Is a man-eating plant realistic?

Question

Carnivorous plants live in nutrient-poor, highly lit habitats, such as bogs or rocks. For example Earth's biggest carnivorous plant, Nepenthes rajah, lives on mountain-tops with high concentration of heavy metals but few nutrients such as nitrogen. However, man-eating trees and triffids are just fiction and don't seem very realistic, because they would need too much energy for catching their prey or even moving.

So what conditions would make carnivorous behavior so beneficial, that carnivorous plants would grow much bigger, enough to be threat to men and similarly big animals? Also how likely is each of the five basic trapping strategies (pitfall, flypaper, snap, bladder and lobster-pot trap)? Or is any other strategy possible and realistic under such conditions? Nutrient-low habitats creating need for carnivores usually don't allow for big growth, so is it realistic at all?

Answer 1

A plant chasing humans would be unrealistic, but I think a plant generating a trap for human-sized animals could be possible in principle, given the right circumstances (maybe it evolved from one eating smaller animals, but those got extinct, and the trap size grew to capture ever larger animals).

The most obvious way would IMHO be if the plant managed to create a concealed hole in the floor where animals would fall in, breaking through the thin cover (which would regrow while the animal would be digested inside the trap). Of course the natural countermeasure would be to be always careful where you go, so animals in a region with many such plants would usually always test the floor before they step; running would probably be quite unusual. If the substance in the trap kills quickly, a careless human (or one not knowing about that danger) could also be caught by such a plant.

Another possibility could be using many small barbed hooks growing out of the floor, and unnoticeable to the animal (looking basically just like the normal floor). When the animal steps on the floor, the hook enters its skin, and if the hook is strong enough, it cannot leave any more. Such a plant could normally be completely under the earth except for the hooks; after an animal is caught by the hooks, it might grow special roots relatively quickly into the caught animal to digest it from the inside. Coevolution would probably make the animal's feet thicker (so they are less easily pentrated by the hooks) and the animals lighter (so they are pressed into the hooks with a lower force), or to avoid the floor as much as possible (like apes living in the trees). Humans living in those regions would probably develop shoes early on as protection against those plants. You certainly should not go to sleep where such a plant grows; maybe humans would predominantly live on trees, too. Humans not knowing about that danger might fall prey to such plants because they walk onto such plants with their bare foots or insufficiently strong shoes, or lie down to sleep on one. Also stumbling on such a place can be deadly (the plants might also grow tripping hazards in order to make animals — and humans — trip on such places and thus involuntarily expose less-protected parts of their body to the hooks).

---

Here's another option, which is actually a worked-out scenario how an actually man-eating plant could have evolved.

Imagine a nutrient-low environment, and a plant which happens to produce a substance that happens to be very beneficial for some man-ancestor animal. Members of that species will then, of course, start harvesting that substance from the plant, which actually harms the plant, therefore it will start to evolve counter measures. Those counter measures could initially have been a contact poison on the bark, which over time got more effective. However, since the substance was so advantageous to those species, they also co-evolved to become more skilled in getting at the substance while avoiding that contact poison. There still were some individuals dying from the poison, but the substance was so advantageous to them, that it more than made up the evolutionary disadvantage of some individuals dying.

However, at the same time those individuals dying turned out to be an evolutionary advantage for the plant, since the rotting corpses delivered sorely needed nutrients to it. So the evolutionary pressure changed on the plant: Now it was not just reducing harm from the harvesting of the substance, but also increasing the number of individuals killed by the poison, to deliver more nutrients.

Of course there's a delicate equilibrium involved here, as the chance to survive the harvesting attempt must be high enough that the advantage of harvesting outweighs the danger of being killed. So the evolutionary pressure is to increase the absolute number of individuals killed, while at the same time not increasing the relative number, at least not beyond a certain point. That means, attracting more individuals. Thus, the substance in question will be produced in more amount, and also in a way that is more easily accessible (which also means that after some time the harvesting doesn't harm the plant any more, because it is offered at a place where it can be gotten at with little harm (but high probability to get into contact with the poison).

At the same time, it will also evolve means to more effectively harvest the nutrients from the corpses, like growing roots into it.

