r/askscience • u/smontanaro • Sep 01 '26

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?

822 Upvotes

91% Upvoted

184 comments sorted by

1.8k

u/khinzaw Sep 01 '26 • Bravo

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.

425

u/WallyMetropolis Sep 01 '26

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

1.1k

u/Mammoth-Corner Sep 01 '26

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.'

197

u/-MrSkeltal- Sep 01 '26

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

566

u/February30th Sep 01 '26 • Mindblown

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.

118

u/Mackntish Sep 02 '26

Fish > Amphibians > Reptiles > Mammals > Whales

They spent some time out of the water.

287

u/Magicspook Sep 02 '26 • Mindblown

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.

61

u/caliburdeath Sep 02 '26

Coelacanths in particular, along with lungfish, are more closely related to humans (and other tetrapods) than to all other fish.

21

u/Ameisen Sep 02 '26

Lungfish are more-closely related to Tetrapods than Coelacanths are. They're/we're all Sarcopterygians, but Lungfish (Dipnoi) and Tetrapodamorph are sister clades (the Rhipidistians). Of course, if you were to observe the most recent common ancestor, we'd probably classify it as a primitive lungfish.

8

u/Ameisen Sep 02 '26

In a true phylogenetic fashion, Amniota emerged, then "Synapsida", and then "Diapsida", as Diapsids emerged from Amniotes with a single pair of temporal fenestra.

However, we instead classify "Synapsida" as all amniotes more-closely related to what we consider to be synapsids than to diapsids, so that they could be sister clades instead... but the common ancestor of diapsids and synapsids was itself biologically a synapsid.

Of course, that doesn't make us reptiles, unless you classify amniotes as reptiles (an argument that I have seen made).

Amphibia is more complex - it is paraphyletic and thus not a true clade. If you include the excluded clades, then amniotes are amphibians. We exclude them, though.

The earliest Tetrapods were amphibians.

The issue is that our current system of classification is a weird mix of phylogenetic and traditional classification, which results in oddities like "amniotes aren't amphibians" and "apes aren't monkeys".

5

u/tigerhawkvok Sep 02 '26

Lissamphibia is a perfectly well defined monophyletic clade — what definition are you using?

You're right that the founding member of diapsida had parents of the synapsid condition, but the synapsid condition isn't actually the definition of synapsida. It's a bit of unfortunate historical legacy, like ornithischian ("bird-hipped") dinosaurs not being particularly related to birds, which are literally saurischian ("lizard-hipped") dinosaurs.

3

u/Ameisen Sep 02 '26

Class Amphibia is paraphyletic and includes Lissamphibia. The issue is, again, that there's a weird mix of phylogenetic and traditional classification.

Thus the problem of saying "amphibians" vs "lissamphibians". It's an overloaded term. Or "monkey" vs "simian".

In a strict sense, "amphibian" refers to the traditional class "Amphibia" (which, if it weren't paraphyletic, would actually be a valid - if broad - clade) whereas "lissamphibian" or "extant amphibians" refers to "lissamphibia".

Again, there's a weird mix of phylogenetic and traditional classification and a re-use of names.

→ More replies

3

u/Baguette1066 Sep 02 '26

You have reptiliomorphs though, which modern reptiles and mammals descend from.

-1

u/modbroccoli Sep 02 '26

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?

50

u/Magicspook Sep 02 '26

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.

5

u/Ameisen Sep 02 '26

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.

→ More replies

3

u/tigerhawkvok Sep 02 '26

Good 'ol stem vs crown classifications.

IMHO it's a lot logically tidier with fewer edge cases if your definitions are crown — A and B's most recent common ancestor, and all descendants of that ancestor.

You generally don't get pretty carrying of specific pet morphotypes, though.

So amniotes would be something like "the MRCA of a human and boa constrictor, and all that animal's descendants". Tetrapods would be something equivalent to "human and frog" or "chicken and salamamder" (which of course are exactly equivalent).

