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Plant poisons in our world (that I'm familiar with) have operational half-lives in the order of seconds to maybe a day to reach full effects. This is because poisons are a defense mechanism and work best when things hurt right away, so that you'll associate pain with the plant. For example, contact dermatitis from the poison ivy family of plants sets in within minutes. An ingested poison, such as from eating the wrong berry, can take hours.

Why would a plant develop a poison (not necessarily a lethal one, though lethal is okay) that takes days to set in? What evolutionary benefit could that provide?

thanby
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    Poisonous mushrooms indeed take days to kill their victims. – Alexander Nov 20 '19 at 20:26
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    To expound on what Alexander said, the Destroying Angel is rather infamous in this regard. Upon consuming it, the subject experience no difference for up to 24 hours and feel perfectly fine. At which point the effects kick in, and the subject dies quite painfully. Don't eat white mushrooms in the wild. – Halfthawed Nov 20 '19 at 20:48
  • See there's the thing, it kicks in and is noticeable within 24 hours. It may take days to kill them, but they know what did it. – thanby Nov 20 '19 at 21:01
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    And now "assassination by mushroom" is in my search results, so the NSA and the FBI have THAT to be curious about... – Morris The Cat Nov 20 '19 at 21:25
  • slow acting is better than none when it comes to detering things you don't want eating you. – John Nov 20 '19 at 22:04
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    @MorrisTheCat there is already a question about using it for assassination, one advantage is it kills a few backyard gardeners each year who accidentally use it salads it is said they taste quite good, but also make your everything stop working. – John Nov 20 '19 at 23:27
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    @john I don't agree with the assertion that "slow is better than none". From the perspective of the plants being consumed, there is no difference whatsoever between a delayed effect and no effect whatsoever. – Morris The Cat Nov 20 '19 at 23:35
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    @MorrisTheCat sure there is, few plants go extinct or even die from a single feeding, if everything that eats you dies, very soon there will be nothing that eats you. Lots of plants and other things have slow acting poisons for this reason. – John Nov 20 '19 at 23:45
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    @MorrisTheCat I mean, it's not like we're not already all on all the watch lists. Worst-case scenario you just got bumped up a few places. – Gryphon Nov 21 '19 at 05:03
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    @MorrisTheCat You're looking at evolution as if individuals were doing the evolving. That's not the case. Look at the genes. A gene for a slow acting poison will become more numerous than a gene for no poison. A slow acting poison isn't going to help an individual plant, but it still has a huge effect on the genes! Just look at how many animals (or plants) merely mimic dangerous species - in your view, that would be even worse than a slow acting poison, while in reality, it's a very effective evolutionary strategy. – Luaan Nov 21 '19 at 09:52
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    @MorrisTheCat But that's still design thinking. Evolution doesn't have purposes. It doesn't find optimal solutions. It's remarkable that it even results in optimization at all, don't try to expect the solutions will actually be optimal. A random change happens. It has effects. Those effects either make those genes spread more or less. The berry didn't choose to be poisonous to humans. Maybe the poison has a different use - e.g. nicotine is a pesticide; it's not "designed" to kill humans. Morphine in a poppy is just an intermediary that happens to work as an analgesic in humans. – Luaan Nov 21 '19 at 10:01
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    Framing challenge: poisons are slow to act on mammals. The evolutionary selection game had been running for millions of years between plants (from 470 million years ago) and insects (roughly the same) before a land animal (370 million years ago) entered the picture. Selection now is largely based on the range of expression established during the first quarter of the war. – Eric Towers Nov 21 '19 at 21:43
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    Carcinogenic plants are just jerks. – Roger Nov 21 '19 at 22:17
  • @Roger Good one, I hadn't considered carcinogens! – thanby Nov 21 '19 at 23:02
  • @MorrisTheCat: Well you're lucky you didn't Google for Ricinus communis or you'd probably be face down on the ground by now with someone kneeing on you. As it happens, that tree has poisonous seeds (and looks similar to a fig tree, yay!) and death takes a couple of days, too. Can find it in not-too-few gardens around here. – Damon Nov 22 '19 at 11:10
  • @Damon Australia? Sounds Australian. – thanby Nov 22 '19 at 13:22
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    You're assuming that anything which is harmful is a deliberate poison selected for being poisonous. Your example of poison ivy is especially apt - the reason it harms us is not at all the fault of the plant but is purely an allergic reaction on our part. The plant developed it for other reasons unrelated to us, yet the effect on humans is harmful. Substances which merely aid in the plant retaining water, or assist in healing damage, preventing fungal infections, etc. could be fatally poisonous to humans merely by happenstance, in which case the speed of onset is irrelevant. – pluckedkiwi Nov 22 '19 at 16:22
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    @Alexander Mushrooms might not try to kill you to stop you eating them, but to use your corpse to nourish its spores – Quasi_Stomach Nov 22 '19 at 19:18
  • @Quasi_Stomach potentially yes, but I'm not proposing that mechanism here. – Alexander Nov 22 '19 at 19:37

18 Answers18

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It slowly kills the animal in order to spread over larger distances across soil with poor nutrients.

