How can I explain tree-like creatures that are only active in the cold of night

Question

In a world close to ours, on a planet, intelligent "Ents" or tree-like creatures live. The problem is, the creatures become dormant during the day; they "hibernate" when the sun is up, which heats up the planet. They become active when the sun goes down and it gets colder. The tree-like creatures need sunlight, oxygen and hydrogen (water), and soil to live and survive. The "hibernation" process is like sleep for humans, only caused by sunlight and heat.

What could be the reason and explain the "hibernation" process of these creatures? The cold doesn't cause them trouble, but without sunlight and heat during the day they will die, but they are awake when there is no sunlight and heat. Sunlight might be a source of energy for them, and they accumulate it while they are dormant during the day. That is the reason they don't go to live underground.

Answer 1

Have you ever heard of a Tardigrade??

They’re hardy, ubiquitous little insects that can survive boiling, freezing, hard vaccuum, hard radiation, crushing pressure and almost total dehydration by entering a sort of hibernative state (with the awesome name of cryptobiosis) whenever the environment gets a bit tough. In one case a payload of Tardigrades bound for the ISS was atop a rocket that exploded. The report said ‘some of the Tardigrades may have died’. Not some may have survived: the default position was that they’d be OK with being blown up. When Tardigrades are hibernating they’re nigh on invincible (ok, that’s hyperbole, but they can survive a damn sight more than anything else I know)

So too your trees. They need the cold. They like the cold. When the environment gets too warm and hostile? They just shut down in self defence, then when it cools down again they shake it off and carry on as normal. If they were to remain active in the heat they might die, so instead they just... stop.

Answer 2

Nature already provides you the answer. If your tree creatures are alive, they require energy. If they host complex neural pathways which give them human-like intelligence, they require even more energy.

If the only time they can generate and store energy is during the day time, then they will have evolved to shut-down all portions of their body not required to produce energy (ATP->PGAL->glucose actually) so as to be more efficient.

Your psuedo-ents, erm, creatures will likely need to produce large amounts of energy to function during the night time. This means that they will turn the glucose in to more complex compounds, such as fats and proteins, which can be metabolized during the dark period. This could manifest in something simple like what we call fruits (maybe interesting for others to visually be able to see the energy levels of your plants), but also just stored internally-to-the-cells in vacuoles.

Why exactly do they need to be stationary to generate energy? Water. They need to "root-in" to the ground to gather enough water to complete the photosynthesis process. This is an energy intensive process which is basically an investment with negative returns over the short-term (i.e. the tree will use more energy rooting than it will gather for say the first 3 hours of sunlight). Therefore the creatures must remain stationary, borderline in stasis/hibernation, for long periods.

Depending on how much you want to stretch science, another, likely secondary, reason why they need to enter a deep sleep is that the membranes binding the necessary enzymes used during the photosynthesis process may become more rigid, and therefore less efficient, when the creature is in the awake state (say do to a higher "metabolic" rate).

For a great and simple explanation see this page on the Calvin-cycle.

Answer 3

Hibernation Is Universally For a Few Simple Reasons

In nature a very large part of evolution revolves around dealing with cyclical resource shortages and dealing with periodic unfavorable conditions. Two major methods are migration or hibernation. A major evolutionary reason why human's and other large mammals sleep at night is to avoid the night period where resources are harder to gather and the risk of becoming prey is increased. Its a major evolutionary advantage to curl up in a safe space and sleep thus conserving energy and avoiding the worst of the predators. The most plausible cause of your tree-creatures "sleeping" is that the resources they require are not available at night.

You see, being a tree is great. You can photosynthesize your own nutrients, no need for risky survival strategies like hunting, or avoiding predators. Best part is, since you aren't moving all of your nutrients can be directed at either growing, reproducing, or storing for later. All of these advantages quickly disappear if you start adding mobility to the mix. Photosynthesis is an efficient method of deriving nutrients when you are not moving, but once you are you find out that a metabolism that supports even basic slow mobility is an order of magnitude more hungry than simply sitting there passively collecting sunlight and absorbing water. If you granted the ability to move to a plant here on earth it could never photosynthesize even a fraction of what it needed to be capable of true mobility. Granted, your tree-beasts aren't really trees, but they are more or less deriving their nutrients from similar processes.

Now you mentioned that they are using hydrogen and oxygen, along with minerals from the soils. This tells me that they are probably reacting oxygen and hydrogen into water and using the energy from that reaction to run their metabolism. (Normally this is a big hot explosion, but we are going to assume that these reactions are taking place internally and at a much slower pace.) The soil contains the minerals and chemicals they need to both build their own structure and also to create the reaction necessary to combine oxygen and hydrogen into water, thus releasing energy. You can't just mix the two gases together, you need to add energy to start the reaction. So my theory is this, your tree-beasts utilize alkali metals in the soils and combine them with water in the presence of hydrogen and oxygen in trace amounts to spark this reaction.