Of course, for the human-precursor species, there's an evolutionary pressure to get at the substance while losing as few individuals as possible, therefore getting better and better at avoiding being killed by the plant; this will be countered by the plant evolving better and better strategies of killing. At this point, both species my already be dependent on each other, so that's the only way evolution may proceed.

Now imagine that the plant, due to some mutation, produces a bit more of the advantageous substance while consuming a killed individual. Now this changes the evolutionary pressure inside the pre-human species: Now it is not only advantageous to avoid getting killed, now it is also advantageous if a competing individual does get killed. Not only does that other individual get removed from the gene pool (that would have been the case even before), but in addition it means more of the advantageous substance. So there's now an evolutionary pressure not only to avoid getting killed by the plant, but also to cause competing individuals getting killed (for example, by pushing them to the tree). This means (according to the egoistic gene assumption) that such a behaviour will evolve. Also it means that individuals get an evolutionary advantage from recognizing times when the plant produces more of the substance.

Now individuals being actively pushed to the plant for being consumed by it is, of course, of advantage of the plant, so it itself will evolve to encourage that behaviour; it will make the difference between "hungry" and "consuming" state more pronounced, eventually stopping to provide any substance while "hungry", and probably over time stop to kill during "consuming" phases. At that point, it will not just be advantageous, but actually mandatory that some individuals are pushed to the plant, in order to get the substance which the species is long dependent on.

For members of the species this means an evolutionary pressure to get more intelligent: First, it is of advantage to understand in which phase the plant is, so you can avoid it while it is hungry, and harvest it otherwise. But at the same time you need social intelligence to both trick others to visiting the plant when hungry, and to avoid being pushed to the plant yourself. So this specific situation should give an evolutionary pressure to become an intelligent species.

Now fast forward, and see how that species turns into (that world's equivalent of) humans. As the intelligence evolves, they will become aware of the fact that they are killing others at the plant, but also that this is absolutely essential for them to be able to live on. Probably they'll rationalize it that the plant is from a god who wants them to sacrifice humans, but who gives them the substance in return.

The specific way they evolved will probably mean they will naturally mistrust each other very much. The main goal of the emerging power structure (with a priest or similar on the top) will likely to be to regulate sacrifices to the plant, so individuals don't have to live in constant danger. I'd also expect tribal wars to be common, in order to get people from other tribes to sacrifice to the plant.

All in all, you'd get a man-eating plant which is explicitly fed by an inherently mistrusting and martial humanity depending on that plant.

Answer 2

If you want a carnivorous plant to eat large animal it needs a quick way to disable and kill them. The venus flytrap relies of it being structurally stronger than the insects, the pitcher plants just outright try to drown the insect to get it to stop escaping along with the slippery walls of the pitcher. So how could a plant quickly disable a large animal? The only answer I can think of is poison. Lots of it quickly administered before the animal can get away. There are spiders and jellyfish that produce enough toxins that can be quickly delivered to incapacitate a human pretty quickly. Combine some of those toxins with long vines/roots and these could be made to work.

Answer 3

Many plants today have attributes that could be useful for trapping larger animals (including humans); thorns and barbs, poisonous plants, or smelling/tasting great. Of course, the reasons for those "traps" are to keep animals away or spread seeds/pollen, not to gather plant food, but with a little "tweaking", it's perfectly possible for plants to "eat" humans, or even bigger animals. Here are four possibilities I've come up with:

The easiest trap to make would be one where the plant doesn't need to move or generate anything: a pit. Of course, an animal could crawl out, so there should be sharp spikes or barbs at the bottom of a short, steep drop. Add some trip hazards and/or slippery film, and all manner of creatures will fall in and be killed, ready for digesting, without any worry of a damaging struggle. The design scales well, so a small man-eating pit-plant can catch insects, rats or mice until it grows big enough to catch larger prey.

A crushing trap would expend more energy, but is still possible; many ferns like the Mimosa Pudica can fold themselves up when touched. A tree with a large hollow area in the middle that is "touch sensitive" could squeeze closed without expending much energy, and quickly crush whatever creature set it off. Again, spikes or barbs would help. Smaller creatures wouldn't be enough to trigger it, meaning it wouldn't waste its energy.