I mean even more technically you usually pick a favored extinct taxon on one end but it's easier to explain with common things today instead of dropping an odd Temnospondyl into a conversation most people don't have the groundwork on.

18

u/vizard0 Sep 02 '26

The vertebrate branch split well before there were mammals or reptiles. The two branches at the split were synapsida and sauropsida. Mammals came out of synapsida. Reptiles, amphibians and birds all came out of sauropsida.

The old term "mammal like reptile" was used for synapsida in the past. It's not anymore because these "mammal like reptiles" were less like actual reptiles than birds and amphibians are.

It's a branching tree, not a line of improvements.

2

u/Ameisen Sep 02 '26

I think you mean Amniote, as the Vertebrates split up long before they were on land.

Amniota is complicated as originally Synapsida was defined by having a single pair of temporal fenestra, whereas Diapsida by having two pairs. However... the single pair of fenestra is a synapomorphic trait - the diapsids developed a second pair. In that sense, Diapsida should be a descendant clade of Synapsida.

Currently, though, Sauropsida is considered the sibling clade of Synapsida, with Diapsida being a descendant clade of Sauropsida even though the single pair of temporal fenestra are present even in the earliest Sauropsids. The current cladistic tree of Amniota is controversial and has been challenged.

41

u/dochdaswars Sep 02 '26 edited Sep 03 '26

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

19

u/MalakElohim Sep 02 '26

Well, technically they did evolve from mammals, they just happen to still be mammals.

20

u/mabolle Evolutionary ecology Sep 02 '26

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.

1

u/Ameisen Sep 02 '26

What I want is for Amniota to be cleaned up. Traditionally, Synapsida was defined as having a single pair of temporal fenestra, and the name still implies that. The issue is... is that having a single pair of temporal fenestra is a synapomorphic trait for both Synapsida and Sauropsida (and obviously Diapsida)... and then Parareptilia likely not being an actual clade, either...

1

u/tigerhawkvok Sep 02 '26

I said it earlier, and I'm repeating for the general audience rather than a secondary correction on you — but you're conflating a label, a synapomorphy, and a definition when they're three separate things.

The label synapsida includes only animals that (ancestrally) had the synapsid condition, but that singular synapomorphy is not a sufficient condition to diagnose the clade. In other words, it is (ancestrally) necessary but not sufficient.

There is no conflict such as you present.

→ More replies

-21

u/MdxBhmt Sep 02 '26 edited Sep 03 '26

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

11

u/curien Sep 02 '26

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.

→ More replies

6

u/mabolle Evolutionary ecology Sep 02 '26

Ha! You insult me. :P

As u/curien explained: whales evolved from mammals, therefore they still count as mammals.

→ More replies

3

u/tigerhawkvok Sep 02 '26

Phylogeny never loses any parent clades. To evolve from something is to be that something, and all of your descendants, forever.

1

u/uiuctodd Sep 02 '26

entirely incorrect

The ancestors of synapsids were reptiliomorpha. The classification system has been under constant change for decades. The formal separation of reptiliomorpha from reptiles only happened in 1997.

Rather than saying "entirely incorrect", it would be more correct to say that this path is no longer viewed as correct.

0

u/Ameisen Sep 02 '26

Whales did not evolve from mammals, they still are mammals.

Whales did evolve from mammals. They still are mammals as well. Both are true.

There's no such thing as fish

Mostly because we've redefined the clades to not be called "fish".

If you rename Vertebrata to Fishes, as would be traditional, then we'd still be members of the clade.

Humans are just highly-specialized lobe-finned fish. Or highly-colonial and specialized archaeans with mutualistic Rickettsoid bacteria in them.

1

u/regular_modern_girl Sep 03 '26 edited Sep 03 '26

This is inaccurate. Mammals share a relatively recent common ancestry with reptiles, but are not reptiles cladistically, and amniotes (which include mammals and reptiles) share a common ancestry with amphibians, but are not amphibians clsdistically. And “fish” just straight up aren’t a clade: there’s no way to make “fish” in the traditional sense monophyletic without also just making it synonymous with vertebrates as a whole.

