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I want to design a creature capable of surviving long periods without drinking water, but I am not sure how that creature could store water.
These are my ideas but I am looking for something more efficient or possible:

  • Increase amount of water in blood: Each litre of blood has almost half litre of water. Increasing the amount of water in blood could be used to store it but thinking in that I notice a little problem: increasing the amount of water in blood would also raise the pressure of blood and I don't know if that could be bad. Also I have read somewhere [citation needed] that if you increase the amount of water in blood, red blood cells start absorbing it autocratically (it's a problem of their plasma membrane) until they "explode" and die - a huge problem.
  • Store water inside fat: Dromedaries store almost 50 kg of fat in their hump; fat isn't water but fat has water. I don't know exactly the process but when they produce energy with the fat, each kilogram of fat combined with oxygen could produce like waste a litre of water used to survive. This idea seems quite good but I don't want to put a hump in them and also a litre of fat doesn't have exactly a litre of water so it is quite "inefficient".
  • Special organ: My last idea is have an inner organ like a bag full of liquid water to store it. Water enter to the organ through the blood system and when it's needed, it's send to the intestine, where it can be re-absorbed. I find the idea excellent but I don't know any specie which does that so I don't know if it's possible. Also I was thinking to store water in an organic material made of water (or at least hydrogen) and more dense than water, so it can store more water in the same volume, but I don't know any compound like that.

Edit:
I don't like the idea of use humps because:

  • A hump isn't "cute", I don't want to put an hump in their back. Also it's quite heavy and it's a hunter creature.
  • A kilogram of fat produces a litre of water, but a litre of fat isn't a kilogram of fat (I can't find the value), so it's less efficient than storing the water itself. Maybe you could say that it has the plus of giving energy and you are right but my specie is fine with that, it doesn't need more energy backup, I am looking for water backup.
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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – James Sep 13 '17 at 16:03

10 Answers 10

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Fat is best.

from https://en.wikipedia.org/wiki/Metabolic_water

Metabolic water refers to water created inside a living organism through their metabolism, by oxidizing energy-containing substances in their food. Animal metabolism produces about 100 grams of water per 100 grams of fat,1 42 grams of water per 100 g of protein and 60 grams of water per 100 g of carbohydrate.

Fat is very much water storage. Except the fat is not water. Fat is the raw material from which water is created. Water is produced when the fat is oxidized for energy.

from http://www.bmj.com/content/349/bmj.g7257 diagram: oxidation of fat

Oxidizing any reduced carbon yields carbon dioxide, water and energy. This is why you can see car exhaust on a cold day: oxidizing the gasoline produces water and CO2, and the water condenses. This is why if you put out a candle by lowering a glass jar over it, you will find water on the inside of the jar.

Your creatures will most efficiently store "water" as fat. The fat will not evaporate. It can efficiently be kept in one place and will not drift around and interfere with the function of other systems / cells. Possible problems - 1: you need oxygen to make water from fat. and 2: the production of water will also produce heat.

LET THERE BE HUMP!

addendum: the hump doesn't have to be on the back...

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    $\begingroup$ "the hump doesn't have to be on the back..." this obviously leads one to imagine a world where all excess fat goes straight to the penis. $\endgroup$ – Coty Johnathan Saxman Sep 12 '17 at 6:00
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    $\begingroup$ oh, finally an explanation how 1 kg of fat could "be" 1 kg of water at the same time! $\endgroup$ – Agent_L Sep 12 '17 at 21:03
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    $\begingroup$ education re "a hump isn't cute"! And also something else for Saxman to think about. $\endgroup$ – Willk Sep 13 '17 at 14:13
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Water volume isn't a problem. The mass is the problem. Your body is somewhere around 60% water. This means even a perfect water storage mechanism can only be about 66% more efficient. You can't even double your water storage per body-mass. This is why water-efficient creatures typically focus on conserving water rather than trying to store more of it.

Even if you could, would you want to? Water storage is literally pound for pound. You can store a pound of water if you're willing to carry around a pound of weight. If you're a 100kg creature, and you want to store an extra 40kg of water, you have to have the musculature, skeletal, and energetic structures to carry around 140kg. There is no solution to this, unless we start to talk about leaving deposits of water around (perhaps buried in the ground?)

As for the "hump," dromedaries don't actually use it for water! It's used to store food/energy. Camels are also very water efficient, so people got used to thinking that the hump was for that. If you think about it, the hump would inflate/deflate with water consumption if it was used for water!

My recommendation would be to store the water inside cells, or inside a constantly flowing extracellular matrix like our lymph system. Water has bacteria in it, and nothing's worse than having your water spoiled while you're carrying it in some bladder. Cellular storage would probably work best, based on what I think you're going for. That would allow us to specialize the water storage cells for maximum capacity. Now this won't buy you much. Soft tissue is already 70-75% water, so you're going to earn yourself maybe a meager 20% more capacity this way. But if you're obsessed with maximization, that'd be how I'd do it.