Sunlight adds heat, which lowers the amount of energy that needs to be added to the hydrogen and oxygen mix in order to create the chemical reaction that is driving the tree's metabolism. So during the day the tree-beasts are actually basking in the sun and sleeping to conserve energy while their bodies perform this chemical reaction. Again we run into the question of why they even evolved to move, so I posit this.

They move at night using the energy stores they built up during the day in order to search for soils high in the reactive chemicals they require to perform the chemical processes necessary for their metabolism. Also, it should be noted, with a bio-chemistry like this they will essentially burn like literal rocket fuel or even explode, so that could be a unique weakness they have.

End Note

I'm just trying to make the fiction sound more realistic because no known biological processes work like this utilizing these chemicals and I don't even know if it's possible, I'm just trying to make it as believable and scientific as possible. For any chemists or biologists on here please don't cringe and scream at the screen too hard.

Answer 4

If your trees have brains they may very well need sleep, which on earth is normally done at night. Most creatures sleep at night because the sun helps them see and being in the dark is dangerous. Your trees however, are nocturnal. They are adapted to being in the dark because there aren't as many living creatures moving around that could potentially hurt them. And when the sun rises, they go to sleep and let automatic processes like photosynthesis take over. Save energy during the day when standing still helps with photosynthesis, have a sleep period for their minds, and avoid hostile creatures by moving at night only. Triple whamy reasons.

Answer 5

This kind of reminds me of a book called "Sentenced to Prism", by Alan Dean Foster, except in reverse.

https://en.wikipedia.org/wiki/Sentenced_to_Prism

In the book, the MC meets silicon based lifeforms that can only move while the sun is up and they are able to convert the sun's rays into electricity the immediately use. They don't have batteries, so no storage. Unfortunately, there are silicon/carbon hybrid creatures that can destroy them at night, but that goes off the point I was getting to.

Maybe your trees can't move while storing energy, or find that it's most efficient to store energy when not moving, i.e. there is a perfect "stance" they take that maximizes their solar intake. Since they can freeze their limbs in place and don't want to move, they simply fall asleep Then at night, they wake up and are free to move around due to their batteries being full.

Possibly the heat simply feels good and they really like to bask in it, like a lizard or cat. The cold is uncomfortable, so they need to move to "keep the blood moving". There's a lot of heat generated when thinking, so movement isn't necessarily a requirement. Maybe the heat of the day is just more than they can handle, so just "shut down" when it gets to be too much.

Answer 6

Camouflage.

These tree-like creatures are a potential meal for a number of large predators. These predators will identify the creatures by sight. If they see a "tree" moving about during the day they will attack, kill and eat it.

So during the day the tree like creatures mimic trees. They become dormant. A predator could pass by and not notice anything. But during the night, the predators can't see so it is safer to move about.

We see something of this adaption among rodents that hide during the day, and become active at night. They do this to avoid the predators (such as birds of prey) that hunt by sight during the day. An "Ent" cannot hide in a tunnel but if it is sufficiently still it can "hide in plain sight" as a tree.

Answer 7

Their internal body temperature is below zero degrees celcius. Ice is essential to their metabolic activities, many essential enzymes use ice-crystals as a catalytic substrate. If they thaw, they die. They are not of course solid ice, their bodily fluids are filled with antifreeze, but they must preserve the ice crystals in the heart of their cells.

Metabolic activity produces heat, more heat than they can afford. During the day, the Tree-Folk rest to keep as still, calm, and cool as possible while it hides from the scorching sun beneath its leaves. Its skin is a reflective white and its thick fleshy leaves are a deep golden color to reflect the heat of the sun, but capture the vital UV rays that are needed for its photosynthesis. The leaves are deeply veined to maintain their cooling bloodflow, and feed the perspiration from the dark undersides of their leaves. When the sun sets, their leaves raise up to catch the wind and show their darker side to the darkening sky.

When night falls the cool air can sustain them, then they awaken thirstily from their slumber and begin to roam the land in search of ice and snow. This is their food and fills them with a vital coldness. They graze the ground for frost and fresh snowfall and store it away in their bodies in preparation for the coming day. In truth, it is the sugars from their leaves made during the daytime that give them energy, but they recognize coldness as the greater virtue. For what good is syrup, when it's too hot to move?

Answer 8

Humans feel sleepy when they have high levels of adenosine as a signature to signal that their energy reserves are lower. Sleeping allows body to filter adenosine, converting them to ATP faster, hence they are recharged. Trees on the other hand, can do photosynthesis during day time only. At night they can burn the storage using regular respiration, but this is not very efficient. Thus they cannot properly rest during the night. During the day time they sleep, during the night they are active. Deep hibernation is an adaptation to further reduce energy consumption during the daytime. After all, trees do not have a predator and can sleep in peace any time of the day, uninterrupted is better.