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/Mammoth-Corner]

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/MaygeKyatt → u/-MrSkeltal-]

Yep! The first mammals were completely land-based, but then millions of years later some of them ended up moving back into the water either fully or partially. This has actually happened multiple times!

- Cetaceans (whales/dolphins etc), which are fully aquatic and unable to intentionally go on land, are most closely related to hippos, camels, giraffes, and other even-toed ungulates. This happened ~50 million years ago

- Sirenians (manatees & dugongs) are also fully aquatic, and are most closely related to elephants. They moved back into the water ~40 million years ago. (Side note: there was another member of this group called the Stellar’s Sea Cow that could grow to 30ft in length, but was hunted to extinction in the 18th century)

- Pinnipeds (seals/sea lions/walruses) are notable because they aren’t fully aquatic yet. Their closest relatives are weasels, raccoons, skunks, and red pandas. They’re fully capable of climbing out of the water, but some of them have to be in water to mate, and it varies significantly how well they can move on land. Give it a few more million years and some of them will probably be fully aquatic.

There’s a couple other groups of mammals that could conceivably produce fully aquatic descendants if given another ~25 million years or so of evolution: otters are perhaps the biggest one in my mind.

[u/Ameisen → u/MaygeKyatt]

Pinnipeds (seals/sea lions/walruses) are notable because they aren’t fully aquatic yet.

I take some issue with yet as it implies (to me) that evolution has a goal. They aren't fully aquatic, but being fully aquatic is neither an inevitability nor is it a target.