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In my story, there is a planet that experiences near constant sunlight. Since sunlight so plentiful and reliable most organisms are, at least in part, photosynthetic.

The only intelligent life on this planet is plant-like people that get their energy entirely from photosynthesis, and absorb nutrients and water from suitable soil (which is not rare).

They are roughly similar to humans in size, shape and appendages, and general intelligence. They are mobile, because they are prey, although this changes as they get older: they never stop growing and can live for several centuries. At some point, the creature becomes so large that it plants itself and never moves again. These elders continue to grow wiser as they age (their bodies essentially becoming shelters for continually growing brains), though not many reach these extremely old ages.

They have never really developed a psychology that promotes warfare for three reasons:

  1. Since they are photosynthetic, they don't need to kill (or even farm) for food.
  2. They are hardy, and younger members can quite reliably regrow, which makes war among the species impractical with their level of technology. This is so reliable that 'murder' in their society is still a crime, but not nearly as terrible as it is on Earth
  3. Their predators are much more capable than them, so they can't feasibly exterminate them.

My question is fairly open ended: these creatures don't need to collect resources for food or shelter, so what would drive them to do anything but lay in the sun all day?

The species prizes the arts over sciences, but I can't see why or how they would have developed the tools to make paints, instruments, sculptures, etc.

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    $\begingroup$ How do they get constant daylight? Is the planet tidally locked? Multiple suns? Artificial light from mirrors in spade or something? Also, I think you will find that photosynthetic higher life forms are not possible, as there isn't nearly enough energy in sunlight to provide the energy advanced beings need. Maybe you could have them 'hibernate' for part of the time, where they unroll a lot of surface area of photosynthetic 'skin', then hibernate while their bodies convert sunlight into stored energy. When 'full', they roll up back to their waking size and go about their business. $\endgroup$ – Dan Hanson Oct 3 at 17:28
  • $\begingroup$ They are on an eyeball planet that faces the star, so at least in the part of the world they know about daylight is constant. Is there really not enough energy in sunlight? I would think that would be the most efficient way of getting energy. $\endgroup$ – Aetherfox Oct 3 at 17:38
  • $\begingroup$ "Their predators are much more capable than them, so they can't feasibly exterminate their prey." did you mean aren't there? because it doesn't make much sense at the moment. $\endgroup$ – Starfish Prime Oct 3 at 17:50
  • $\begingroup$ @StarfishPrime Whoops. I updated the question. Basically I meant their predators are much bigger and stronger than they are, so they can't realistically use their greater intelligence to get rid of their predators. $\endgroup$ – Aetherfox Oct 3 at 18:01
  • $\begingroup$ Bears, lions and tigers are bigger and stronger than humans. Yet with out intelligence we almost drove them to extinction. $\endgroup$ – L.Dutch - Reinstate Monica Oct 3 at 18:33
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The best way to figure out what motivates your creature is to consider the constraints it lives under.

Energy Requirement

First, let's see if your creature can get the energy it needs. An average person consumes about 2000 calories per day (food calories, or 2000 kcal). That's equivalent to 2.3 kilowatt hours of power.

Energy in sunlight at the equator is about 2300 Watts per square meter. A human has about 1.7 square meters of surface area, but only half would be facing the sun at best. Let's say it's a big creature, and therefore has 1 square meter of surface area facing the light.

The most efficient plant at converting solar energy is sugarcane, and it converts 3.5% of light into energy. So the available energy to the organism will be 2300 Watts * 3.5%, or about 85 watts. So to get the amount of energy that a human consumes in a day would require your creature to absorb 27 hours of direct sunlight, assuming the sun was as bright as the Earth's at the equator.

But that's a very, very optimistic number. Every detail from there makes it worse. For example, maximum solar efficiency requires that the collector be tilted to the angle of the sun. Then there's cloud cover, and shade, and...

On Earth, a large tree on average collects maybe 200 calories of energy in a day. That gives you an idea of the real world efficiency of photosynthesis.

Planet Problems

My understanding is that a tidally-locked planet close enough to a star would have any water evaporate from the day side and freeze out on the night side. If the planet is far enough away for liquid water to exist on the day side, it's not going to get nearly as much solar energy. On such planets, the only areas where a plant species might survive would be along the terminator, but the solar energy on the terminator would be much lower. Also, there would be a lot of clouds in that area.

On a cooler tidally locked planet you could have water on the 'day' side, but probably only if there was enough water for a global ocean, because the water on the 'day' side would be melted ponding on top of the ice. If there wasn't enough water to go around the globe, it would still all be transported to the 'dark' side where it would freeze out into a permanent ice cap. Perhaps on such a planet there would be a larger habitable zone along the region where ice turns to water. Again, you would expect lower solar energy there than at the hottest place on the planet.

