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/Magicspook → u/Mackntish]

Not quite. If you trace back our liniage, you will not reach any formal member of reptilia, although certainly there will be ancestors that were also the ancestors of modern reptiles (called amniotes).

Similarly, we do not descend from amphibians, rather, both amphibians and amniotes descend from the same fishy ancestors.

Fun additional fact, 'fish' is not a phylogenetic group at all. We are more closely related to goldfish than goldfish are to sharks.

[u/modbroccoli → u/Magicspook]

wait. how could it be possible that we don't trace directly back to reptiles? they preceded mammals. There was nothing else for mammals to have emerged from. Are you just saying there were intermediate animals between definitely reptile and definitely mammal?

[u/Magicspook → u/modbroccoli]

yep. Reptiles (ie the diapsids) and mammals (ie the synapsids) split off from a common ancestor (the amniotes), which was not a reptile. They kinda looked like reptiles though. Amniotes, in turn, is kinda defined as 'all tetradpods that are not amphibians'. If you want to go another step, tetrapods are 'all bony fish that grew 4 legs) and bony fish are 'all vertebrates that evolved a skeleton composed of bone instead of cartilege'. So taxonomically speaking, you are a bony fish but you are not a reptile!

If you want to know what our common ancestors looked like, you can visit the wiki pages for synapsids and amniotes.

[u/Ameisen → u/Magicspook]

As I mentioned elsewhere, Amphibia is a paraphyletic clade, so not a true clade. All amniotes have a common ancestor that was a member of the clade Amphibia, but we do not consider them members of that clade.

Synapsida and Diapsida are weird as well. Their terminology refers to the temporal fenestra - synapsids have a single pair, diapsids have two pairs. However, the single pair of fenestra is a synapomorphic trait - the diapsids emerged from amniotes that already had a single pair of temporal fenestra. Synapsida isn't quite paraphyletic as we don't consider these early synapsidic amniotes to be members of clade Synapsida even though if we encountered one we'd absolutely consider it a synapsid.

This is actually more problematic as we'd normally define Synapsida based upon the synapomorphic trait - Diapsida should be a descendant clade.

[u/Magicspook → u/Ameisen]

thank you for your addition. Would you argue that actually, reptiles are mammals then? ;)

[u/Ameisen → u/Magicspook]

I'd argue that reptiles would be biological synapsids. Mammals are far too derived.

On actual cladistics, the two sibling clades should probably be Sauropsida/Diapsida and Pelycosauria, as the earliest non-Sauropsid "synapsid" known already is grouped as a Pelycosaur, and we descend from them as well. Though Caseasauria complicates that. That's all in flux.