Premise: In a world similar ours, whose culture is like our medieval era, there is one key difference. There is a "Darkness" (at night or in very dark places) which forms into "beasts" through some means, and they can be held back by any form of light.

I'm thinking of Brandon Sanderson's The Way of Kings as an example:

The world itself has flora and fauna which is based on surviving the common, and extremely powerful highstorms. Most animal life is based on crustaceans, most of which can burrow into the ground to survive a highstorm. Plant life is also mobile in that it retracts into the ground to survive highstorms. (Wikipedia)

Question: How might the animal world adapt?

Could organisms adapt to emit light or would they have to rely on lantern and torch light? I see a problem with luminescent animals in the night, because they'd be easy to spot for predators, even though they might repel the "Darkness."

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    How dangerous are those beasts? Can you compare them to, let's say, wolves? How many of those beasts come up in a normal, moon-lit night in the wilderness? – Liquid Oct 18 at 8:13
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    Why would there be a need to repel the "darkness"? What qualities do these darkness-made creatures have that make them so dangerous? – nullpointer Oct 18 at 8:16
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    Why do animals need any adaptation to this "darkness" thing, in the first place? – enkryptor Oct 18 at 10:27
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    What is the darkness made of? There has to be some substance, otherwise it's just no-light and no physical body and thus cannot harm anyone – Alexander von Wernherr Oct 18 at 11:35
  • Sanderson would tell you to set your beasts with limits :-)… Is light your only protection against Darknessies? Then light you'll have to use to defend against/deter them. But if you wanna make the story a bit more interesting... Set strengths and weaknesses for the bad guys, and use counter measures for these. Perhaps dark-fire, or holy water, or magical repellents or another kind? – Nahshon paz Oct 18 at 12:01

Most organisms in the deep sea don't glow all the time. They emit light only after being touched or disturbed in another way.

In your world, if "darkness" transforms into "beasts" somehow, simply standing in the dark without emitting light is not harmfull. Standing in the dark and not emitting light while a "dark beast" approached is potentially lethal, so plants and animals might adapt by lighting up only when stressed. Predators and prey alike can hide and hunt in the dark and only light up if a "dark beast" approaches.

The light is emitted by mixing certain chemicals inside the body or on the outside of the skin, just like in fireflies. These chemicals have to be produced by the body, which takes resources and energy. Not wasting these resources (by lighting up if no "dark beast" in nearby) is beneficial for the survival of the individual and would probably drive evolution in this direction.

Humans (that cannot emit light) need to light fires or lanterns to protect against these beasts. Onyz made a very interesting comment about how humans couldn't evolve to the state of making fire in this world without going extinct, so you could imagine them domesticating light-emitting animals from very early history or maybe developing a symbiotic relationship with a light-emitting species.

Some animals that cannot produce light might seek shelter near human settlements (like birds, rats and foxes) and others might hide underground (like rabbits, lizards and insects).

  • How would humans have evolved before the discovery of fire though, if these darkness beasts are dangerous enough to drive the evolution of such light-producing species? Maybe they learned to use the light produced by other species while still in their primitive state? I imagine a lot of species would piggy-back like that, given the opportunity. (Light without investing energy into it, wow!) – Onyz Oct 18 at 11:07
  • But what if a prey animal became stressed by a normal predator? Then all any predator has to do is scare its prey, wait for it to light up, and then easily follow it. – David K Oct 18 at 12:38
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    @DavidK For most animals it's not much different from how they live in our world. Most of them either need to run faster, disappear into a hole in the ground or fly away. – Elmy Oct 18 at 12:42

Plants produce so much excess energy from photosynthesis that they make fruit (an intensely rich energy source) as a bribe, so animals will eat it and spread the seeds in their dung. At least some plants will develop the ability to glow at night on a constant, ongoing basis. These plants will spend lots of energy doing this, but in exchange animals will hide out in very close proximity to them, and herbivores will be very unlikely to eat them. Burr seeds will work well to spread them (the point of a burr is the seed gets caught on fur then falls off eventually, sowing the seed potentially a great ways off). The downside is they will be slow growers (because they spend lots of energy on things that aren't growing, there's less available for that purpose).

There is a more complex series of trade-offs involved for animals.

Animals without the ability to shed light at all will have to either be good at running away (very good), or be able to hide from the Shadows, or rely on other light sources for protection (which will make them easy to find, a problem for a prey animal, but not necessarily an insurmountable one).

Most animals will not want to shed light constantly. For prey, that would be like walking around with a blinking billboard saying "Eat at Joe's", and predators that are constantly lit up like a glow-stick will have similar problems. The only animals that could get away with being lit up constantly are ones like elephants that no predator will dare to mess with.

Most animals will not want to be completely without the ability to shed light, either. So that leaves various ways of generating light briefly or intermittently.

