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Water bears are about the most impressive organism on our planet. They can withstand extreme temperatures (both hot and cold), can survive for a time in the vacuum and cold of space, are resistant to chemicals and radiation, they can go very long periods without nutrients, can dehydrate until they are in a glass state and then simply rehydrate later... they are near indestructible.

Part of their incredible ability comes from their size, but how much? How large could one of these critters grow before it is just another bug, prone to the same woes and concerns as any other critter?

Could there be a cat-sized water bear that maintains its impressive indestructibility?

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    $\begingroup$ The square-cube law is not kind to giant animals of any sort. $\endgroup$ – Draconis Feb 2 '17 at 18:15
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    $\begingroup$ Nice try, but your plan to take over the world with your unstoppable water bear army ends here, Dr. Mkinson. $\endgroup$ – Jeroen Mostert Feb 3 '17 at 6:39
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    $\begingroup$ @Draconis The square cube law just sucks in general. Ruins all the fun. $\endgroup$ – SGR Feb 3 '17 at 8:24
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    $\begingroup$ If you need a large physical presence based on these critters then maybe a hive or swarm is your best bet, rather than large individuals. They could even sometimes clump into an (apparently) single entity in the manner of a Portugese man of war when conditions are not completely horrendous. $\endgroup$ – Grimm The Opiner Feb 3 '17 at 14:26
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    $\begingroup$ You know what they say - if a flea was the size of an elephant, it could jump ... as far as an elephant. $\endgroup$ – Steve Ives Feb 3 '17 at 14:52
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Sadly, (for they would be adorable) no.

One of the main things that's going to hinder your dear "water boars" (hey if normally sized tardigrades are called water pigletts why not call these ones boars...) are predators. Dehydrating yourself to survive without water/nutriants is absolutely useless if a predator can come around and kill you anyways (especially since the method used to do this involves loading your cells up with tasty sugar; your animals would be turning themselves into hard candy).


Another huge issue is that a large creature needs a complex bloodstream to move nutrients around. Tardigrades, being so small, don't. Their cells just diffuse oxygen and nutrients without the need of a heart, a bloodstream or even lungs. Larger tardigrade (which are still absolutely nowhere near as large as a cat) have to deal with their size by literally stirring their insides so that all their cells can have access to enough food. This is a neat trick but it can only get you so far (to about 2mm in size).

Now at this point, you might be thinking: "So? Let's just add a cardiovascular and pulmonary system. Problem solved." Sadly things are not so simple as the less complex your organism (the less organs and body wide systems it has), the less things that can go wrong. Dehydrating a heart, bloodstream and - oh god - those tiny alvioli will without a doubt kill your creature.


There are many many other problems, but these are arguably some of the most significant.

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    $\begingroup$ So it doesn't work with normal biology...time for the unexplained cryogenics organ! $\endgroup$ – DonyorM Feb 2 '17 at 19:43
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    $\begingroup$ How large can they be? "no" $\endgroup$ – Zxyrra Feb 2 '17 at 22:51
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    $\begingroup$ @Zxyrra Haha! Good catch, but it was an answer to their definite question "Could there be a cat-sized water bear that maintains its impressive indestructibility?". $\endgroup$ – AngelPray Feb 2 '17 at 22:59
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    $\begingroup$ Adorable? Perhaps for certain values of 'adorable!' $\endgroup$ – Catalyst Feb 3 '17 at 11:08
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    $\begingroup$ Giant Tardigrade-based candy seems like a super awesome element to throw in into sci-fi work, however. Or into a weird sequel to Charlie and the Chocolate Factory. $\endgroup$ – T. Sar Feb 3 '17 at 11:58
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Yes

The largest arthropods to 'walk' the earth were Arthropleura that could reach 2.3 meters in length. https://en.wikipedia.org/wiki/Arthropleura

The challenges that AngelPray identified are the reasons why vertebrates out-competed large invertebrates.

Provided that there are no large vertebrate competitors nearby your giant water bear should be fine.

The giant water bear will be very slow with a 1-2 second reaction time and require an oxygen rich atmosphere.

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    $\begingroup$ First, welcome to worldbuilding! Second, arthropleura are awesome. I'm not sure I see the connection with water bear though, as mkinson is interested in their overpowered resistance (that the arthropleura didn't have AFAIK). Maybe you could try to expand your question showing how an arthropleura could be that kind of beast? $\endgroup$ – PatJ Feb 3 '17 at 14:31
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    $\begingroup$ Tardigrades aren't arthropods. They're likely to have different constraints to their maximum size compared to arthropods, as a result of differences in their anatomy. Most arthropods have hearts and breathing organs, for example, which tardigrades lack. $\endgroup$ – ckersch Feb 3 '17 at 16:27
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I suspect that if there is a niche, something will evolve to fill it. The question is why is extreme toughness at a cellular level the right way for a larger animal? The square-cube law means that the larger an animal is, the more easily it can maintain its body above freezing point all winter.

If we look at the Arctic, there is a frog which survives freezing solid every winter. It hasn't evolved extreme heat or vacuum tolerance because it doesn't need them. It is also an outlier. The approach used by most Arctic critters is good thermal insulation. Many hibernate to a greater or lesser extent. They reduce their metabolic rate and body temperature so they can survive winter on little or no food without freezing.

This approach works (for birds!) even in the harsher Antarctic. The Emperor penguin is perhaps the ultimate example. It goes somewhere so cold abd barren that there are no predators in order to breed safely, surviving purely on insulation and body fat.

If Earth's seasonal variations were even greater or longer I suspect there would be larger animals managing to survive freezing solid for lack of any alternative evolutionary adaptation. The frog is cold-blooded so this is the only avenue open to it to survive in the Arctic niche. For warm blooded creatures, it is easier to evolve insulation, fat reserves, and hibernation.

Earth does not have vacuum or high radiation areas so any ability to survive these is an evolutionary accident, rather than something selected for. There are geese that fly at altitudes where airliners cruise and humans would pass out, but there is no evolutionary reason for them to go any higher.

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  • $\begingroup$ A lot of citation needed here. Which frog? And is it really because it has good thermal insulation? Or does it just have an enzyme in its body that acts as anti-freeze but otherwise reaches a sub-zero body temperature during its frozen hibernation? $\endgroup$ – Shufflepants Feb 3 '17 at 15:25
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    $\begingroup$ Wood frog Rana Sylvatica. National Geographic say it freezes solid. Antifreeze probably, prevents ice crystal formation so cells not damaged. I said frog is an outlier in that it does not rely on thermal insulation. Cold-blooded, unlike mammals which hibernate. $\endgroup$ – nigel222 Feb 3 '17 at 16:42
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    $\begingroup$ Just looked up highest recorded bird altitude and it's not a goose. It's an unfortunate Griffon vulture which collided with a jet at 37,000 ft. $\endgroup$ – nigel222 Feb 3 '17 at 16:52

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