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I am considering the possibility of giving the creatures on my planet fur with UV-florescent pigments as an extra defense against the high levels of UV radiation from their star, an F3.5V

This planet is, for all intents and purposes, nearly identical to earth in size, gravity, atmosphere etc. It orbits a 1.3 stellar mass star at 1.7 AU.

My question is this: Firstly, even on our planet, florescent materials "pop" at dusk or on cloudy days. I'm assuming that around an F-type star with much more UV radiation, the effect would be more dramatic. How MUCH more dramatic could you see this being?

Secondly, even if it were not more dramatic, this seems like it would wreck the camouflage of any hunting predator or hiding prey. I could give the plant life the exact same adaptation so that the whole landscape glows, allowing glowing predators and prey to blend in at dusk, but this would still cause problems in rocky, barren areas and something about having most of the biosphere wearing high-visibility jackets seems... risky? I would like the thoughts of anyone interested in evolutionary theory.

To head off some potential questions: I am not asking about the habitability of the star, but the visual and practical affects of having florescent fur in this environment. Some of the UV radiation is being handwaved away with a thicker ozone layer, but not all of it. I have backup protection measures if florescence is not plausible and won't mind too much if it turns out to be impractical, but I was obligated by the coolness factor to explore the possibility.

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    $\begingroup$ (1) Why would fluorescent pigmentation provide extra awesome protection compared to ordinary non-fluorescent UV absorbent pigmentation? (2) There is basically no UV in the daylight of cloudy days. (3) One thing is for sure, ruthless natural selection will quickly resolve any conflict between fluorescent pigmentation and the need for camouflage. $\endgroup$
    – AlexP
    Commented Sep 30, 2023 at 14:36
  • $\begingroup$ The idea of it helping with protection is based on the idea that some pigments absorb the harmful UV and re-emit it as visible light. I was aware it was a bit of a stretch, but I felt the need to at least investigate it before moving on to potentially more practical ones. Agreed about natural selection. But what informs your assessment of basically no UV on cloudy days? I've been told time and time and TIME again that you can sunburn on a cloudy day. $\endgroup$
    – Lynx
    Commented Sep 30, 2023 at 14:56
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    $\begingroup$ (1) UV light is strongly attenuated by water. A light cloud cover, such that the sun is still visible through it, does attenuate UV radiation but only partially; on such a lightly cloudy day you can get sunburn, if your skin is lacking pigment. If the cloud cover is enough so that you cannot guess the position of the sun, then there is no risk of sunburn. (2) Yes, to confer protection against UV the pigmentation of the skin must absorb it or reflect it. What I don't understand is what does fluorescence buy you in comparison with ordinary black pigments which absorb UV and convert it to heat. $\endgroup$
    – AlexP
    Commented Sep 30, 2023 at 15:26

3 Answers 3

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Compare your creatures that live under UV light to creatures that live in deserts...

Your star gives off more UV than the sun does. For some short wavelengths, it gives off a lot more. The short wavelengths have enough energy to break carbon-carbon mods which is why your need sunscreen. Maybe life evolved in cracks and shadows, or under the sea. But they have adapted to survive on the land, and maybe some tougher ones have adapted to survive in full sunlight.

Desert creatures survive too much sunlight. The risk in our deserts include overheating and dehydration. These creatures also have to avoid UV exposure. It seems likely they will adopt the same strategies as our desert creatures: stay out of the sun, if you can't, carry your own shade with you, or put on sunscreen if you are human.

Human sunscreen is not fluorescent. Or, at least not visibly so. Organic fluorescent dyes are fugitive - they bleach since the photon energies they absorb are enough to break bonds. So, any creature that uses fluorescent organics will have to keep renewing them. That is the kind of dumb strategy that mating often encourages.

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My question is this: Firstly, even on our planet, florescent materials "pop" at dusk or on cloudy days. I'm assuming that around an F-type star with much more UV radiation, the effect would be more dramatic. How MUCH more dramatic could you see this being?

You'd still get a lot of visible light, so it's more like having very powerful black light lamps everywhere. Might be quite anticlimatic, in that you get a little more glow but it still gets outshined by daylight.

Secondly, even if it were not more dramatic, this seems like it would wreck the camouflage of any hunting predator or hiding prey.

Meet the scarlet macaw.