There might be a way around this - we don't know much about such planets, and there are probably variations we haven't thought of. But it will take some work in planet design to come up with an eyeball planet that can host sentient plant-creatures.

Creature Design

But I think that's still do-able. Your people would have to be large, spread out, maybe with leafy sprouts all over to maximize surface area. They'd probably also be slow moving and slow thinking, as thinking takes lots of energy.

Consider creatures like a Tree Sloth. A Tree Sloth only uses about 110 calories per day. So a large, slow-moving creature with lots of surface area could possibly work.

What would motivate such a creature? Lots of things. For example, first contact with a faster thinking, faster moving species like us. Or a natural disaster, such a plague of mites or bugs that starts eating them, requiring them to up their game. Or another natural problem such as drought or CO2 depletion.

If you are asking what motivates them as a species absent any external shocks, that's a bit tougher. Why would such a species evolve intelligence in the first place? What problem does that solve?

Perhaps they have a symbiotic relationship with another species that can tolerate heat better. Maybe there's a type of vine that grows underground for long distances, then sprouts where it's hot and very bright. The symbiosis is that the sentient species provides water to the vines, and the vines return nutrients for the people or something.

Here's a potential idea: They are networked. Rather than move around and consume energy, they are motivated to connect to each other either through roots or temporary physical contact. Because if they can share their chemical resources and concentrate them , they can advance and prosper.

Perhaps they all started out equal and non-sentient, but once they started connecting together some evolved the ability to extract extra nutrients from the network, and they became smarter and faster thinking. Eventually they all became sentient, but the creatures in the 'super clusters' in the network get more than their share of energy and therefore evolved more intelligence to manage and improve the network.

That creates a society where the powerless have more than their share of energy extracted for use by the 'advanced' ones. So you have armies of plant people who move slowly and without much thought, providing their photosynthetic energy to the 'haves' who need it to think at higher levels. This creates a stratified society where only a few plant-people can really think and work at a high level, but only because the majority have to do with less than they would on their own. But it's stable because the powerful ones do more good for the society through invention or organization that the individuals could do on their own. An analogy would be an anthill with lots of workers, except the 'work' involves optimally spreading out to collect as much light as possible. This feeds energy back to the 'hive', or cluster of higher-level members of society (although in an anthill that's not true - queen ants aren't any smarter than any other ants - they just use the excess energy to breed more). Still, the same idea - masses of creatures that aren't motivated to do much more than find good places to spread out and relax, and their thinking is constrained by lack of energy. In the meantime, creatures in the 'hubs' have plenty of nutrients, so over time they became smarter, faster, etc.

Then one of the smart ones invents a new energy source the masses can use to bring themselves up to the same level. As this tech spreads and members of society can supplant their own photosynthesis with extra energy, they start to think better and faster, and decide they no longer need the 'smart' ones. And the 'smart' ones are going to do everything they can to hold on to their power over their society. Wars break out, etc.

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  • $\begingroup$ +1 for your consideration of energy budgets. That said, the fact that the OP describes the species as mobile and that they have brains and energy to spare for art suggests that some or more of your objections were of necessity overcome, probably starting with photosynthetic efficiency. $\endgroup$ – Starfish Prime Oct 4 at 13:03
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Part one, your actual question:

My question is fairly open ended: these creatures don't need to collect resources for food or shelter, so what would drive them to do anything but lay in the sun all day?

The species prizes the arts over sciences, but I can't see why or how they would have developed the tools to make paints, instruments, sculptures, etc.

A few things spring to mind. Firstly, seasonal changes might require migration (even eyeball worlds can have eccentric orbits that could drive a summer/winter cycle) and constructing shelters, clothing or foodstocks to reduce or avoid the need for migration requires some engineering and possibly some form of hunting/gathering or even farming.

Secondly, expansion into less amenable landscapes might require some work as well... irrigation networks, clearing unwanted vegetation that might compete for light or secrete inhibitory chemicals, etc. Not quite warfare, but slash-and-burn is a bit more aggressive and closer to home if you're a plant-person.

Thirdly, predation. Recognising the presence of, or seasonal arrival of predators and then taking steps to avoid them, evade them or build defenses against them would be an imporant part of their lives

All these things go together to form the foundation of a technological society that has to at least occasionally do stuff, and when it doesn't have to do stuff then maybe energy (or food) is sufficiently abundant that leisure time activities could be enjoyed instead of simply vegetating.

Which brings me to idea number four: reproduction. Maybe the art wasn't originally for its own sake, or for entertainment, or to impress other plant-peeps. Maybe it was part of a display to make the artist seem more appealing to other species who might be opportunistic or maybe even symbiotic. Pollinators, fruit-eaters, pest-eaters and seed-spreaders, oh my. Maybe even sources of nutrients, either directly (by being eaten) or indirectly (by providing nutrients from their excrement, or maybe even corpses).

Self decoration and modification to attract more birds and insects and so on, with a knock-on effect that it would also attract "mates" in the form of other members of the plant-peep species who'd like to bask in the "halo effect" of the ecosystem around a particularly talented individual, and so increase their own chances of reproducing with them.

(you could just rub flowers together and avoid pollinators, of course, if you were some kind of deviant, but eew)


Part two, nitpicks:

They are mobile, because they are prey

This is a somewhat important detail. If they need to be very mobile, they'll probably need a source of energy a bit more concentrated than sunlight (or alternatively, vastly more efficient photosynthesis than you'll find on earth). A sloth isn't going to outrun a pack of wolves, for example (or some other suitably arboreal predator; probably some kind of ape or monkey).

Since they are photosynthetic, they don't need to kill (or even farm) for food.

Are they exclusively photosynthetic? Or can they eat and digest stuff as well? Because being able to supplement your diet is quite a useful thing.

Also note that plants are made of more than just carbon, hydrogen and oxygen, and as such need a good supply of many more materials than just air and water (nitrogen, phosphorous and potassium just to start with, and they need to be in a suitably bioavailable form, too). A plant doesn't need to be a predator like a venus flytrap or pitcher plant to benefit from the nutrients tied up in the body of other animals... just murder them somehow (coconut on the head?), smash the body up a bit, bury it in a shallow grave and fill yer roots.

This is so reliable that 'murder' in their society is still a crime, but not nearly as terrible as it is on Earth

Y'know, if you had to work really hard to murder someone, really smashing them up and dismembering them and burning the bits in order to make it stick, wouldn't that be a worse crime? You don't woodchip someone by accident, or in the heat of passion...

Their predators are much more capable than them, so they can't feasibly exterminate them.

Intelligence is an extremely powerful lever. Predators may be more effectively evaded, but they may also be poisoned and trapped and their eggs or young (or saplings, as it may be) found and destroyed whilst the parents are unavailable. The most useful thing an intelligent species can do to prolong its existence and propagate itself is to remove overt threats.

Unless those predators are themselves just as intelligent, if not more so. In which case it is in their advantage to farm the dumber and tastier species rather than just letting them run around the place willy-nilly. The situation as you describe it does not sound entirely stable.

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Their peculiar energy economy might cause some interesting behavior. As noted in other answers, the energy density of photosynthesis is going to be a challenge when it comes to cognition. Perhaps it is the case that your creatures are very dependent on the Elders.

The Elders are probably going to be subject the square/cubed effects (surface area doesn't scale as quickly as volume, so big creatures are relatively heavy). Despite this, they still get much more energy than their smaller comrades. There's no reason cognition needs to scale with anything in particular other than energy, aka surface area (well there's the need to satisfy motor control which is less significant in your sedentary Elders -- sidenote, movement for an Elder could be a task that requires them to activate their abstract thinking brainparts, which would make it so they couldn't "walk and talk at the same time" so to speak).

If we look at Machine Learning (despite the fact that ML/true intelligence analogies are a bit fraught), there's a split between "training" and "inference." Training when the computer looks at the data set and tries to build a model, while inference is when it applies the model (so training might be looking at all the cats and picking out the common features, while inference would be identifying cats). Training is generally more computationally intensive, slow, and massively parallel. So, it might be the case that the Elders receive information from the Sprouts, and use that to build models. Because these are true intelligences and not ML algorithms, these models are more like ways of thinking. The Sprouts go out into the world and try to gather more information/experiences to improve the Elder's datasets.

The degree to which this upload/download process is physical or, whatever, psionic, is up to you. But it could be the driver for a significant amount of art building. Perhaps it is really concrete -- the Sprouts re-enact the stories of their adventures to the Elders in little plays or spoken word poetry. Perhaps it is more symbolic -- they draw beautiful patterns that represent a compressed form of knowledge (perhaps these patterns are also Elder specific, so communicating to far-off Elders needs to be more concrete).

So, the Sprouts are mostly executing the worldview of the Elders. This makes them extremely dependent on the Elders. They might spend a significant amount of effort tending to the Elders -- cleaning them of parasites, gathering resources, etc. This might also induce some strange behavior on their part. For example, assuming their Elder isn't under some immediate nutrient crisis, the value of a Sprout is mostly tied to the information they carry (because information lasts forever). So a sprout at the beginning of an adventure, who's just info-dumped to the Elder, is relatively valueless and might exhibit extreme risk taking behavior (in an attempt to get more information to bring home), while one nearing the end of a journey just wants to get back.