I'd store the tissues made from these cells in the same way we store abdominal fat. It's a part of the body that can expand efficiently, and it's close to our center of gravity so we waste the minimum amount of effort possible if we store extra stuff there.

As for trying to do chemical tricks to store water more densely, I don't think you'll have much luck. I'm unaware of any molecules that would do what you'd like, and I have good reason they do not exist. The fact that we see hydrogen and oxygen bond together into water and virtually nothing else is a hint that this is a highly favorable structure. Any structure which did better than this would also have to have other atoms (such as the carbons in a hydrocarbon). This would increase the mass, and since mass is vastly more important than volume for these sorts of problems, it would be a horrible trade. Your cute creatures are going to have to be a bit pudgy once they gorge themselves on water! However, several of the comments did point to an interesting solution, which is to draw the oxygen from the air and only store the hydrogen. If that qualifies as "storing water," then you could gain some benefits that way.

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    $\begingroup$ Oil is the most compact (MJ/L) hydrogen storage. $\endgroup$ – Cees Timmerman Sep 12 '17 at 9:08
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    $\begingroup$ "Dehydrated water! Just add water!" (Sorry, I had to...) $\endgroup$ – Shaamaan Sep 12 '17 at 10:39
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    $\begingroup$ There are compounds which can dramatically increase the density of stored hydrogen, and the hydrogen alone is what, 20% of the mass? You could do a lot better than pound for pound if you stored hydrogen only. $\endgroup$ – Puppy Sep 12 '17 at 13:20
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    $\begingroup$ @Shaamaan Would you like to buy some? $\endgroup$ – Izkata Sep 12 '17 at 20:32
  • $\begingroup$ @Izkata I like the suggested uses... Dry ice? Dry Martini? Dry cleaning? Such a flexible product! I'M SOLD! $\endgroup$ – Shaamaan Sep 13 '17 at 7:19
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You dont have to store more water...

Just lose less water. Camels don't store water in their humps; that's just fat. They store water in an internal bladder.

  • Animals lose water either through general/unintended I'm gonna say osmosis but thats not quite right. With water being leeched from their skin from the dryer air. This happens most when we open our mouths.

  • They lose water because of perspiration. We sweat to cool ourselves down.

  • They also lose water because of secretion. We gotta get rid of waste somehow.

All your creature has to do is not need to sweat, capture lost water from when opening its mouth (a lot of hair maybe), and defecate dust.

Some animals evolve spines to capture moisture and draw it closer to their body for collection in desert environments.

Saharan snakes are cold blooded so they don't need to sweat in the desert. They also burrow into the sand at noon to avoid to much heat.

You could justify having a cold-blooded unicorn with reflective skin, a long snout it breathes in and out that is packed with hair and acts as a filter and water catch. It can also poo rainbow colored dust. Would be perfectly adapted for the desert.

Edit: Since there is a desire for a chemical approach to this:

Hydrocarbon chains!!

Hydrocarbon chains is practically synonymous with oil. They are composed of chains of carbon with hydrogen caping the ends. Your creature could just drink water and convert carbon and hydrogen into these. It could then perform a controlled combustion reaction to convert these into H20 and CO2 by taking in air. This is an incredibly dense way to store energy and water. It is also not completely unrealistic. Plants store energy in glucose which is essentially a hydrocarbon and some nitrates i think. When it undergoes respiration at night it indeed release this water which is what can often, in combination with other conditions, result in muggy or foggy conditions.

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  • $\begingroup$ I didnt feel the need to edit and clarify this, however, you dont have to achieve all those goals to be desert proof. Your unicorn could be warm blooded and perspire losing some water slowly. $\endgroup$ – anon Sep 12 '17 at 17:56
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Adaptation

Depends alot in the environment in wich the creature will live to give a correct answer, because one solution for an arctic tundra is most likely not going to work in the Sahara desert.

Behaviour

  • Never move when the temperature is at its height
  • Burrow below earth to absorve moist from the surrounding land
  • Stay active only during the night time or dawn.

Acquiring Water

You don't need to carry all that water around; just being able to get it from something in particular would help you.

For example the Kangaroo rat would obtain water mostly from seeds.

Retaining water

Ultra efficient management of water excretion, which means:

  • Lack of sweating; you dissipate heat using longer ears/legs.
  • Liquid excretion is being stripped from most water as possible leaving only uric acid.
  • Highly efficient kidneys that help obtain as much water as posible from any food.

Storing water

Is a valid option, but sadly you need to put it somewhere, and since humps isn't an option, maybe you could consider an atrophied limb or wings. Like a species that is adapting to this "new" place, and their wings are for heat dissipation and water storage instead of flying now.

