25
$\begingroup$

I'm trying to come up with a land animal that can drink saltwater but for some reason cannot drink freshwater. The advantage of drinking saltwater would be that they can get drinking water straight from the ocean and salty seas. The disadvantage of not drinking freshwater is that they cannot get potable water from sources like rivers, lakes, groundwater, or rain. Such a land animal would flourish well on islands and peninsulas. On the other hand, this creature couldn't really live inland due to saltwater being hard to find away from the ocean. I'm imagining this creature to be an amphibious one similar to a saltwater crocodile.

Is it biologically plausible (no magic or divine intervention) for a land animal to be able to drink saltwater and brackish water but not freshwater? There are saltwater fish that can survive in saltwater but not freshwater, but those are aquatic fish. The marine mammals don't even drink saltwater, they get freshwater from food instead.

$\endgroup$
5
  • $\begingroup$ This seems highly dependent upon the critter you've created. Without knowing the specifics of your critter this seems like a question asking to brainstorm, generate ideas, something that isn't permitted on this site. Can you try describing your critter in detail? $\endgroup$
    – sphennings
    Aug 30, 2022 at 17:12
  • 2
    $\begingroup$ @spennings I think the OP question is intended to drill down the specific needs of such a creature. Applying brainstorming to this question on such a narrow topic would invalidate the creature design tag entirely. $\endgroup$
    – Gillgamesh
    Aug 30, 2022 at 17:42
  • $\begingroup$ @Gillgamesh The moment you said "I think" you uncovered a fundamental problem with the question. It's not our job to assume what the OP intended. The moment you believe "I think" is applicable is the moment you discovered the question should be closed for needing more details. $\endgroup$
    – JBH
    Aug 30, 2022 at 18:30
  • 3
    $\begingroup$ @itm_coder Your hypothetical creature could also exist inland around highly-saline lakes. Examples include Utah's Great Salt Lake, Jordan and Israel's Dead Sea, possibly the Caspian Sea, which is 1/3rd as salty as most oceans, but x12 saltier than most freshwater sources. Basically most Endorheic basins: places where water drains to with no ocean route, so water partially evaporates leaving extra salt. $\endgroup$
    – Jamin Grey
    Aug 31, 2022 at 1:39
  • 3
    $\begingroup$ drinking saltwater is fairly easy not being able to drink freshwater is much harder. $\endgroup$
    – John
    Aug 31, 2022 at 3:05

6 Answers 6

39
$\begingroup$

In animals found on Earth, the ability to drink sea water is dependent on the ability of the kidneys to export salt without damage. Sea mammals can do it -- seals, sea lions, manatees and dugongs, and all cetaceans. Surprisingly, house cats can, as well (within limits) -- which means I wouldn't be surprised to find that at least the smaller wild cat species can do so.

Now, being unable to safely drink fresh water is an interesting restriction. It would imply that their blood salinity might need to be much higher than any existing animal (even most fish have blood less saline than average sea water -- 2.5% salts by weight for modern sea water vs. about 0.9% by weight for body fluids -- because cellular salinity became fixed before the oceans had become as salty as they are now). If that were the case, drinking too much fresh water could lead to hyposalinity -- "water poisoning" as sometimes happens in college hazings -- which is potentially fatal.

How animals could evolve with much higher body salinity is open for discussion -- on Earth, it would imply that their cell chemistry became fixed much later than is the case for other animals (even invertebrates). Perhaps they evolved from halophiles at the shores of a hypersaline sea, as opposed to free-swimming animals in Earth's half billion years less salty oceans.

$\endgroup$
1
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Sep 2, 2022 at 8:37
22
$\begingroup$

Selective Pressure:

Can they drink salt water? Yes, it's certainly possible.

Must they NOT drink fresh water? Now that's complicated. Or is it?

Nasty Water:

The fresh water on your world is hopelessly contaminated. With What? Pick your poison. Animal (parasite), vegetal (biotoxin/disease) or mineral (localized poison). The best (in my mind, story-wise) would be some sort of biologically-derived material like a neurotoxin produced by a common bacterium. The spread of the organism was relatively quick and resulted in a mass extinction-level event on land.

So your world has effectively become a wet desert. Some animals survive by sucking dew off of plants, or getting all of their water from eating plants that don't absorb the toxin. A few might find clean sources and survive there. EVENTUALLY, some or most animals will evolve resistance to the toxin or other strategies to overcome the effects.

Curiously, the toxin is rapidly broken down by microorganisms living in salt water.