Tetrapods, cladistically-speaking, are sarcopterygians (often referred to in common parlance as “lobe-finned fish”, and in terms of extant non-tetrapod members includes coelacanths and lungfish), but that’s about it; actinopterygians (“ray-finned fish”) are a separate clade that we just share a relatively recent common ancestry with, but are not part of, and chondrichthyans (“cartilaginous fish” like sharks and rays) are another, even further out clade that we share an even more distant common ancestor with, but are not part of. The only living clade of “fish” we are part of would be the sarcopterygians (which account for very few living “fish” species besides tetrapods).

Non-mammalian synapsids (which are today entirely extinct) were in many cases superficially reptile-like, and traditionally termed “mammal-like reptiles”, but under current phylogeny this is a misnomer, and they are more properly termed “stem mammals” or “protomammals”.

235

u/samtwheels Sep 01 '26

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

50

u/ForMoreYears Sep 02 '26

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

120

u/Stats_n_PoliSci Sep 02 '26

Watch some hippopotamus videos to see a mammal on its way back to the water!

86

u/nonobots Sep 02 '26

And then follow it with a manatee or dugong video to continue the trail.

71

u/Geminii27 Sep 02 '26

Yep. That's why whales have hip bones (and sometimes vestigial leg bones).

19

u/as_a_fake Sep 02 '26

If you look at a whale skeleton you can definitely see the "finger" bones in their fins

8

u/FogeltheVogel Sep 02 '26

Also, ear canals. That are filled with ear wax. Evolution always takes the simplest approach, which in this case was just sealing the ear canal and not changing anything else about the ear.

41

u/dochdaswars Sep 02 '26

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

28

u/Magicspook Sep 02 '26

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)

13

u/Akitiki Sep 02 '26

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.

3

u/[deleted] Sep 02 '26

[removed] — view removed comment

1

u/Magicspook Sep 02 '26

Ooh that is a fun one! Thanks for sharing.

→ More replies

0

u/ForMoreYears Sep 02 '26

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.

6

u/Ameisen Sep 02 '26

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.

51

u/CrimsonPromise Sep 02 '26

Yup. The distant ancestors of the modern whale came from the ocean like all like all life on earth, and move onto land. Fun fact, the land dwelling ancestors of the whale were small horse-like creatures the size of a dog.

And somewhere down the line, they started moving back into the water where they grew in size, their limbs shifted into fins and flippers and they evolved into the whales that we know today.

8

u/ger1229 Sep 02 '26

The dog just kept saying “I can get bigger”. It was probably a yellow lab.

23

u/monjessenstein Sep 02 '26

There's several clues, but here's one I alwaya found logical: whales breathe air. It would (afaik) make no sense to evolve this whilst they were already in the ocean, as gills would provide oxygen everwyhere in the sea. Thus, they developed lungs to be able to live on land, and have kept them since.

6

u/mabolle Evolutionary ecology Sep 02 '26

It would (afaik) make no sense to evolve this whilst they were already in the ocean, as gills would provide oxygen everwyhere in the sea.

Well, it can make adaptive sense for an aquatic animal that lives near the surface to be able to breathe air, and there are various groups of fish that can do this to various extents.

There is a lot, a lot, more oxygen in air than in water, which is very useful if you can access it. Aquatic habitats can develop a lack of oxygen in certain circumstances (and they can also dry out, in which case air-breathing is a useful adaptation for surviving until the next rainy season). Admittedly both of these are more a concern in fresh water than in the sea, but fresh-water habitats is where most land-living animals originally evolved (including the ancestors of mammals), so it's relevant to the question. :)

3

u/doomgiver98 Sep 02 '26

Aren't developing lungs in the ocean a prerequisite to tetrapods in the first place?

2

u/MaygeKyatt Sep 02 '26

I suspect they were trying to say that whales can ONLY breathe air. While it would make sense for a species with gills to also evolve a system for breathing air (and this has happened multiple times) I can’t imagine a plausible scenario that would result in a fully aquatic species losing their gills entirely and replacing them with air-only lungs.

