Justifying why a troll's skin would turn to stone in sunlight

Question

It's a fairly common legend that trolls turn to stone when exposed to daylight, and I was thinking about using such a creature in a story. I'm searching for a plausible way of justifying such a weakness - there's nothing similar existing in nature that I know about.

For background; I'm visualising these trolls as primitive, ape-like creatures that can grow to very large sizes. They are nocturnal / subterranean animals, and the sun is bane to them. They can move around on the surface at night, but in the daytime they have to take shelter under bridges or in caves.

Direct sunlight is their big weakness. These trolls are like lobsters - they are biologically immortal and they will continue to grow for as long they live. The trolls keep on growing and getting larger until they can't support their own mass, or until they can't hide from the sunlight any more.

When they die in the sunlight, their skin calcifies and they become rock. It's a slow and painful death; their skin turns to stone from the outwards inwards, until they finally crack apart.

The very largest trolls might become mountains. In this world, there are lots of strange rock formations that have been left behind by dead trolls.

It's possible that young trolls have resistance to sunlight and can operate in the day, but as they get older they lose this resistance. Young trolls are born in droves, but very few of them reach large sizes.

So; biologically speaking, what could justify this fatal trait of trolls? I want to avoid just using magic as a reason, and I'm more looking for broadly feasible suggestions about how and why trolls might develop like this. Is there any deficiency, medical or evolutionary reason on why would sunburn would cause troll's skin to calcify?

Answer 1

Their skin contains a set of proteins that will bind to each other and harden when exposed to certain wavelengths, much like dental composite polymers:

Composites are placed while still in a soft, dough-like state, but when exposed to light of a certain blue wavelength (typically 470 nm), they polymerize and harden into the solid filling (for more information, see Light activated resin).

And from the link in the quote:

A photopolymer or light-activated resin is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum. These changes are often manifested structurally, for example hardening of the material occurs as a result of cross-linking when exposed to light.

So they are not becoming actual rocks. Their skin are becoming a very hard organic material that has much the same properties as most kinds of rocks.

Kinda like porphyria, but you become a statue rather than getting 3^rd degree burns from sunlight.

For more effect, rather than hardening on the skin, the proteins can break loose from it when exposed to sunlight. They fall in the bloosdtream and then harden within other tissues as well as the skin itself.

Answer 2

Another classic trait of trolls, is a tremendous regenerative ability. Sometimes to the point where only fire can kill them.

So I'd suggest trolls create a chemical in their body that boosts their regenerative abilities, and any damage will increase the levels of this chemical. Sadly (for the troll) this chemical reacts with UV light from the sun, and hardens. Creating the resemblance of turning to stone.

That could be an explanation for why only sunlight, and not other kinds of light (fires usually don't emit UV light) it also explains why they haven't evolved out of such a vulnerability, as they would loose their regenerative ability.

It can be adjusted from only their skin turning to stone, and at night they can break free (loosing their skin, but it'll regenerate almost immediately) all the way to turning completely to stone (UV light can penetrate pretty deep, that's why we use sunscreen to not get cancer)

Answer 3

The frozen troll is the second stage in a two part lifecycle.

Part 1:

The troll is mobile, and goes around eating minerals to store in its body, and killing and eating animals (or hobbits) to sustain itself. It gathers and stores sperm from other trolls to enable it to reproduce in its sessile stage. The mobile troll focuses on growing, eating and generally causing havoc. It also carries dormant algae in its skin.

Part 2:

The troll is exposed to sunlight, which activates the algae on its skin. This triggers a biological reaction in the troll which causes it to rapidly harden into an armored algae fortress, by precipitating all the minerals it stored by eating rocks. The algae now live and thrive off sunlight and stored minerals, while enjoying the protection from herbivores afforded by the troll's frozen body. They produce lots of sugar, which is stored deep inside the troll. Most of the troll's cells die; however, the reproductive system is sustained by the algae. The troll-rock then can exist indefinitely, producing a steady stream of tiny trolls, which gather algae spores on the way out, then grow up and continue the cycle.

Becoming a troll rock is incredibly unpleasant, as the brain of the troll survives while frozen in place until it starves as resources are diverted to reproducing. For this reason, trolls do their best to avoid getting frozen, but eventually they slip up and continue the cycle.