As BSteinhurst mentioned, a fast-acting poison would also work. A shroud of barbed nettles, each containing a nerve-blocking poison, hanging over an exposed root system would kill anything it managed to touch, and allow the nutrients of the decaying body to be absorbed. A cluster of seedless fruit that smell sweet (or like raw meat) would attract various creatures to their doom.

The man-eating plant doesn't have to be big, though. A simple mushroom could do the trick. It would be designed to grow on flesh, and smell sweet and tasty to attract being eaten, but contain a deadly toxin that would kill even large animals. When eaten, the spores from the mushroom would cover the creature (hands, mouth, etc.), and eventually begin to grow on the recently deceased.

However, all these methods (apart from the mushroom) require a substantial investment of energy. Growing a large pit or crushing trap will take a lot of resources. Plants like that would only be able to survive where there is a large amount of incoming food, enough to make an investment in traps and digestion more worthwhile than grabbing energy from the sun and nutrients from the ground. Then again, in fiction, there are always adventurers ready to travel into killer-plant-infested caves and dungeons, so it may very well work out.

Answer 4

It depends on what you mean by realistic, in particular humans might be too rare and too smart. However, if you want "any big animal", then here are some ideas:

• The simplest is a plant that uses tiny aerial spores to be breathed-in and take root in the lungs, then killing the animal and taking root in its corpse (more like a fungus, which technically is not a plant, but may actually work too). I've seen this idea several times already.
• A plant that has leaves/flowers that act as triggers, when touched they spray a paralyzing agent to breathe-in, the victim dies by cardiac arrest or lack of oxygen. There's no need to "devour" it, the remains will provide enough nutrients.
• A plant with giant leaves, that are slippery to create a pitfall trap filled with some kind of sticky gel. Once you are in, it closes and keeps you from breathing (very few big animals can survive longer periods without oxygen).
• A plant that creates a cheese-like structure using roots with leaves covering the holes. Once some animal steps onto it, it collapses, making the leg hard to get out (perhaps using thorns, or something similar). Then, before the victim can free itself, some symbiotic creatures (ants, spiders?) step in, and deal the real damage.
• A plant that is a semi-parasite that grows on trees (like mistletoe), it has thread-like tentacles falling from above when it detects a victim below (e.g. by carbon-dioxide) and like some jellyfish it paralyzes its target, which will provide the nutrients for the tree, and indirectly for the plant.
• A plant symbiotic with some insects: the insects carry the spores on them and kill the animal, while the plan uses the nutrients to grow and provide optimal environment for new nest and larvae to grow.

I hope this helps ;-)

Edit: Added clarification about fungi not being plants (thanks @pluckedkiwi).

Answer 5

Realistic? Probably not, at least not with our earth plant biology, however if there were a plant that was 'more' than plant you could get to that level.

Here's a scenario that might work. A fruiting tree that generates very large fruit as a way to spread its seed, but it uses tremendous amounts of energy and resources to produce. So it finds a way to 'fertilize' itself by adding animals to its diet. One way it might do it is year around it has a pleasant odor to attract animals and an anesthesia that affects them when they get close and they fall asleep on the root system, where they are absorbed to produce the fruit. Of course once the fruit is ripe it would have to turn off the anesthesia to allow the fruit to be taken and seeds to spread.

Answer 6

Consider symbiosis. A man gets tangled in roots of a tree and carnivorous but weak birds, who live in the tree, swoop down to eat him. Or wolves who live under the tree eat the man's flesh and then the tree's roots absorb the man's organs.

Answer 7

There seem to be more than a few plants that rely on their seeds being eaten to spread and reproduce. Usually herbivores eat the plant and pass the seeds through their digestive systems and deposit the seed on the ground with a lump of fertilizer.

Well, what if the seed simply opted to sprout earlier and take root while still inside the unsuspecting animal? Starting its life cycle more like an intestinal parasite and eventually growing to a point where it would block and eventually burst the intestine killing the animal. Then it could take advantage of the corpse as fertilizer.

Of course the process would be really slow and painful for the animal, so one would assume that animals would avoid the plant and just not eat it, but I'm thinking that if the process was slow enough most animals wouldn't make the connection between the fruit eaten and the death weeks later.