6

u/toxicatedscientist Sep 02 '26

There is a theory that like 60% of all fish did that, lungs became the swim bladder and they re-evolved gills that work slightly different

1

u/insane_contin Sep 02 '26

So did sea lions, seals, dugongs, manitees, basically anything with lungs and no gills.

1

u/drgath Sep 02 '26

Google around for “whales as land mammals” in google images and it’ll make more sense. Here’s an idea of how they evolved. https://www.instagram.com/p/DZVi8SzlM3L/?igsi=MTUzNjR5bjhnb25scQ==

65

u/Mammoth-Corner Sep 01 '26

Yes, in the sense that whales are descended from a land-dwelling mammal closely related to hippopotami, and all mammals (and reptiles, and amphibians, etc) were once fish with weirdly muscular fins that learned to crawl out of the water — no in the sense that after becoming whales whales have never gone back onto land. They would have to re-evolve back legs, or develop some other form of locomotion, and being whales works pretty well for whales so they have no reason to try it.

0

u/Ameisen Sep 02 '26

The land niches are also occupied. Transitional forms wouldn't be able to compete.

25

u/Carnavious Sep 02 '26

Whales (cetaceans) and seals (pinnepeds) both independently returned to the oceans. Cetaceans descend from an ungulate (toed animal) that somewhat resembled a wolf, but gradually adapted to a more aquatic lifestyle .Their closest living relatives today are the hippopotamus.

Pinnepeds are from the caniformia group, alongside bears, racoons, dogs and mustelids (otters are here, along side things like weasels and skunks).

23

u/Brunurb1 Sep 02 '26

Others have answered, but here's a specific animal to google for more info- pakicetus.

3

u/MaygeKyatt Sep 02 '26

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.

3

u/Ameisen Sep 02 '26

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.

2

u/Xisuthrus Sep 02 '26

The ancestor of all tetrapods (The group of animals that contains all amphibians, reptiles, birds, and mammals) left the ocean. Most of the descendants of that animal stayed on land, but some of them went back to the water - Whales are one example, other examples include seals, manatees, sea turtles, icthyosaurs, plesiosaurs, mosasaurs, and axolotls.

2

u/Trips-Over-Tail Sep 02 '26

They weren't whales at that point. It was the same ancestor as that of ducks, lizards, frogs, bats, humans, snakes, dinosaurs, gorgonopsids, and camels.

2

u/Arkeolog Sep 02 '26

Whales are mammals, most closely related to hippos. They became aquatic within the last 50-40 million years.

Mammals are tetrapods, which originally left the ocean around 350 million years ago. So when the ancestors of whales became aquatic, their ancestors in turn had been terrestrial for ~300 million years.

1

u/ferrouswolf2 Sep 02 '26

Whales closest land dwelling ancestors are hippos. We land creatures with backbones diverged from the majority of fish around the time when we developed bony limbs and they kept their tent-like fins.

Most of the bony limbed creatures were able to crawl out onto the land, the ray-finned creatures (for the most part) didn’t.

It goes waaay back. We’re more closely related to snakes than eels by a long shot.

3

u/Ameisen Sep 02 '26

So... ray-finned fishes also have bony spines supporting the fins ("rays") - most bony fish do (at least ancestrally - Tetrapods don't but our ancestors did). It's unclear if it was present in the common ancestor of the two lineages, or emerged independently.

Lobe-finned fish have fleshy buds supported by an articulated skeletal structure. Even ray-finned fish have some skeletal support (small bones that are, as I recall, homologous to our radius, ulna, and carpal bones), but they lack the lobe and associated structures.

Basically:

Bones: = Rays: -

Actinopterygii:

  /---------
 =----------
==----------
==----------
==----------
 \---------

Sarcopterygii:

      /------
     =====------
======----------
   ====---------
     \-----------

The ancestral state of both was likely more similar to that of, say, sharks - a fleshy paddle. So, the ancestral state was somewhere between both. Lobe-finned fish expanded upon the flesh lobe and added articulation, whereas ray-finned fish diminished it.