For prey animals, doing this will draw large amounts of attention. Attention means hungry nocturnal predators that now know you exist and have a pretty good idea of where you are. (The flip side is they also know you just encountered a Shadow creature, and they are likely to want to tangle with it as little as you do.) Predators have similar issues (just backwards). Shining a floodlight on your presence is not a good way to get a meal.

In general, a wide variety of animalss would have flare defenses they'd be reluctant to use, since using them probably brings almost as much trouble as not.

Predators would also have to emit light else they'll be the hunted.

If animals glowed and formed schools or herds, the light would be confusing to predators like the stripes of the zebra.

At the end of the day everything would have to glow else be eaten so it's a level playing field.

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    This. I'm thinking of the Avatar movie, where EVERYTHING glows somehow. Predators won't stand out, because they can camouflage their glow to match their surroundings. – GreySage Oct 18 at 18:31
  • Depends if the plants also have to glow or get eaten by the darkness – Thorne Oct 18 at 22:13

they can be held back by any form of light.

they can be held back by any form of light.

they can be held back by any form of light.

Warm-blooded animals will be selected by evolution since they naturally emit more intense infrared radiation. In regions where it's not cold they might even evolve to have less fur/feathers and more skin area exposed.

Also spending nighttime in the open will be more favorable than doing so in caves or burrows, since in the open there is also starlight and moonlight to protect them critters (except when on cloudy/rainy nights).

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    The pedant in me appreciates this answer, but the asker was probably using the colloquial definition of light, which is limited to human-visible light. Though I don't speak for them, of course... – Onyz Oct 18 at 14:12
  • @Onyz it would have sufficed to say that the beasts can be held back by light, no? – Renan Oct 18 at 14:14
  • Likely he meant to generalize between sources of visible light like fire, bio-luminescence, sunlight, etc. that many sources of fiction will make arbitrary distinctions between. – Onyz Oct 18 at 14:24

TL;DR Evolution is unlikely to come up with one solution to a problem, but rather a varied seletion of species which each try to solve the problem of survival a different way.

That's already the case for real life. Every type of animal in existence has a different genetic feature set, each of which is a different solution to the problem of perpetuating life.

This immediately reminds me of the Vashta Nerada from Doctor Who. In short, they are flesheating creatures that live in the shadows. I do suspect that your shadow creatures are less seemingly-omnipresent than the Vashta Nerada, but this is a good starting point for inspiration nonetheless.

How would the animal world adapt?

At some point during the episode(s), the Doctor specifically mentions that the Vashta Nerada are why creatures are conditioned to be afraid of the dark. Being afraid of the dark is a common fear, especially for children and some types of animal. And it makes sense from an evolutionary perspective, as it is pure darwinism:

  • Those who are afraid of the dark will go out in the dark less than those who are not afraid
  • As a result, those who are afraid of the dark will be killed less (by the Vashta Nerada).
  • As a result, they procreate more often than their peers who are not afraid of the dark.
  • Over many generations, an innate fear of the dark is bred into the species. They no longer know why they fear the dark, but their instincts are set in stone.
  • This fear is instinctive, but it can be rationally overcome, which is why adults can lose their fear of the dark if they consistently do not observe any negative consequences from being in the dark.

This is already the case. There is a logical reason for our instincts to tell us to be afraid of the dark: you can't see it coming ("it" being whatever is going to imminently kill you).
The only difference between your world and reality is that the cause of death is a "shadow creature" instead of just an "unseen danger".

There are many other rational instincts whose justification is similar: fear of heights, fear of being alone, fear of animal howls, fear of being around dead bodies, ... ALL of these situations are situations in which you're more likely to die soon, and thus we instinctively learn to avoid them.

Could organisms adapt to emit light or would they have to rely on lantern and torch light?

There are many ways to evolutionarily solve a problem.

Bioluminescence is one such reason, but it's a rather intricate one. The more intricate solutions tend to be less likely to emerge. There will be some species that evolve into being bioluminescent, but it won't be ubiquitous.

You haven't really addressed any other weaknesses of the shadow creatures, so it's hard to tell you what would counter them. I'm just going to invent some things to showcase the point:

  • If the shadow creatures cannot travel through water, animals could learn to sleep underwater (using the "upside down bucket" style air bubble - or simply by evolving waterbreathing).
  • Can these creates walk through walls or are they bound by the physical realm? Because if it's the latter, any form of closed hole/house/door will keep them out.
  • If the shadow creatures cannot stand even the dimmest of light, other creatures may flock to places with light: a pool of lava, a mountain of phosphorus, anywhere near bioluminescent creatures, ...
  • If the shadow creatures cannot fly or jump high, animals can safely sleep in trees whose bark is covered in bioluminescent creatures.
  • How do the shadow creatures track their prey? Camouflage, hiding your smell, looking like a different (to the shadow creatures inedible) animal, ... are all viable options to counter whatever method the shadow creatures rely on to hunt prey.

