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Planet Swamp. As the name suggest, it is a planet fully covered with swamps.

A new kind of carnivorous plant has been discovered on this planet: when its leaves are touched by an animal or moved by the wind they emit a sweet scent, something like vanilla and sugar. This sweet scent attract unaware animals close to the plant.

Its fern like leaves are covered with tiny and fragile thorns, and as soon as an animal touches one of these thorns, it gets injected a small amount of toxin.

This toxin, when injected in small amounts initially enhances the sensitivity to the smell emitted by the plant, giving pleasure and euphoria to the animal inhaling it. Therefore the animal keeps touching the plant leaves, rolling in them, and it's not scared when the leaves and branches start enveloping and immobilizing its body until the toxin leads the animal to death. The enveloping movement is triggered by multiple local torn ruptures, and forces the branch to bend in the direction of the broken thorns, and can be as fast as in the Mimosa pudica.

The same toxin, once injected in large amounts as consequence of the enveloping, speeds up the body decomposition, resulting in a slurry that will feed the plant, providing it nutrients else not available in the swamp soil.

What can be the optimal size of preys that this plant should target? In other word, does it make sense that such a plant grows until it is able to catch an elephant sized animal, or it would be better for it to target a sheep sized (or even smaller) animal?

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It'll probably stick with fairly small creatures.

There's a reason why carnivorous plants haven't yet grown to large sizes (or at least the traps created by those plants) - the amount of prey animals decreases as the size grows larger.

Currently the largest carnivorous plants (pitcher plants) still eat insects and small frogs.

Although you have a different design here, the numbers of scale still apply and the plants will probably benefit more from more and smaller meals than limiting itself to larger prey that occurs less often (and has more capability of damaging the plant in question).

Plants are also largely self-serving - they'd want resources for themselves and probably wouldn't emit a toxin that would end up benefitting another plant should the prey animal wander off in it's stupor.

Unless the toxin also carries pollen.

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    $\begingroup$ @Pete good notes about the size. I would add that also with smaller creatures the toxins would act faster and therefore be more effective. Big creatures would take much larger doses and the dose would take longer to act. $\endgroup$ – Paul TIKI Mar 7 '17 at 16:11
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I like a fungus for the approach you describe.

1: Fungi are mostly underground in mycelial webs. I think I read that the largest organism on earth is a colossal underground mycelium extending for many acres. The above ground pieces are dispensible appendages (like mushrooms). It is no big deal to the fungus if a mushroom gets squashed - that might even help with its function to disperse spores. Your creature needs to have it be no big deal for a cow to roll on it. For a plant with a large above-ground body it has grown, it is a real misfortune if a cow rolls on it.

2: You already require the prey to decompose into the ground before ingestion. That is unlike plants and animals which break down prey within their bodies. This internal route prevents competition from scavengers who might want to come and take the prey. Fungi already break down nutrients external to the mycelia and absorb them. They have mechanisms to prevent competition which involve toxins (e.g. penicillin) to poison competitors for nutrients.

3: Hallucinogenic and other toxins are a good match with fungi.

4: I like a possible tar pit aspect: fungi grow within the decomposing corpse and use that as a springboard to put forth new thorns / above ground structures. A scavenger attracted by the corpse might very well itself wind up prey to the fungus.

5: Once you have introduced such a creature I like the idea of an assassin using it as poison. The protagonists enter a castle which has been taken over by the fungus after it used to poison a meal. The few inhabitants who remain alive are intoxicated and euphoric as the fungus grows over them.

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You plant doesn't want to catch large animals, because a rotting corpse (or slurry) kills the soil macro and micro-fauna which are keeping the plant alive. Decaying corpses turn the soil anaerobic (oxygen-less) as the gloop uses up all the free oxygen. So useful burrowing creatures like worms will flee or die of suffocation. Anaerobic bacteria flourish, but aerobic bacteria will die. On Earth, plants need oxygen for their roots, so all this will make them very unhappy and/or kill them outright.

Thus the corpse of a big animal rotting will temporarily decrease soil fertility. After the gooey phase is over, the soil fertility will slowly return to normal, then eventually be boosted as the phosphate from the bones leaches into the ground.

If your plant can stop the slurry from reaching its roots, it can avoid this problem. It would have to have a 'stomach' to contain the slurry in. Like the pitcher of a pitcher plant. But to eat an elephant it therefore needs an elephant-sized pitcher. (Alternatively it needs aerial roots like a mangrove tree).

Soooo... you are down to energy balance/budget. Can your plant afford to use lots of resources to grow elephant-sized pitchers in the hope that an elephant comes along? Would it be better growing lots of rabbit-sized pitchers instead? Like a Venus fly-trap invests in several leaves.

If it manufactures a 'pitcher' on demand - small for a rabbit, large for an elephant - it still needs enough stored resources to make the elephant-sized one. It might only need to be the size of a sunflower to have enough leaves to wrap up a rabbit, but would have to be oak tree sized to wrap up an elephant. To get oak tree sized it would have to be very good at catching rabbits... so why not just stay catching rabbits?

Also are elephants smart enough to spot the huge clump of leaves/huge pitchers and keep away? :-)

And a 5 tonne euphoric elephant rolling about on your leaves will do a lot more damage than a rabbit! An elephant-eating plant will have to invest more precious resources in becoming physically tough.

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Small animals are best for all the reasons above and the fact that the drug needs to affect the animal before it leaves the area. Taking an elephant as an example, the skin is so thick that the toxin may not reach past the layer of dead cells. If it does affect a large animal, it may take too long. It does the plant no good if the animal is far away when it starts feeling "cuddly."

Another aspect of large animals is how much damage can it do to the plant. An elephant that that rubs against a plant often damages that plant. You can see this when you look at photos or video of trees that elephants decide are good back scratchers.

Also, you have to take evolution into account. The plants with the better toxins will survive the best and reproduce. The animals that best resist that toxin or avoid the plant will survive to reproduce unless the plant takes a very small percentage of the animals. The animals that reproduce at a large rate tend to be the smaller ones.

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  • $\begingroup$ You said "The animals that best resist that toxin or avoid the plant will survive to reproduce unless the plant takes a very small percentage of the animals." -- but what if the plants also provide some benefit to the prey animals? Perhaps the plant carries essential elements in its fruit, or a prey-aphrodisiac in its alluring scent. $\endgroup$ – Charles Rockafellor Mar 13 '18 at 10:00
  • $\begingroup$ @CharlesRockafellor, what benefit can the plant provide to a dead animal? As for alluring scent, like a toxin, those who are most susceptible are removed from the gene pool, leaving the rest to become more resistant over time. Unless the plant takes a statistically small percentage of the prey, it will either kill off its food source or breed resistant prey. At that point, it's methods have to evolve as fast as the prey's resistance. $\endgroup$ – ShadoCat Mar 13 '18 at 20:22
  • $\begingroup$ I don't know; I'm sorry, it was just a thought, and I was trying to be constructive. $\endgroup$ – Charles Rockafellor Mar 13 '18 at 21:15
  • $\begingroup$ @CharlesRockafellor, no worries, I couldn't see any benefits that the plants could give the prey but I've been thinking about it for a bit and that's good. As for my question back, I was hoping you had an idea that I hand't considered. $\endgroup$ – ShadoCat Mar 13 '18 at 22:46

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