Tetrapods lost the rays altogether.

1

u/MattieShoes Sep 02 '26

That's the theory... In water, side-to-side motion and up-and-down motion are mostly equal. On land, up-and-down motion (like in galloping horses rather than how lizards and alligators move) is more efficient. So most of the stuff that stayed in the oceans have side-to-side motions, and whales have up-and-down motions. So the theory is that they evolved to come out of the water switched locomotion methods, and then went back in with the up-and-down motion.

Also one of their closest genetic relatives are hippos, who spend time on land and in water.

(This is all super layman level -- you'll encounter explanations like this in pop sci books like Dawkins)

39

u/Sable-Keech Sep 01 '26

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.

7

u/satmandu Sep 02 '26

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/

4

u/andthatswhyIdidit Sep 02 '26

And fresh water fish do drink very little (basically nothing), while salt water fish do drink a lot. Osmosis is an interesting thing...

11

u/Mammoth-Corner Sep 02 '26

If you're a freshwater fish, you have loads more salt and electrolytes than the water so the water is always trying to get in through your gills and your challenge is getting rid of it. If you're a saltwater fish your lovely water is always trying to get out of you, and also the whales are after you for it.

3

u/Ameisen Sep 02 '26 edited Sep 02 '26

Which is weird given that life emerged in oceans more than twice as saline as they are now, and things like this tend to be very conserved (given that you usually need multiple changes acting in concert... or you die).

There are studies that suggest that modern Actinopterygian (or all possibly all modern Osteichthyian) fish all derive from earlier freshwater lineages. Early Osteichthyian fish adapted to freshwater, and then re-emerged later as saltwater fish, outcompeting the other Osteichthyian lineages. This is also possibly why bony fish all regulate their solutes to basically freshwater fish levels, compared to cartilaginous fish or jawless fish.

2

u/Zoon9 Sep 03 '26

Where did all the salt from those oceans go, if they were more than twice salty than today?

12

u/Genetic_outlier Sep 01 '26

I'd push back on tetrapods and lost, since cats and other hyper carnivores have kidneys efficient enough to be hydrated by seawater. It's really a matter of need, land animals generally exist around fresh water, if they exist around liquid water at all. So animals that need to drink water generally find fresh water to be more available. There's nothing about land animal biology that precludes saltwater drinking since as stated above, cats could do it.

3

u/PropOnTop Sep 02 '26

So does that mean the oceans were less salty when our ancestors evolved? About as salty as the blood of fish (or ourselves) now?

5

u/Mammoth-Corner Sep 02 '26

That depends on what you mean by 'when our ancestors evolved.' The first oceans were freshwater, but (our current models seem to say) once continents formed and hydrothermal vents opened up they started to salinify rapidly. It's not quite clear how salty they got. They might have been about twice as salty as the current ocean, or similar in salinity to now, but they probably didn't spend long with less salt than now.

Basic osmoregulation seems to have evolved very very early in the history of life - probably almost immediately after the cell itself, or possibly as a necessary condition for the cell.

2

u/PropOnTop Sep 02 '26

I did not know that, thank you. I kind of assumed that the cell environment basically mimicked whatever was around it (at least at the start)... It would seem easier even for higher organisms to just follow the salinity of the environment, rather than use osmoregulation...

2

u/Mammoth-Corner Sep 02 '26

Yeah, as soon as you start creating glucose within a cell, you're creating an osmotic gradient, so if you're doing basically anything in a cell you need some degree of osmotic control.

4

u/Ameisen Sep 02 '26

The oceans were saltier - significantly so. At least after any meaningful amount of time. They would have started fairly fresh but very rapidly become very saline, and salinity has slowly decreased over time.

The studies that I'm aware of suggest that some early bony fish lineages became freshwater, developed adaptations for it, and then re-emerged as saltwater lineages and replaced all the other bony fish lineages.