Your species had the ability to drink salt water as a result of their ancestor that was once a desert species. So when the calamity came, the individuals living near the ocean were the only ones that survived. Selective pressure to only drink salt water would have been intense. The species is physically capable of drinking water, but doing so is like asking people to eat dirt. The species has either learned behavior to avoid fresh water, or has developed a deep genetic aversion to drinking fresh water.

All who fail to follow this imperative die.

$\endgroup$
5
  • 2
    $\begingroup$ Does there even have to be a toxin? The bacterium could just be a particularly virulent pathogen that infests nearly all fresh water, but that can't survive in saltwater. $\endgroup$
    – Hearth
    Aug 31, 2022 at 20:09
  • 1
    $\begingroup$ @Hearth The exact form is up to the OP. A pathogen could certainly work. It could also just affect the one species rather than most. It could even be a mineral common to the area, and this species evolved elsewhere where the mineral wasn't common. The choices are wide open. $\endgroup$
    – DWKraus
    Aug 31, 2022 at 20:41
  • 2
    $\begingroup$ Great answer for the main idea, but re “The fresh water on your world is hopelessly contaminated.” — this is falling into the “jungle world” fallacy/habit. All the world’s fresh water being contaminated is a huge break from reality / suspension of disbelief — but it’s also not necessary. All that’s really required is most fresh water in the animal’s native habitat being contaminated — that’s enough to justify the species having evolved an overwhelming instinctive aversion to fresh water. $\endgroup$ Aug 31, 2022 at 22:28
  • 1
    $\begingroup$ @PeterLeFanuLumsdaine I'd say it's a matter of semantics, and I do suggest not ALL of the water is contaminated. But I can't really dispute the argument - and I am as guilty of arguing semantics from time to time as anyone else. :) $\endgroup$
    – DWKraus
    Aug 31, 2022 at 22:44
  • $\begingroup$ I came to post something similar. The creature might be able to drink from fresh rain puddles and not know it because all lakes and rivers are harmful. I think a living pathogen makes more sense than a toxin, because that could easily be killed by saltwater while a toxin would have to be broken down once washed out to sea, or the seas would be toxic too $\endgroup$
    – Chris H
    Sep 1, 2022 at 13:50
8
$\begingroup$

There are already plenty of answers regarding the ability to drink salt water, but I have a good idea of why a species would evolve a need for salt water.

This species evolved a biological process that consumes minerals from the body to produce an exoskeleton. This process is passive and can't be stopped. If the animal drinks an excess of fresh water, it will rapidly run out of salt in it's body. Salt is essential for animal bodily functions so this would lead to its death. Because this animal has a thick mineral exoskeleton, it also doesn't have much room to sweat out the excess water.

$\endgroup$
6
$\begingroup$

Thousands if not tens of thousands of species of saltwater-only fish prove that what you want is plausible

As well as getting water through osmosis, saltwater fish need to purposefully drink water in order to get enough into their systems. Where their freshwater counterparts direct all of the water that comes into their mouths out through their gills, saltwater fish direct some into their digestive tract.

But fishes’ bodies, just like ours, need a certain concentration of salt to function best. They can’t just allow the water to diffuse freely through their gills; the saltwater fish would shrivel up and the freshwater fish would explode!

To stop the exploding fish phenomenon, their gills have special cells that selectively pump salt in, or out of their blood. In freshwater fish, the cells constantly pump salt in, and in saltwater fish, they constantly pump salt out. Saltwater fishes’ kidneys also help to filter out some of their salt. (Source)

There are also some mammalian creatures that can drink seawater:

Sea otters, seals, sea lions, and manatees have been observed occasionally drinking seawater. But they’re capable of concentrating and excreting highly salty urine, so they can handle it. (Source)

However, I couldn't find an example of any land-based animal that did. Why not?

So according to my very very rough calculations only around 0.1% of the land area is located within 5 minutes of sea water while 100% is located within 7 minutes of fresh water! I don't think that extra two minute walk is going to be that much of an evolutionary drive for any species. (Source)

I quoted only a small portion of a larger answer, and that answer points out the weaknesses in the calculations, and obviously the nature of averages is that 100% of all land (before humanity invented drainage and irrigation) is unlikely to be within just 7 minutes of fresh water. But his point is very well taken. There aren't any examples of land-based saltwater-drinking animals because there was no evolutionary reason for one to come to pass.

But do we care?

Nope. The fish, sea otters, seals, sea lions, and manatees prove that the creature you're contemplating is plausible, even if that creature is land-based. At worst, you would need to rationalize geography or behavior that give reason to the ability to process saltwater — like being land-locked on a small island or land-locked by cliffs in an area where no fresh water can be found.

