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What would animals that dig through solid rock look like? How would they differ from animals that dig through dirt?

So basically I am trying to populate ravines and canyons with fauna that digs through rock and make extensive networks of tunnels to live in. They don’t need to be fast diggers by any means, solid rock is far harder to dig than dirt or sand. However the burrows would be permanent and offer better protection against predators and the environment. Predators that can fit through the tunnels may still be a danger but it’s far easier to defend oneself in tight spaces by blocking entrances or lunging at the attacker. The creature would occupy the niche of a herbivore that live in communities, stashing food into the tunnels chambers. These animals are essential for the ecology of that environment thanks to their burrows creating homes for many other species.

What I would like to figure out is the body plan and necessary adaptations of such a creature.

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    $\begingroup$ When you say rock... (you may grow to hate me), how hard a rock, are we talking - in the range of sandstone/shale or more at the granite end of the hardness scale? $\endgroup$ Aug 14 at 22:35
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    $\begingroup$ The Horta in Star Trek used acid. Note that @ARogueAnt. is right, we need to know the kind of rock we're going through. We also need to know the approx size of the tunnels you're interested in. There's a big difference between a creature that moves boulders aside and one that actually grinds/breaks/dissolves rock. What is your specific goal (you need to be specific, the more "I'm open for anything" you are, the more the question deserves to be closed)? $\endgroup$ Aug 15 at 3:08
  • $\begingroup$ Is this a [hard-science] question, or is [magic] allowed? $\endgroup$
    – Drake P
    Aug 15 at 16:30
  • $\begingroup$ @Drake P Do you want me to add a [hard science] tag? So far answers have been pretty grounded (pun not intended). $\endgroup$ Aug 15 at 17:02
  • $\begingroup$ @LiveInAmbeR if that's what you're wanting, something along the lines of [hard-science] or [science-based] could be warranted (though not necessary). I was just asking because if magic was on the table, that would make a big difference. $\endgroup$
    – Drake P
    Aug 15 at 17:09
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How does a species of animal dig through rock instead of dirt?

Slowly, very very slowly.

Some already do but none of them are fast.

Some clams & sea urchins can burrow into rock, a couple of examples, there's a species of bee that does it & there's a species of ship worm that does it.

None of those examples really look all that different from normal animals of their type, of course If you want something that does the same thing at speed then that may be a different issue.

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    $\begingroup$ “Researchers placed single sea urchins on flat pieces of soft mudstone, moderately hard sandstone and tough granite. After a year, they measured the weights of the rocks and how much the rocks were eroded. The sea urchins had eaten holes in all the rocks, although they made slower progress on the harder ones. Field measurements showed that holes in mudstone were about 220 cubic centimetres, whereas holes in sandstone were 63 cubic centimetres and holes in granite were just 45 cubic centimetres.” $\endgroup$ Aug 15 at 8:27
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    $\begingroup$ @LiveInAmbeR I did say very very slowly ;) $\endgroup$
    – Pelinore
    Aug 15 at 12:42
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    $\begingroup$ @LiveInAmbeR: I think the urchins also made more holes on the harder rocks as they were trying to find a soft spot. Even the most amazing rock-diggers care more about shortcuts than showing off to boost their ego. $\endgroup$ Aug 15 at 23:14
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    $\begingroup$ @LiveInAmbeR trying to put it into perspective for myself, and leaving it here for others... the sea urchins used apparently grow to a diameter of 10cm, which (assuming sphere) gives us a bit over 500 cm³ of volume. So in a year, they dug enough to hide half of themselves (ignoring spines?) in mudstone, or 1/10th in granite. The numbers sounded big/fast to me which is why I checked, but putting them in relation to their body volume means yes, that is slow. Source for LiveInAmbeR's quote: www.newscientist.com/article/2161771-sea-urchins-can-drill-holes-in-solid-rock-with-just-their-teeth/ $\endgroup$
    – Syndic
    Aug 16 at 7:05
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With giant claws.

claws

https://www.discovermagazine.com/planet-earth/get-lost-in-mega-tunnels-dug-by-south-american-megafauna

Paleoburrows are giant tunnels through rock, excavated by extinct South American megafauna. Opinions differ as to whether these tunnels were excavated by giant sloths, giant armadillos. I favor both, in a team effort.

Giant Paleoburrows Attributed to Extinct Cenozoic Mammals from South America

These burrows are found in continental settings, in substrates that include consolidated sands, sandstones, and weathered granitic and basaltic rocks ranging in age from the Precambrian to the late Cenozoic. The size of these structures precludes their assignment to any living species, and therefore, they are considered paleoburrows (i.e., produced by extinct organisms)... Large (about 2 meters in height and up to 4 meters in width), subelliptical paleoburrows that can surpass 50 meters in length and become narrower towards their ends. These megatunnels are subhorizontal; so far, no body fossils have been found inside, but scratches produced by the diggers are present along the walls...

Tunnels that have not been choked with mud and silt are big enough to walk through and some are big enough to drive a car through. The longest one found so far is 2000 feet long! It is not known how long it took for digging animals to excavate these tunnels. It is known that they are super cool. This would be perfect for a fiction and the body plans of both giant sloth and giant armadillos are known from their skeletons.

sloth and man

https://www.reddit.com/r/Slothfoot/comments/heac7e/lifesized_jeffersons_ground_sloth_model_by_jaap/

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    $\begingroup$ It looks so cute and cuddly! Untill it starts eating you alive... $\endgroup$ Aug 15 at 3:05
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    $\begingroup$ @JoinJBHonCodidact Very, very slowly :P $\endgroup$
    – Corey
    Aug 17 at 4:46
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With tools. There are plenty of actual animals that can use simple tools, so any animal picking up a rock and bashing it into a wall isn't hard to imagine. And a Mantis Shrimp is capable of punching at 23 m/s, with a force of 1500 N. They have been known to smash the glass out in aquariums. If giant Mantis Shrimps picked up hard sharp rocks (like flint buried in chalk), they could probably progress quickly compared to other methods. It's likely how early humans did it.

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    $\begingroup$ +1 because that was my first pitch too :) $\endgroup$ Aug 15 at 14:16
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Acidic sweat.

Like some animals sweat pheromones (territory, sexual marking), or saline solution (cooling), through their paws/hands, theres no reason why a creature couldn't have mildly acidic sweat/secretions. Not on the scale of Aliens, just enough to weaken and burrow into a very alkali rock such as limestone.

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In a number of places in the world, people have cut tunnels and homes into volcanic tuff. That is a fairly soft stone and I think that a number of animals could dig into it. For example, Cappadocia has both homes cut into volcanic tuff towers and into deeper areas. Bandelier National Monument has a history going back 11,000 years. Both places have deeper valleys.

Your story can have both critters with claws and soft stone to dig into.

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Like humans, using explosives

If you want to spice up the story, have the creatures secrete an explosive compound. There are real animals that can do that. The explosive compounds will be different from what real animals make, but I don't think there's any reason why an animal could not evolve that.

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    $\begingroup$ That is tempting, a creature that farts explosions and can cause an avalanche. But to actually shatter the rock it would first need to drill a hole into the rock to deliver the explosive compound. Surface explosions don't do much. An alternative to explosives would be to release a compound that expands within the rock, shattering it through erosion. Or heating the rock and then pouring cold water on it. $\endgroup$ Aug 16 at 15:35
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    $\begingroup$ Yes they'll first need some way to drill into the rock. This is more of a way to speed up digging when combined with one of the existing answers. I was thinking either secreting acid or a rock digesting protein, or using a hard claw on a long thin finger. The drilling would be slow, but then they could quickly enlarge the dig using explosives. $\endgroup$
    – JanKanis
    Aug 16 at 15:39
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They don't

Unless they enlarge existing tunnnels or cracks in the rock which are ventilated they would not have enough oxygen. The soil is porous and lets some air go through, the rock is not, they could not stay very long in a deep unventilated burrow.