Answer 4

Gypsum trolls

Trolls really are softies (literally) on the inside. As soon as a baby is capable of moving, troll moms pound gypsum with their clubs, heat it up, and roll them around in the resulting plaster of Paris. As they grow older, the old layers slowly crack, and more plaster is rolled on, giving the trolls a tough outer shell. As the plaster sets, it also cools, thus helping the massive creatures cool down. This is why the biggest baddest trolls spend all their time wallowing in plaster pools.

Now, this isn't a problem -- trolls live in moist caves, by continuing to move and keeping the joints wet, they ensure that the plaster sets slowly, and only sets in desireable parts of the body. However, once they go outside in the sunlight, their eyes (so perfectly adapted to the dark) get blinded; making it hard to find shelter before the plaster sets into all their joints, leaving them immobile, dying a slow death.

For added fun: gypsum also forms beautiful crystals. Trolls can "wear" these crystals either by sticking them onto still wet plaster or by encouraging crystal growth on their bodies. Hardened trolls look distinctive in the landscape: blinding white structures adorned with glittering multi-colored crystals.

Giant gypsum Crystals in Naica Mine, Mexico (See person for scale). Source: Alexander Van Driessche

Answer 5

Let's assume that trolls are unusual creatures, in that they consume and metabolize rocks, allowing them to access an abundant, yet generally low-competition material, as a food source.

This is what allows them to grow to huge sizes, as they gradually assimilate more and more of their mountainous surroundings.

However, it does come with a disadvantage: the chemical processes that allow them to integrate rock material in their organisms are expressed throughout the body and must continuously remain active in order to preserve tissue viability. Since trollkind has evolved deep underground and only in recent millennia has emerged to the surface through cave systems, their special metabolism is particularly sensitive to UV light. Once exposed to UV rays, even at low levels, a cascade reaction is triggered starting from their skin, turning the chemicals responsible with tissue viability into an inactive form.

Without these chemical processes, rock material reverts to its passive, inorganic state and the troll quickly and painfully turns to stone, from the outside in.

Answer 6

Specific photosynthesis

Trolls can continue to grow for as long as they live…

I know another sentient being that have the same property: Trees.

What if trolls were closer to trees than to animals?

Yes, trolls evolved a lot since that time where they had roots instead of foots but they still share a lot with their ancestors… Especially photosynthesis!

But trolls have a particular sort of photosynthesis, the oxygenic photosynthesis:

CO2 + H2O + photons light energy → [CH2O] + O2

This reaction is pretty useful for them as they are living in caves where the CO2 concentration is very high! That’s the reason why this photosynthesis has to be very efficient: without it, trolls would suffocate in their lairs.

But… There is not light in their cave so how is this possible? In fact, there is light in these caves: from fireflies and glowing mushrooms… And trolls have evolved in such a way that this tiny amount of light is enough for them to produce a lot of O2.

So… When a troll is exposed to direct sunlight, the amount of light it takes is insanely huge for him and its photosynthesis reaction gets instantly mad!!!

Problem : This reaction consumes water! Therefore, the troll is desiccated in a matter of minutes and the remaining corps just looks like a sort of mummy or a rock…

By the way, this tree-affiliation and the photosynthesis would explain their green skin and their mossy look...

Answer 7

One aspect of common sunburns is the body's inflammatory process. Perhaps the biology of your trolls is such that their inflammation redirects calcium from their bones to the surface of their skin, where it merges with their skin cells to calcify it. Then, like a hemophiliac response to a bruise, hemorrhage or cut, the process continues in a runaway fashion until their whole bodies are turned to "stone" (or bone).

Your trolls would need a large supply of calcium for this to make sense, so they should have especially large, thick, and dense bones.

Answer 8

You might also consider why they would react to light by turning to stone. My suggestion would be as a defense mechanism against some other type of creature that produces light. The troll's skin turns to stone as a defense against that creature. After a few nights, the stone skin would slough off and allow the new skin under it to grow out.

Assuming the attacking creature uses a reddish, long wavelength, the problem with sunlight might be that it includes a more penetrating short wavelength ultraviolet light. This penetrates into the interior of the troll, turning the organs and muscles to stone. Trolls might be naturally somewhat translucent in strong light, although most people never get a chance to see it.