A scarlet macaw resting on a branch

This critter lives in, among other places, the Amazonian jungle. It is the only place in the world that compares to Australia in terms of everything that moves wanting to kill you.

The jungle foliage is green. These [redacted] are [redacted] bright red with bright yellow and bright blue wings, as if saying to all JAGUARS and BOA CONSTRICTORS and HAWKS:

COME AT ME [REDACTED], I AM RIGHT HERE WHERE YOU CAN SEE ME

As if these birds weren't scandalous enough, they are also loud. Not only in decibels but like they can't shut up.

So how does a critter that only eats insects and small fruits manage to survive without becoming the easiest meal ever to any predator? Specially in a place that is full of jaguars, which are tough enough to hunt anacondas underwater and who are known to mimic bird calls?

You see, flamboyance among macaws is a means to show that they are tough and cunning enough to defeat any predator that comes their way. It's like saying "I am a living neon sign that says free food and yet I manage to live an average of forty to fifty years in the wild". All the weak ones were removed from the gene pool millions of years ago. The survivors are some of the nastiest animals I've ever met.

I would let an owl or hawk or even an eagle perch on my shoulder or forearm anyday. I don't want to get within three feet of a macaw. They aren't as murderous as the birds of prey, but they can grasp with a lot of strength too and have very sharp claws, and unlike the predators macaws have no respect to other animals, their own lives or even God in Heaven when triggered.

I once saw a DC comic where some villains were comparing heroes. They came to the conclusion that Superman is usually nice, and Batman can break some of your bones but Flash is the worst because after he defeats you he will talk to you and that's absolutely humiliating. What does this have to do with macaws? Well, like parrots they can mimic the sounds of other animals. And the way they do this is by imitating the calls of predators. The kicker though is that while sometimes they do this to prevent conflict, they also love doing this AFTER winning a fight against a predator. Pure sadistical taunting.

That is not to say that they don't get eaten here and there. Jaguars sometimes get desperate, and boas are too dumb to ponder if they can get dinner unharmed. But overall macaws survive because usually they are not worth the hassle.

Your day-glo fauna might just have the same vibes. A predator might look at something that shines like a pop singer wearing a disco-ball costume under a floodlight and then, either by instinct or prior experience say to itself: "eh, I'd rather eat dirt".

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    $\begingroup$ Yeah, I've heard macaw beaks described as "bolt cutters" before, just to add to the ways they can put the hurt on anything that tries to mess with them. $\endgroup$
    – Shalvenay
    Commented Oct 2, 2023 at 2:31
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    $\begingroup$ It won't let me flag multiple answers as correct, but your information combined with the other answer is very helpful. I feel relieved that I don't have an excuse to make everything glow (the project is bloated enough with details already) but now I have information for why some creatures might glow, which gives me lots of fun macaw-like possibilities. $\endgroup$
    – Lynx
    Commented Oct 2, 2023 at 22:05
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Fluorescent or eye-catching patterns aren't always a downside. Sometimes they're actually a defense mechanism.

  • Confusion: Zebras' stripes make it difficult for predators to immediately tell where one individual ends and another begins, making it hard to single one out and focus on catching them. There's also evidence that the stripes might help confuse biting insects looking for something to feed on.
  • Warning: many species of amphibians and invertebrates use bright colours as a warning, saying "I am poisonous—don't eat me!" This can be safer than being disguised: predators have evolved to be very good at finding their prey, even through camouflage. If a bird sees a small amphibian hiding in the shallows, it probably won't care if it's a toad or a frog or a salamander, because they're all delicious. A bright red poison dart frog, on the other hand, it will actively avoid eating.
  • Intimidation: animals often will puff up or make themselves look larger to ward off threats, and winged insects sometimes have eye-like markings on their wings. An organism on your planet might normally have the fluorescent parts of their body held close to themselves, like feathers or fur folded up, giving them a sort of striped or striated appearance. But when threatened, they could flare out, revealing a large, dazzling pattern that suddenly is bright and intimidating.
  • Camouflage depending on the other species, fluorescent colours might actually help blend in. If all the plants nearby are bright and patterned, a single moving dark spot will probably stand out like a sore thumb.
  • And, as mentioned already, showing off: an organism painting a target on its back and still being able to survive and thrive is a good indicator of fitness, and so might become a desirable trait in mate selection, with brighter colours indicating both better nutrition and the ability to survive despite more risky patterning.
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