So if your fruit kills the host 1 meter away, not much gain. Let the host migrate for days.

Now if you kill the host while inside the digestive tract, the seed can spawn a large plant using the decomposing host's body. Nature is full of examples. Wasps place bugs inside other insects, and it eats away the host.

Nature is ruthless.

Gustavo
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    Damn you Gusty! this would have been my answer [+] seed dispersal with it's own traveling grow bag :) – Pelinore Nov 20 '19 at 21:52
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    @Pelinore There is something shocking about some natural adaptations like carnivorous plants. In that case is due to poor soil. If someone says Australia got this kind of stuff, with all the weird animals they already got? I would believe them. – Gustavo Nov 20 '19 at 23:04
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    ... and we have hippos at the beach, bring togs. But alas no, plants find it quite difficult to kill animals using a seed/newly germinated plant. The animal can quite easily brush it off. That pretty much leaves the interesting services to breathing, fornification, and poop. Fungus however loves killing animals, because food. Cordiceps do this to all sorts of insects, and there are cases of humans dying because fungus was literally growing in their bodies. It is often mistaken for a drug overdose. Still +1 to the answer. – Kain0_0 Nov 21 '19 at 00:49
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    We have examples in nature of such strategies. I can't recall the plant but it's objective is to weigh down the bird. The seed gets the nutrients of the dead bird to help it grow. (It's a mechanical attack, not a toxin, but the same idea would apply.) – Loren Pechtel Nov 21 '19 at 01:30
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    Animals typically crap out any poison that exists in less than 1 day. I honestly don't believe it's statistically influential for the 0.01% of seeds that are spread by a dying animal compared to adapted species... animals can develop tolerance to nearly any poison by habituation. No need for a host to migrate for days, i.e. many kilometers, if a plant can travel 100 meters every year (ten minutes to most animals), that's a lot faster than climate changes can account for. It's not mechanically a very convincing list of arguments. – bandybabboon Nov 21 '19 at 14:03
  • @com.prehensible You could conceivably have a faster-acting component that induces constipation. – ceejayoz Nov 21 '19 at 14:38
  • Yes it's true, and the seed will be disseminated far and with fertilizer regardless of whether it keeps the animal healthy, damages or kills it... curiously, toxic peas are used in Australia because they are safe to local species and fatal to invasive species. – bandybabboon Nov 21 '19 at 14:46
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    @com.prehensible : You could also conceivable have delayed release, a coating on a fruits tiny tomato like seeds that takes a while for digestion to get through perhaps & the poison under that coating. : it's also worth noting there are natural poisons out there that show no symptoms for a considerable time & then kill, the Destroying angel for one, you can survive days without treatment when the organs it attacks fail & less damage from a small dose can take longer to be fatal with initially less severe but worsening symptoms. – Pelinore Nov 21 '19 at 15:07
  • The problem with this answer is that you'd also need to propose a trick that the seed uses to stay inside the host's digestive system for 'days'. Most animals pass solid waste within half a day. Possibilities include migrating out of the digestive system (possibly part of the fatality effect) or inducing constipation of some sort. Of course, just sticking around for 6 hours before killing the host would be enough to gain a good distance. – Adam Miller Nov 21 '19 at 15:11
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    @AdamMiller : ceejayoz already gave one, another (fast acting this time) 'poison' causing constipation : or the seeds might start growing instantly they hit stomach juices sending out rootlets to anchor themselves in the stomach or gut wall, it's really not hard to come up with a plausible 'trick' for that. – Pelinore Nov 21 '19 at 15:21
  • @AdamMiller : but that aside the question is not *how* but *why*, so this answers lack of how isn't a problem. – Pelinore Nov 21 '19 at 15:27
  • A friend of mine gave an gruesome description of a destroying angel... He said, victims start to feel ill the same day and twist around in bed with impossible digestive problems for an entire day... Then, they suddenly feel better again and there is a sense of calm and stillness... And on the third day, they die. – bandybabboon Nov 21 '19 at 15:42
  • Incidentally, the answer here sounds like a fatal interspecies generalistic parastic plant relationship, which aids the plant in survival by dispersion... That kind of theory is generally contrary to the forces of selection. – bandybabboon Nov 21 '19 at 15:43
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    @com.prehensible : *"That kind of theory is generally contrary to the forces of selection"*, you should tell Cordyceps that, cos it's clearly not heard yet :) incidentally probably the inspiration for the Yellow musk creeper I always thought. – Pelinore Nov 21 '19 at 16:23
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    @LorenPechtel : I did some temping @ a brewery's maltings once, a big industrial sized operation, pigeons all over the shop, always a few wandering around in serious distress that had got into some spilled undried malt & stuffed themselves, it continued to swell & sprout inside them bloated them up something awful until they couldn't fly, it'd swell so much you'd even see sprouting malt pushing out of their bum, would eventually kill them by bursting their stomach : not a deliberate dispersal strategy but does show how it might happen. – Pelinore Nov 21 '19 at 17:35
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    This makes sense too from an evolutionary point in that if they can spread farther there is less fighting over resources so a better survival rate and thus more slow-poison berries. – DasBeasto Nov 21 '19 at 21:19
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The simplest answer is that the delayed poisonous effect is a secondary (and from the plant's perspective, inconsequential) effect compared to the primary evolutionary purpose of the compound in question.