$\endgroup$
12
  • 4
    $\begingroup$ All those animals can however drink freshwater, the fish can't live in freshwater but that is a separate problem. $\endgroup$
    – John
    Aug 31, 2022 at 3:04
  • 1
    $\begingroup$ The reason fishes are often stuck living in either fresh or salt water is because they literally swim in it and are constantly exposed to it through their skin. That means their kidneys (or their analogues in non-fishes) are constantly being strained by either trying to lose as much water as possible in freshwater, or retaining as much water as possible in salt water. Land animals don't have to deal with this because they all have the problem of retaining water and filtering waste, and so they all have to have some kind of functional kidney. $\endgroup$ Aug 31, 2022 at 3:53
  • 4
    $\begingroup$ @JBH The point is that the answer you gave is non-analogous to the issue that OP raised. OP wants to know if a land animal could drink only salt water. Fishes having problems with living in certain salinity has is due to other biological issues that very specifically do not occur in land animals (or marine tetrapods, which are descended from land animals), which is the type of animal that OP is creating. $\endgroup$ Aug 31, 2022 at 3:57
  • 1
    $\begingroup$ The issue is you gave an answer that is a non sequitur by biology standards. Biology has rules. It would be like using an incorrect principle in physics to explain something. This is not what OP asked. It's possible to say "you're the author, do what you want for the story", but if that's the case why would OP come to the stack to ask if their ideas are biologically plausible in the first place? Your logic creates a Morton's Fork where every question is either "it's your story, do what you want" or is "too story based" and must be closed, leaving next to no questions suitable for the stack. $\endgroup$ Aug 31, 2022 at 12:45
  • 3
    $\begingroup$ I.e., "The fish, sea otters, seals, sea lions, and manatees prove that the creature you're contemplating is plausible, even if that creature is land-based." The problem is that drinking freshwater is easy, drinking saltwater is hard. There is little reason to lose the ability to drink freshwater in a land-living animal. This is the challenge OP is facing. Your example does not show if and why OP's idea is biologically plausible, regardless of if it is possible. $\endgroup$ Aug 31, 2022 at 12:48
5
$\begingroup$

This land animal evolved directly from saltwater fish. It still needs salt water, as its biochemistry and internal organs are not that different from those of the fish. It may have evolved lungs and adapted to walking on land, but needs the same food and drink as fish do.

Saltwater fish do not expect to ever run out of salt, not even for short periods. They may lack any mechanism to retain salt. Same with this animal. It looses salt through urinating/sweating, and needs to replenish the lost salt as much as it needs to replenish the water. Drinking fresh water quickly leads to salt deficiency illness.

Contrast with the usual land animals, believed to have evolved from freshwater fish. Of course they prefer fresh water, and are good at retaining the salt they need.

$\endgroup$
1
  • 2
    $\begingroup$ Salt water fish have the same intracellular salinity as fresh water fish and land animals. Fresh water fish have just lost the ability to export excess salt, as have most land animals. Even land animals need salt, though, especially herbivores. That's why deer and cattle will visit salt licks -- because their plant diet doesn't contain enough salt. $\endgroup$
    – Zeiss Ikon
    Sep 1, 2022 at 15:48
1
$\begingroup$

Is it possible - yes. Lots of mamals that drink exclusively saltwater.

Why is freshwater bad?

To answer this question, we need to make the "salt" part of salt water extremely valuable. For this we can:

  1. set the setting - The reason your creates must drink saltwater, but not freshwater, is that saltwater is their only source of sodium.
  2. Make symptoms of hypernatremia - more essential
  3. Make symptoms of hyponatremia - deadlier
  4. Kindeys!

Kidneys:

Kidneys have two functions that work well for creating an overwhelming need for salt - regulating blood pressure, and filtering toxins. Your creature might require a high blood pressure for... something. Or maybe, it has a diet in which salt is the only thing that raises blood pressure, and everything else brings it down (mint?).

Kidneys filter toxins, eventually secreting them through urine, but maybe the creature needs kidney and urine stones, so that the toxins don't damage their kidneys or urine tract.

Result:

Freshwater is deadly, because it dilutes the concentration of sodium in the organism, and salt is already sparse, so drinking freshwater will cause the onset of hyponatremia (lack of salt), which can lead to death. Kidney stones won't be forming fast enough, so the toxins will start to damage kidneys and urine tract, leading to other complications, infections, and ultimately - death. Also, lack of salt will lead to hypotension, eventually leading to ... death.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .