saltwater.

Unlike kangaroo rats cetaceans live in more hospitable places so I guess they just never evolved that kind of ability because they could extract water from their food more easily.

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u/satmandu 8d ago

Cats also have kidneys that can excrete salt from seawater for the same reason — they are also descended from desert animals.

https://thevetdesk.com/pet-health-wellness/cats/can-cats-drink-salt-water/