For example: Let's assume that the poison in the berry kills by initiating a chain of biochemical reactions in a human who consumes it that causes renal failure. This makes it useful for humans to kill other humans because renal failure kills relatively slowly which makes it difficult to know when or by who the target was poisoned.

However, that compound exists in the berry because it has a completely different kind of effect on the insects that would otherwise consume the new shoots on the plant before they can fully develop. Having the shoots consumed harms the plant because it prevents the fruit from developing and thus prevents distribution of the seeds.

Plants with this compound in the berry don't have their shoots consumed by grasshoppers or whatever, thus the berries can be consumed and the seeds deposited by birds and other larger animals a distance away, which is evolutionary beneficial. The fact that the birds and other larger animals subsequently die of renal failure isn't relevant as far as the plant is concerned.

Morris The Cat
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    Lethal berries would very quickly cause evolutionary adaptation in animals. Animals which don't consume them, don't die, thus every species would quickly evolve to avoid them, preventing spread of berry bush. You need something more targeted, compare: Capsaicin. – M i ech Nov 20 '19 at 21:33
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    @Miech well, that's a question. Capsaicin provides IMMEDIATE deterrence to the consuming organism. Mushrooms by comparison absolutely do not. By the time an organism is aware of the negative effects of having eaten that mushroom, it's FAR too late, so the evolutionary adaptation is more difficult. – Morris The Cat Nov 20 '19 at 21:42
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    The evolutionary adaption would still occur. Evolution doesn't care if the creature figured out why it died. Creatures that eat poisonous plants would tend to die before reproducing, which is all that matters. Other than that, this seems like a good answer. The modification I'd suggest: Plant wants to be eaten by cows, which it uses to help spread seeds around. It wants to scaring away... prairie dogs, because their digestive systems don't let the seeds through whole (too small). It incidentally kills humans slowly, because the dose is just enough, and it hasn't adapted either way to us yet. – Zwuwdz Nov 20 '19 at 23:05
  • @Morris The Cat: WRT capsaicin, there's not a whole lot of deterrent effect (on humans, anyway) in most naturally-ocurring varieties (AFAIK). You have to get into selective breeding of things like habaneros to get that. But there are lots of examples where the poisonousness seems only incidental. For instance, the solanine in green potatos (that have been exposed to sunlight), rhubarb having poisonous leaves but edible stalks... – jamesqf Nov 20 '19 at 23:06
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    @jamesqf I think you're missing an important point. The purpose of the deterrent effect isn't to prevent the plant from being eaten. It's to prevent the plant from being eaten in ways that are detrimental to the propagation of the species. Please think about whether capsaicin levels have been detrimental or beneficial to the propagation of habenero peppers. =P – Morris The Cat Nov 20 '19 at 23:12
  • @jamesqf note the vast majority of herbivores are not as selective nor inventive as humans, what part of the plant is poisonous does not matter for a lot of herbivores because they either eat all of it or none of it. in fact leaves and fruit are often the most commonly eaten part of a plant focusing the poison there makes a lot of sense. an animal might eat the leaves and never touch the tuber but eating the tuber and not eating the leaves is does not make much sense unless you are smart enough to know one is poisonous and the other is not, which most animals are not. – John Nov 20 '19 at 23:57
  • Also you might have a poisonous effect happening due to the animal in question being exposed to a plant / berry they're not exposed to in their evolution and when exposed to it, the organism try to use whatever tools (enzymes) they have on hand to metabolize it which might lead to the creation of something that can be lethal (e.g. grapes and methylxantines - coffee, chocolate, tea - in dogs and cats) – Juliana Karasawa Souza Nov 21 '19 at 10:17
  • It might even evolve this way because humans cultivate it as a tool of murder. – R.. GitHub STOP HELPING ICE Nov 21 '19 at 14:05
  • @R.. well that's not terrifying or anything... – Morris The Cat Nov 21 '19 at 15:02
  • @Morris The Cat: But capsaicin existed in capsicums long before humans came along and started doing selective breeding to increase the levels, so the evolutionary function must be something other than a human attractant (or poison). – jamesqf Nov 21 '19 at 16:40
  • @jamesqf: There is significant deterrent effect (burning mucus membranes suck), even from naturally occurring levels of capsaicin, in most mammals. It's there for humans too, but humans are smart enough to distinguish unpleasant from harmful, and to know when unpleasant can be worth it. Among other things capsaicin is an anti-fungal (it likely evolved to protect the seeds from fungus), so humans use it as a preservative; slight burning in exchange for "food you mix with it doesn't go bad as fast" is worthwhile, but not something any other animal has had the opportunity to try. – ShadowRanger Nov 22 '19 at 01:11
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    Interestingly, capsaicin is toxic to most insects (so again, protects the plant), extremely unpleasant to most mammals, but entirely unnoticed by birds. So things that can disperse the seeds widely are allowed to eat them, but everything else is strongly discouraged or killed. Of course, evolution isn't perfect; bats might work as well as birds for seed distribution, but being mammals, they're repelled by the capsaicin. Evolution finds local maxima, not global maxima after all. – ShadowRanger Nov 22 '19 at 01:12
  • @ShadowRanger: If it is unpleasant to humans, then why do so many humans seek it out, adding it to their food in considerable quantity (enough to cause ME pain), and eating the peppers themselves? – jamesqf Nov 23 '19 at 02:46
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    @jamesqf: Because it served a purpose before refrigeration, and we developed whole cultures around it? To prove how tough they are initially, eventually acquiring a taste for it? Pain triggers endorphin release, so finding ways to "hurt" yourself without actual damage (or with minor damage) can actually become addictive, or help cope with other problems (people who cut their wrists for non-suicidal reasons are frequently doing so to trigger endorphin release to deal with depression)? There's a lot of reasons people do mildly self-destructive things; this is a basically harmless one at least. – ShadowRanger Nov 23 '19 at 03:27
  • I mean, objectively, a lot of stuff people eat/drink tastes terrible to basically everyone the first time they try it (black coffee, alcohol, smoking a cigarette, and strong spices to name a few). But humans can get used to a lot if there is some other benefit to it, be it health, stimulant effect, or simple social lubrication. – ShadowRanger Nov 23 '19 at 03:30
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Dosage is wrong.