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    $\begingroup$ This is why insects like ants, termites and bees are experts at ventilating their nests. A bee keeper once forgot to add the frame into the bee nest he was keeping and the bees optimised air flow. Ants also open and close the 'doors' of their mounds depending on the suns position for optimal temperature regulation. $\endgroup$ Aug 16 at 14:35
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Take a cue from Tremors, and just assert that it happens:

an enormous burrowing wormlike monster suddenly erupts out of the ground

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  • $\begingroup$ Ah yes! The Graboid, a creature with a mysterious life cycle. And that burrows through dirt, not rock... $\endgroup$ Aug 16 at 14:37
  • $\begingroup$ @LiveInAmbeR more like sandstone (or other soft rock), as shown when one digs out of the cliff face and falls into the canyon depths. $\endgroup$
    – RonJohn
    Aug 16 at 14:41
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As mentioned in comments, this is going to depend on the type of stone and how quickly you want it done.

Sandstone, soapstone, some pumice and a number of other types of stone are less than 3 on the Mohs scale, meaning you can at least scratch it with a fingernail. Most animals with good strong keratin claws can dig through these with a little effort using nothing other than their claws. It'll still be slow, but with some persistence it's not hard... sorry, had to be said.

For tougher types of stone nails aren't going to be enough. Up to ~5 on the Mohs scale you're working with material that's softer than human tooth enamel and the more dense bone structures of the human body. Animals with stronger teeth aren't terribly uncommon, and there are several types of animals in the world that grow new teeth throughout their lifespan. Properly configured this will get you almost as high as granite, letting you tunnel through marble or dolomite.

From Mohs 5 up you're going to need something better than the norm for hard biological structures on Earth. At this point I'd switch tack and look for a chemical solution (yeah, I know, just ignore the puns). Acids sound like a good option, especially as acid production is a routine function of most animals. You'll need to adjust the particular type of acid to better fit the rock type you're attempting to tunnel through, both for efficacy and to minimize the toxicity of the reaction products. And you'll want a decent airflow.

On the plus side you don't have to actually dig the tunnel with acid, just use it to soften up the work surface until it's weak enough for you to dig out the next layer. It's a slow process, but still faster than trying to scratch or chew your way through granite.

For more esoteric options, you can do a lot with ice in a rock face filled with microfractures. In a sub-zero C environment (< 32F for the Americans in the audience) all you need is an organic antifreeze that either breaks down quickly or with some enzyme action. The antifreeze lets water stay liquid long enough to seep into the cracks, then when it breaks down the water freezes and breaks the rock via expansion. Repeat the process until you have friable rock that can be easily cleared to allow access to the next layer.

If ice isn't really your speed, how about something biological that's a bit more explosive? Bombadier beetles mix hydroquinone and hydrogen peroxide and combine the mixture with an enzyme to produce an extremely energetic reaction. While it doesn't get much above the vaporization point of water, up to 1/5th of the mixture of hydroquinone and peroxide can be vaporized. If you could externalize the reaction then it might produce enough heat and pressure to crack the surrounding rock. With the right combination of reactants you could perhaps end up with an acidic compound... or cut out the middle-man and just use the peroxide to eat the rock for you.

Stepping out of the normal biological mold, there are all sorts of creatures in fantasy and science fiction whose natural processes grow crystalline teeth rather than the less durable enamel-coated structures we use. Imagine a process that lets an animal grow teeth and/or claws out of quartz (7 on the Mohs scale), corundum (9) , or diamond (10). Even if they were fragile to impacts and therefore not much use in a fight, they'd still be good for grinding away at the rocks.

And since I've already gone on far longer than you're probably interested, let me leave you with one final option: bacteria. No, hear me out. I always save the weirdest for last...

Your tunnelers have a symbiotic relationship with a type of bacteria that requires specific minerals to reproduce or as a necessary part of their normal metabolism. As waste product they leave behind a compound that either has nutritional value or acts as a drug to the tunnelers. The bacteria grow through the rock following veins of their mineral, cracking the rocks to something the consistency of chalk as they convert the bulk of this mineral to biomass. The tunnelers come along and slurp up the mess, eating the biomass or getting a good buzz, and incidentally spreading the bacteria to new veins in the uncovered surfaces.

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  • $\begingroup$ +1 For your puns, don't beat yourself down! And yes, I did read the whole thing. That last one in particular was interesting, I feel like you're onto something. $\endgroup$ Aug 17 at 6:06
  • $\begingroup$ @LiveInAmbeR I get bored writing dry answers. It's more fun to play with words on the way :) $\endgroup$
    – Corey
    Aug 17 at 6:08

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