An alternative would reverse things. Instead of light being the issue, the real issue is heat. It's the heat from the sun that turns the troll to stone. As more and more heat is transferred, it effectively cooks the troll from the outside in. Heat of course can be conducted through stone, unlike light. This of course would also allow adventurers to set trolls on fire to turn them to stone.

I'll leave it to others to explain how the conversion to stone occurs. I'm just trying to explain why it would evolve.

Answer 9

So; biologically speaking, what could justify this fatal trait of trolls?

I'm more looking for broadly feasible suggestions about why trolls might develop like this.

It's an accidental feature.

(I'm going to assume polymers hardening in the skin is what causes the solidification as per other posts, and add the possible reason the trolls have survived while still being sensitive to light.)

Originally trolls grew up in the earth, where sun would never reach. Trolls would die frequently when young, so they never got programmed to stop growing, even though this would be a major defect in a small space underground. Call it a side effect of high breeding potentials, they grow fast and loose. Sorry, anyways these small cave dwellers eventually get better at surviving longer, until some of them are large enough that they start to migrate more, looking for bigger passages. Some will get stuck and die, but others will find big places to live, and still others will reach the surface. Of course, as soon as the sun shines upon them their skin starts to harden, so they quickly learn to avoid it, and those who are better at avoiding it can even live on the surface. But that can also be a risk that potentially ends with them being turned to stone/hardening polymer/whatever. Point being, sometimes evolution is more of an art than a science, and not every creature is the best adapted for new environments.

Answer 10

It doesn't need to be a natural feature in order for it to be non-magic. It could be the result of genetic engineering. In the distant past, trolls didn't have this feature. But, some trolls enslaved others to work underground in mines. The trolls that were slaves were genetically modified so that they calcified when exposed to sunlight. This limited their ability to rebel. Eventually, a surface catastrophe (e.g. asteroid impact) caused the surface trolls to become extinct. The former slaves emerged from the mines and found that they could get by just fine at night but would die if caught exposed to sunlight during the day. They have been that way ever since. Since they are well-adopted to a nocturnal life there hasn't been sufficient genetic pressure to remove this engineered trait.

Answer 11

Declare trolls to be immensely specialized to their lifestyle. Everything about them is fine tuned like a shark, so that they can reach unbounded sizes. Its not easy to design a creature that can function at utterly any size, so you're going to have to give up some flexibility in terms of the environments. Some environments are simply inhospitable to what's left after you focused on unbounded grown for that long.

So the trolls have a natural solution. If they find they are in any environment which is not suitable for troll life, they go into defense mode. They literally freeze in place, and wait for the environment to change. It's a similar response as we see in armadillos, which curl up into a ball in response to a predator and wait for the predator to go away.

For the most part, this is effective. If a proto-troll, earlier in their genetic history, were to get caught out in the sun, this freezing-stone-form effect would keep them safe until the sun went down and the environment was once again troll friendly. It was very useful from an evolutionary perspective.

Now what people don't know is that trolls are rather sensitive emotional creatures. They have developed a level of empathy which we can only dream of. And, as such, it is emotionally painful to lose contact with the outer layers of the skin as they enter stone form. As such, inner layers may also turn to stone, in an attempt to avoid the sadness from the loss of contact.

Old trolls eventually develop this empathy so greatly that when the outer layers of their skin freeze, they try to hold onto them so hard that their inner core eventually freezes as well. And this leaves you with a stone troll, forever marking the moment in time where they just couldn't let go.

Answer 12

You said you want to avoid just using magic as a reason, but maybe it would provide an interesting additional layer in your story if you gave the facts a mythic explanation as well (that perhaps your characters grapple with by contrast with the scientific explanation).

So here goes:

Trolls were not part of the rest of creation but were the work of a rebellious power, one opposed to the power responsible for the sun.

With normal creation, the sun has a purifying effect that causes the creatures to regenerate and the plants to thrive, and killing off pathogens: a system set in place during the original creative work.

But because trolls didn't come from that work, the sun's purifying function is against them. So, just like a virus or a bacteria can't survive under UV exposure, neither can a troll.