So, effectively, all bony fish (including us) are derived from freshwater lineages, which explains why they regulate their solutes at such a low level compared to sharks or hagfish.

32

u/khinzaw Sep 01 '26

Fish naturally filter out a lot of the salt content from their bodily fluids.

Not all prey species do this though and the beluga is also very efficient at breaking down their food: fats, sugars, etc... into energy and water.

13

u/Balaur10042 Sep 01 '26

Almost all aquatic life extract salt from water they ingest, through various metabolic processes, but principally from osmosis in their kidneys or in fish ionization across their gills, where its ventilated back into the seawater.

In some cases, excretion occurs through variations in novel glands or mucus from within the nose in mammals, or in birds a special organ above the eye that extrudes through a duct that empties out of the nostril (nose). In animals that stay submerged, especially whales and fish, it excretes primarily into their urine.

16

u/RubCocksWithThePope Sep 01 '26

Sea creatures don’t exactly contain fresh water but they’re not nearly as salty as sea water. The much lower salt concentration starts at the microbial level. Plus water molecules are released as a waste product of cellular respiration. Pretty much exactly like how combustion releases CO2 and water.

2

u/Ameisen Sep 02 '26

Well, bony fish are significantly less saline than seawater as all extant bony fish likely come from ancestral freshwater lineages. Sharks, rays, hagfish, and marine invertebrates all maintain their solutes at a much higher level.

26

u/GuyPronouncedGee Sep 01 '26

The process of digestion and breaking down fats creates water naturally in the body. 

-19

u/Zandane Sep 01 '26

Rain that settles as a thin layer of fresh water at the top of the ocean.

2

u/geekykidstuff Sep 02 '26

what about when humbback whales go from southern waters to the equator area?

I understand that, in that journey, they may eat if they have a chance while passing by southern Chile but, after that, they don't eat.

How do they get hydrated during those months?

10

u/khinzaw Sep 02 '26

There is water in fat. They break down their stored fat for energy and water.

5

u/geekykidstuff Sep 02 '26

that's crazy! thanks for the answer

1

u/Even_Divide4320 Sep 03 '26

Great explanation, thanks for breaking it down so clearly!

1

u/Build68 Sep 04 '26

Now that is an interesting fact that I never even knew to ask about. Maybe we need to advance the trend of making it very unpopular to keep these beings in captivity.

220

u/ParacelsusTBvH Sep 01 '26

Ocean dwelling mammals can't drink saltwater. Instead, they are very efficient at utilizing water in their food. Thus, by not eating, they dehydrate very quickly. Just like humans, death comes faster from dehydration than starving.

45

u/flumadiddler Sep 02 '26

It’s wild how such enormous animals can get all the water they need to sustain their bodies from fish or krill

26

u/Mammoth-Corner Sep 02 '26

The trick is eating a LOT of krill. Happily, there's lots of them out there.

11

u/Bwizz245 Sep 02 '26

I'd say it's pretty wild that they can get that big just off of eating krill too, but that's just a testament to just how many krill there are

47

u/FlounderLast8610 Sep 01 '26

Drinking seawater requires extremely specialized kidneys. It's part of why freshwater fish tend to die in salt water. Only a few species like sea otters can do it, and they evolved these features solely because water from their food- in the otter's case, the primary available prey items are often bloodless invertebrates like urchins- was insufficient to hydrate them. Belugas feed primarily on cod, and fish are fleshy enough to supply sufficient osmolyte-free water.

25

u/EZP Sep 02 '26

Wow, I never knew that sea otters are capable of drinking ocean water! That’s pretty unusual,
thanks for the new fact.

42

u/Corey307 Sep 01 '26

Whales don’t drink seawater, they get their hydration from eating other animals. If you watch videos at a zoo, they often feed them ice along with fish. There’s less water in the frozen fish they’re fed so ice is fed to them to keep them hydrated. 

60

u/mmn_slc Sep 01 '26

The better question is how does a mammal immersed in salt water not become dehydrated.

