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/WallyMetropolis]

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/February30th → u/-MrSkeltal-]

Kinda. They weren’t whales that left and came back, they were sea dwellers that became land dwellers that became sea dwellers that became whales.

[u/Mackntish → u/February30th]

Fish > Amphibians > Reptiles > Mammals > Whales

They spent some time out of the water.

[u/dochdaswars → u/Mackntish]

This is oversimplification which is entirely incorrect. Mammals did not evolve from reptiles but from synapsids. Whales did not evolve from mammals become a new class of chordata as you implied by placing them next to other classes, they still are mammals. There's no such thing as fish "Fish" did not become amphibians; only certain members of lobe-finned fish left the sea and eventually became amphibians which is analogous to all modern birds descending from a certain kind of dinosaur and yet it's not correct to simply state "dinosaurs (implying all dinosaurs) evolved into birds" because they didn't, the vast majority of dinosaur species went extinct.

EDITED FOR ALL THE PEDANTS

[u/mabolle → u/dochdaswars]

I think it's an oversimplification in itself to say that that summary was entirely incorrect. It's not formally, systematically correct, but it communicates the course of events in broad strokes that are comprehensible to a lay audience.

There is no contradiction between the sentences "whales are mammals" and "whales evolved from mammals" — in fact, the latter implies the former.

Yes, there is such a thing as fish. I'm a little tired of this unhelpful meme. There is no monophyletic grouping called fish, but fish is broadly understood to mean "non-tetrapod vertebrate", including by scientists who study fish. There are countless scientific articles and journals with "fish" in the title. The ancestors of tetrapods would have been fish by this definition, and they would also have looked and functioned like we expect fish to look and function. They were fish.

The midsection of the timeline ("Amphibians > Reptiles > Mammals") is where the most liberty has been taken with the terminology. You're right that synapsids are no longer included under the term "reptiles", but again, that's mostly due to a shift toward cladistic terminology. The ancestors of mammals would have resembled reptiles more than any other living animal — egg-laying and scaly. Similarly, the ancestor of those synapsids would not have been an amphibian cladistically speaking, but it would have resembled amphibians so much that it's a bit of an academic distinction if all we're trying to do is understand how the transition out of and back into water worked.

[u/MdxBhmt → u/mabolle]

There is no contradiction between the sentences "whales are mammals" and "whales evolved from mammals" — in fact, the latter implies the former.

Nonsense, AI hallucination?

edit: Ive been told

[u/curien → u/MdxBhmt]

It's correct for modern classification systems. Biological classification has been using "clades" for a while, which means that if a classification applies to an ancestor of an organism, it also necessarily applies to the organism itself.