One other thought about the energy situation is that, because the Elders gather much more energy, they might save some (in the form of sap) for lean times. They might also be able to give this out to Sprouts as necessary, to lessen the tyranny of their poor energy economy.

All in all, it seems like you've got a pretty good setup for various "worship the great old tree of wisdom" tropes.

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One driver for conflict could be disease. Plant beings that would be dormant for large amount of time during their lives (as people are suggesting from the amount of energy one can collect through photosynthesis), they would be quite easy hosts for parasites. Assuming they have no technological means for getting rid of these parasites (antibiotics, pesticides, etc), quarantine would be the most efficient way to keep infestation to a minimum. You said that they are more artistic than scientific, so locking up a large number of these individuals probably woudln't sit too well with them.

EDIT: You might want to check out, if you haven't already, this species of sea slug, if you want a slightly different take on the photosynthetic sentient beings. This animals eats leaves (or algae, can't recall) to absorb their chloroplasts and use them to generate energy through photosynthesis. Interestingly, coevolution of this slug with its source of chloroplasts means that it has integrated some genes that code for proteins that help sustain the chloroplasts it absorbs that the plant usually produces to maintain them. Very interesting biological phenomenon!

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Well, when you think about it a plant needs more than just sunlight and water, and a plant is also at risk of other factors around them. If you're making the species part plant, then I'd recommend adding

What they need:

Water: a plant always need the right amount of water, and I noticed how you wrote down rivers so you have that covered.

Oxygen: plants need to be exposed to clean air, and since they can produce it as well it would be a type of natural exchange.

Nutrients: I know you said nutrients, but it's a bit more than just dirt. Some plants like trees or grass can absorb nutrients from a deceased animal [insert Lion King joke].

But to answer for what could motivate them to move around? well...lots of reasons.

Predators: I like how you brought up how predators , but doesn't it explain why they should worry about finding shelter? You also mentioned how they plant themselves when they get bigger, so what says a large animal or an insect could harm them as they stay still in the ground?

Pollution: I know you said that they're the only intelligent beings on the planet, but what about natural pollution, like animal waste in the rivers or volcanic smoke rising to the air? They could possibly absorb non-clean nutrients and could possibly get sick or die, right?

Over-energize: What are the chances that too much water, nutrients or sunlight could harm them? Every living thing has limits on what they need, and too much sunlight is an actual thing for plants for it could affect their photosynthesis and cause them to wilt.

I won't provide changes to your story, for your ideas are your own, but I hope that what I provided helps you out in some way.

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Trees and all other photosynthetic plants are in a constant state of warfare with one another. A walk in a forest might seem peaceful to us humans, but all the plants are throwing elbows, racing to get as much sunlight as possible. It's just that their timescales are so much longer than ours that they seem to be at peace. Trees live and die by slight advantages that allow them to grow faster, crowding the canopy or otherwise sidestepping the straight-line race to the top. Some have evolved to get ahead with some pretty nasty tactics, too. Look at strangler figs, which literally strangle other trees. Or black walnuts, which poison the soil around their roots so that no other tree species can grow nearby. I like the idea of a peaceful plant people, but I wouldn't rush to the conclusion that your plant people would necessarily be pacifists. Maybe there's some middle ground. (I'm reminded of the hermaphroditic aliens in The Left Hand of Darkness who aren't brash men and thus don't engage in outright war, but who do assassinate each other in small numbers when the need arises).

Here's something else to consider: photosynthetic plants do not simply absorb nutrients from the soil, and thus are not self-sufficient in that regard. Their roots are intertwined with the hyphae (hair-like threads) of fungi, which provide minerals and other nutrients in exchange for the sugars made by the plant. So as Dan Hanson wisely suggested, it might make sense for these photosynthesizing plant people to develop some sort of symbiotic relationship with another organism (i.e. MUSHROOM PEOPLE!)

And unless the plant people's version of photosynthesis were a lot more efficient than terrestrial plants', or unless they supplemented their sun snacks with some other energy source, their metabolisms would be much slower than ours, and so they would likely move around very slowly, like Tolkien's Ents (though I'm guessing you would want to avoid that parallel).

I'm wondering: How plant-like are they? And given the staggering variety of Earth's plants, how would you define plant-like? Is their skin bark? Do they photosynthesize through their skin or through leaflike growths? If they are that plant-like, do they also produce flowers or fruits? If so, that could create an opportunity for some sort of artistic expression. On earth, flowers evolved to attract pollinators, but maybe on your world, they're a means of self-expression.

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