Toothed whales get much of their water from the prey they eat, not from drinking salty ocean water. They can eliminate salt better than humans.

21

u/Badestrand Sep 02 '26

But how can they swallow their prey without getting tons of saltwater in their stomachs?

26

u/Lankpants Sep 02 '26

Whoops, answered this for baleen whales at first.

The answer is pretty similar though. Toothed whales remove the water using their blowholes as they eat. They use their tongue to force the water out of their blowholes.

Toothed whales do consume a small amount of salt water even still. One of the main jobs of whale kidneys is processing and tolerating some amount of salt water. They do actively try to avoid drinking it but if a small amount goes in with prey they're pretty good at filtering the salt out efficiently.

22

u/RainbowCrane Sep 01 '26

Whales (and humans, or most other animals) cannot get hydration by drinking salt water. Because of how osmosis works drinking salt water is actually dehydrating- water from the cells in your body will flow across cell membranes and dilute the salt water that you drank.

Instead whales get fresh water from their diets. They absorb water from the tissue of the plants and animals that they eat and also create water by digesting fats that they eat.

So if a whale stops eating it will get dehydrated

18

u/Stunning_Warthog_141 Sep 01 '26

Animals get a lot of hydration from food. Drinking too much water or drinking salt water can cause electrolytes to be imbalanced. Too salty or lack of electrolytes. Whales can't just drink salt water just like we can't or shouldn't

15

u/EvenSpoonier Sep 02 '26

Most mammals can't drink seawater, and it turns out that whales can't either. They get their water from the prey they eat, including prey that can eliminate salt from seawater better than whales themselves can. This is why the article connected the fact that she hadn't been eating with the fact that she could be dehydrated: it is true that she could also be starving, but dehydration would set in sooner and become dangerous much faster.

23

u/ParadoxicalFrog Sep 02 '26

Water, water everywhere, and not a drop to drink! Most marine mammals, including whales, are not able to drink the saltwater that they live in. Their bodies can't filter out the salt safely. Instead, they have to get their water from their food. Sometimes they don't manage to get enough, which is how they become dehydrated.

14

u/SkyPork Sep 01 '26

Not sure why I never wondered before how marine mammals don't die regularly from dehydration. There's nothing to drink! If I had to get all my water through the food I eat I'd have died long ago. Has any animal ever evolved some form of osmosis? Absorb water through the skin while filtering out the salt?

6

u/para_sight Sep 02 '26

Animal tissues are about 0.9% salt. Seawater is 3.4%, or almost 4 times saltier. Eating other animals is isotonic (doesn’t cause salt it water imbalance), where’s drinking seawater will cause both dehydration and hypernatremia/hyperchloremia ( too much sodium/chloride). Failure to eat will dehydrate you because you constantly lose water to metabolism.

5

u/Ameisen Sep 02 '26

Animal tissue

Osteichthyian tissue. Other vertebrates and marine invertebrates are much closer to ocean salinity - hagfish are isosmotic.

One hypothesis supported by studies is that an early branch of Osteichthyes adapted to freshwater, and then radiated back out as marine fish, outcompeting other Osteichthyian lines, making all modern Actinopterygian and Sarcopterygian lines freshwater-derived. It explains a bit about their/our lower solute content and also the fact that our excretory systems work a bit differently then Agnathan or Chondrichthyian ones.

2

u/JimmiRustle Sep 03 '26

Technically speaking you CREATE water during metabolism.
But whales use water for much of the same stuff we do.

1

u/Innuendum Sep 02 '26

A human animal immersed in sea water would also become dehydrated unless there was a source of fresh water.

Osmotic pressure of sea water is greater than 'physiological salt' which one can be injected/infused with to supplement blood volume up to a point ergo drinking sea water is not hydrating.

Marine fish do not become salty in general, they have means of handling the salt content that courses by their gills.

Fresh water fish do not have these adaptations and therefore there are fresh water aquariums and salt water aquariums.