Answer 8

If you are dead-set (pun intended) on an animal-eating plant, I suggest another option I do not see here.

We have a garden with pumpkins in it. Their stems are really thick, and their leaves humongous, designed for transporting nutrients in volume several feet. In order to anchor down the leaves so they do not flop over, the plant stem sends out fairly tough, strong tendrils that wrap around whatever they contact, and hold on tight. Very tight. The leaves do not flop over in even very high winds. I posit that if there were a plant that used large animals for food, it would use a similar system for capture. That is, tendrils that snake out from thick stems, and wrap around anything that contacted them. They would be very effective for up to 5 or 6 inches in diameter - good enough to snare a leg or small animal. Give them barbs that emit a toxin (pumpkins already have barbs on their stems for protection, sharp prickly little devils, but they are not toxic). Once the prey was captured and subdued by the toxin, the stem itself could curl up around the prey, covering it with biomass from itself, and slowly ingesting it by injecting digestive fluid and sucking it out, somewhat like a mosquito draws blood, but only mega-sized. Or like a spider captures, encapsulates, and digests prey in a web. All of these techniques have antecedents in earth biology, except for the scale and that they cross the plant-animal lineage (not entirely unheard of - there are genes that have crossed between plant and animal, with viruses the suspected transport - this is proposed in genetic modification of organisms), and are evolutionary supportable. If you have ever been in a pumpkin patch, you will understand that it is not hard to envision the plant scaling up to such proportions. None require any particular intelligence or deliberate advanced planning that requires a brain, as long as you do not make the mistake of having the tendrils deliberately seek out and hunt down prey. Everything is reflex. It would be a good way to get calcium in a calcium-poor environment. Perhaps give the plant a calcium-based shell or infrastructure for protection - again a cross-over from animal biology. Or perhaps the seed pod is encapsulated in calcium, like an egg. Animals are a good source of large amounts of calcium.

Answer 9

The long and the short of it, is no.

Humans are biologically a very poor food choice. Skinny, and low in nutrition. Mainly bone mass and hard to digest calcium.

From an evolutionary perspective, if it were realistic to have a man-eating plant, one would have evolved by now.

And if the environment were nutrient-poor, how could larger animals have survived in the first place? If a plant could not survive, how would a more nutritionally demanding animal survive in the same environment? Plants evolve based on the nutritional sources available, not hypothetical sources that may just happen to wander by on an unreliable basis.

Answer 10

Chances are that in the past, plants did indeed catch small mammal and eat them. However, the advantages that this might concur would be quickly outweighed by the fact that small mammals would avoid this plant, and thus would not help it spread.

Historically when this kind of thing happens, one of two things happens:

1. It is unable/refuses to adapt, and thus dies out.

2. Alternatively, it gradually adapts to become less deadly.

An example of the latter are Chili Peppers. They were probably once poisonous, but eventually adapted to become less so. Consequently, they have become much more popular.

Answer 11

A plant that solely preys on large mammals/humans is not very likely, but if you allow the plant to get its nutrients from other sources as well it could be possible.

Something like the venus fly trap would be a good start, something that sits still and lets its prey fall into its trap will not need the energy to move around. The problem with only preying on large mammals in this scenario is that the mammals, especially humans, will be smart enough to avoid the trap once they learn about it. However, if the plant is not solely reliant on large mammals, if it can get enough nutrients to spread through other sources, then it can continue to propagate even during a lifetime of not preying on any humans.

One way this could theoretically evolve would be to start with something like a venus fly trap that eats insects which fall into its trap. If your world is thick with eatable insects the plant could evolve to grow bigger and bigger so it can trap and hold more passing insects. It could grow so large that it happens to also be able to eat large mammals that fall into the trap. It wouldn't rely on large mammals as its main diet, but every now and then when one stumbles into its trap it could eat it.

Answer 12

I've come up with a plant that can see, and is big enough to eat humans. It's an alien, of course. And I've also come up with a sea creature that's basically the Sarlacc, without the absolutely ridiculous "digests its prey over a thousand years" bullcrap. The sooner you end your prey, the better. That's why predatory beings are designed for an insta-kill.