Remember, you don't need to perfectly counter the shadow creatures, you simply need a statistically significant population to survive between birth and procreation.

  • Maybe once they've procreated, the parents leave the nest and give off a different musk, one that smells tasty to the shadow creatures, so that the parents are more likely to get eaten instead of the children, thus perpetuating the species.

In absence of any viable biological solution, instinctively fearing the dark is always possible. In other words, if an animal does not develop any biological countermeasures, maybe the instincts of the animal merely adapt so that the animal only feels safe in locations that minimize the odds of being killed by the shadow creatures.

I see a problem with luminescent animals in the night, because they'd be easy to spot for predators, even though they might repel the "Darkness."

It's a numbers game. Which kills your species the most: the shadow creatures, or other "normal" predators?
If the shadow creatures are the biggest natural enemy, then the animals will evolve to counter the shadow creatures more than the "normal" predators. If the shadow creatures only rarely kill (compared to the other predators), the animals will evolve to counter the other predators.

Evolution always favors the statistically highest survival chances.

Actually, a night in the wilderness can be already quite dangerous as is, since many predators are nocturnal:

  • Nocturnal birds
  • Bats
  • Canids (from foxes, to coyotes, to wolves)
  • Felines (from the wild cat to tigers, ocelots, lions)
  • Some musteliade (like badgers)
  • Some rodents

And this came up just with a quick search (source: nocturnal-animals-wikipedia. Funnily enough, that page doesn't mention wolves). Assuming your setting is the typical european like medieval country, at least some of those animals will be active at night.

What's most interesting is that there are nocturnal predators for vary "scales" of life - from bats, mostly interested in insects, to wolves. So I'd say that our everyday fauna has already, to some extent, adapted to nighttime being dangerous.

The real question is how those darkness beasts of yours will change the game. First of all, what is their size? What's their behaviour? "Created by darkness" isn't really explanatory in this sense. Are they created with a innate hate for life? If that's so, do they attack blindly everything alive (plantlife included)?

Or rather, do they have some more structured habits? Large, scary predators don't bother with small prey - you won't see a lion hunting for insects (and I suppose most lions won't go after small rodents, if not during playtime).

So, first of all, you have to figure

  • how big / dangerous the beasts are
  • how do the beast behave
  • how frequent those beasts are

Now, it's true that bioluminescence is a cool concept and would make some very alien-unique looking animals, but having an entire bioluminescent fauna would look a bit ... strange, imho, and cheap. Of course, in the worst scenario (the beasts are ravenous killers, frequent, and impossible to deal with if not with light) this may be the only option. But if the beasts have more realistic hunting habits or a preferred prey (that may as well be humans), the fauna could work around with less drastic measures.

Consider that the example you bring - Sanderson setting in the "The Way of Kings" is pretty much on the extreme side, since the storms there hit everything and everyone indiscriminately.

For example, your apex predators will change behaviour. Wolves or other large carnivores will probably strenghen their pack habits: during the hunts, a few of the pack members could always stay on the lookout for darkness beasts. Lone predators will behave more like feral cats. Cats are a good examples beacuse they are both predators and prey - so their behaviour reflects both the need for hunting aggresively, both the need to protect themselves from predator (e.g. hiding in small spaces, climbing trees, and so on).

Of course, I'm not excluding that some animal may develop intermittent light, as other users have already suggested. Maybe you could build up some interesting symbiotic relationships between animals and bioluminescent plants or fungi: a small nocturnal animal (like a rodent) may live around a set of those plants, and strike them when it senses a darkness beast coming. The plants flare a light when it, and may receive benefits from the rodent presence (since it eats parasites and so on).

Ethereal Essence of Darkness

...a "Darkness" which forms [presumably by means of magic] into "beasts" through some means...

For the sake of the response, I assume that some sort of tangible link exists between the physical world and the source of the Darkness to permit the Dark Beasts to form.

Therefore, plants (and animals) could, theoretically, adapt to and utilize the ethereal presence that forms the beasts, borrowing some attributes of its existence for the sake of adopting survival traits (and techniques).

Examples of how elements of the Darkness -- used to manifest the beasts -- can (and/or cannot) be adopted by a local species:

  • Blending into the Darkness by exhibiting no characteristics other than that of the surrounding Darkness, or by emitting an "aura" of the Darkness essence.

  • Releasing a pheromone for the Darkness to be lured into destroying the plant, thus spreading the seed. (This, of course, would not help animal survival. Although it would be an interesting style of survivalism for a critter to rely on the luminosity of the plant, until it releases the pheromone.)

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