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Symbiosis

Ants and aphids, symbiosis, yadda yadda. Your creature makes/lives in an underground network of tunnels, like a groundhog. A certain kind of beetle or mouse or something shares the tunnels, parasitically scavenging leftovers, or being cultivated-- let the details fit your creature's emotional role in the story. Glowing pretty fungus? Adorable mouse babies? Woooorms?

The beetles store water. Your predator doesn't store water in its body; it stores it in theirs. When times get dry, the beetle-ites get eaten, or milked, or whatever.

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  • $\begingroup$ That is impossible to adapt for my story or creature. But I like your idea, it's interesting, you have my +1. $\endgroup$ – Ender Look Sep 12 '17 at 19:59
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I would suggest a slight variation on your internal organ idea. I would have them have a second stomach that holds nothing but water, and instead of them having water absorbed through the intestines as you suggest have the circulatory system pass by that organ to gain the needed water. You could have the valve that we use to separate food/liquid from air function as normal,but then have a secondary valve/flap that allows water to pass down a third tube to the water bladder, while the non-water molecules that are the food stuffs go down the regular tube to the stomach. Cell membranes work in a very similar manner, only allowing certain molecules through although if I remember correctly ours are set up for oxygen to pass through, not sure about water. A way to make it more efficient, instead of a single flap have the esophagus act kind of like an intestine where it absorbs the water all the way down, and then sends it to the bladder.

Having the large interior repository, say 2-3x the size and capacity of the creatures normal food stomach, would allow them to go quite some time without drinking once filled completely.

You could even make it a part of their normal digestion that any water they get from food gets filtered out/absorbed in the intestines as normal, but is sent to the repository organ instead. This would help to supplement the water gained through drinking and allow them to go even longer without drinking, provided they were still able to eat.

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  • $\begingroup$ With the second paragraph are you saying that water could be splited from food like food is splited from air? I mean, from the mouth you could get Air, Food and Water, when their reach the Pharynx, the Air is sent to the Larying (lungs) and the others two to the Esophagus (stomach), Are you saying that I shall make a third path to the water? $\endgroup$ – Ender Look Sep 11 '17 at 19:00
  • $\begingroup$ @EnderLook That is what I was thinking, I will adjust my answer for more in-depth of how I would do that. $\endgroup$ – Daishozen Sep 11 '17 at 19:01
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    $\begingroup$ Marine mammals get their water supply from the food they consume (since consuming seawater would not be a good way to go about it). If the water were then excreted into a bladder, that would work fine for Daishozen's idea. Side note: when you - or an animal - is eating, air and food are not interleaved and separated; when you have a mouthful of food, no air makes it to your lungs. $\endgroup$ – jdunlop Sep 11 '17 at 19:05
  • $\begingroup$ I was thinking in your idea but I note something quite strange: What would happend if the creature drink for example orange juice? It has water so it goes to the second stomach but it has organic matter and that would star rotting inside. Also even if it drinks normal water, How the creature would "clean" the second stomach from slighty dirty water? Water isn't 100% H2O, it contains some minerals. $\endgroup$ – Ender Look Sep 11 '17 at 19:23
  • $\begingroup$ @EnderLook I am currently thinking that the osmosis style esophagus would probably be best. They drink the orange juice, it bypasses the lungs as normal, and as it slides down the esophagus the water is absorbed one molecule at a time, while the rest of it goes to the stomach. Then having a redundancy backup in the slower-moving intestines to get what the esophagus missed. $\endgroup$ – Daishozen Sep 11 '17 at 19:25
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Have a Jellyfish type creature:

Only about five percent of the body of a jellyfish is solid matter; the rest is water. Fascinating, elegant, and mysterious to watch in the water, take a jellyfish out of the water, and it becomes a much less fascinating blob. This is because jellyfish are about 95 percent water.

Source: National Ocean Service

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We already store water

As other answers have said, we're already mostly water, hard to improve on storage without having weight problems

Continual Input

To avoid "drinking", your creature could get small amounts from the air itself, filtering it through some kind of skin pores or gills or something. That would be a weightless way to slow major intakes (drinking) by a lot.

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Your creature could have adapted some sort of (mostly external) organ that allows it to create a rudimentary solar still, to extract moisture from the ground. Perhaps skin or a thin shell-like membrane on a vestigial wing or fin. It would place this organ over a hole in the ground, and use a long snout or proboscis to drink the collected water, or maybe tubes built into the membrane. It could even bury itself nearby (but not IN the hole, it doesn't want to bake!) to get out of the sun itself while it waits. The creature would need to stop periodically on its journey to dig a hole, set up the still, and rest and wait for its drink, but it wouldn't need to find surface water nor carry water along.

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Don't store it.

Your creature is a modified egg-layer. In rich times it gorges itself and produces a large number of infertile eggs. These are stored to be eaten during lean times. If lean times only apply to water then the eggs are basically water with little nutrition in them.

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