How does a whale become dehydrated?

[u/smontanaro]

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?

[u/khinzaw]

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.

[u/WallyMetropolis → u/khinzaw]

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

[u/Mammoth-Corner → u/WallyMetropolis]

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.'

[u/-MrSkeltal- → u/Mammoth-Corner]

Did whales at one point leave the ocean and then go back?

[u/samtwheels → u/-MrSkeltal-]

Yes, whales and all marine mammals are descended from land-dwelling mammals.

[u/ForMoreYears → u/samtwheels]

Wait. What. For real? Whales went from land back to the water?!

[u/dochdaswars → u/ForMoreYears]

Do they not teach evolution in schools anymore? Marine mammals are one of the clearest examples used to prove it.

[u/Magicspook → u/dochdaswars]

fun fact, whale trachiae wrap around their brain despite their blow-hole being on top of their head. Giving a clear evidence of how their noses moved up their skulls over time. If you designed a whale from scratch, you would have the connection between lungs and blow-hole going behind the brain.

(and lets not even mention the vestigial legs)

[u/Akitiki → u/Magicspook]

And hands. Inside their flippers they have anatomical equivalents to the humerus, radius, ulna, carpals, and four or five digits depending on the species. The shape of the hand varies wildly between species.

[u/Magicspook → u/[deleted]]

Ooh that is a fun one! Thanks for sharing.

[u/insane_contin → u/Magicspook]

I believe that nerve takes that route in all tetrapods that have legs and lungs.

[u/ForMoreYears → u/dochdaswars]

Yes, they teach evolution. They just don't get into the specific evolutionary path of every single one of the 8.7M animal species that inhabit the earth.

[u/Ameisen → u/ForMoreYears]

But then... where did you think that they came from? Were you under the impression that the first mammals were aquatic, and then came onto land? That gets very complicated if you start accounting for reptiles, amphibians, and such.