Poison dosage is a function of size. There's a very humorous urban legend involving the legendary wrestler Andre the Giant when he was diagnosed for anesthesia, which they needed to base off his alcohol tolerance of 2 liters worth of vodka to give him a buzz. (The story is false, the tolerance isn't.)

If a berry was developed with the poison needed to kill, say, a fox or some other small mammal, it could develop a dosage that would kill said small mammal fairly quickly. When a human would eat said berry, the poison dosage would be too low to kill them instantly, but would cause severe damage that would kill the target over time.

Halfthawed
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There are two evolutionary deterring mechanisms: "Teach a lesson" and "Don't develop a habit".

In "Teach a lesson", the effect is quick and not necessarily fatal. Animals who tasted "wrong" fruits learn to avoid them.

In "Don't develop a habit" an animal may eat what it wants - it only happens that somehow there won't be any animals that have developed a taste for the "wrong" fruits. The effect can take long, and it'd better be fatal.

In the long run, both mechanisms achieve the same goal - there are no animals who may consider the "wrong" fruit a valid food source.

Also, there are many hazards in the animal world which are usually fatal, like attacks from birds of prey. There aren't many surviving animals which "learned the lesson", but over time, species as a whole learn well to stay hidden and avoid being in the open.

A Lambent Eye
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Alexander
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There are plenty of toxins that take quite a bit of time to harm humans, or aren't even harmful at all in reasonable doses,because they are more immediately harmful to other creatures. Examples include chocolate, tobacco, poppies, marijuana, willow, peppers, and coffee.

A poisonous plant's typical targets are insects and caterpillars, so if it's not poisonous to humans, the evolution of the plant is hardly affected -- though being less poisonous to humans can tend to keep us from clear-cutting it away (as in the case with poison ivy, poison oak, etc.), and in the plants that I've listed, having a secondary effect on humans that is not overtly toxic, and possibly beneficial (willow bark is the basis for aspirin), humans may actively cultivate it.

In some carnivorous plants, especially certain species of pitcher plants, there are some traps that are inactive most of the time... such as nectar-bearing flowers where only about a third are slippery at any given time. This attracts more insects overall, as there is a potentially high benefit to individual insects that are lucky enough to pick the right flower, and a consistent benefit to the plant, in the form of the micronutrients that insects bring. (The main macronutrients for plants, of course, being water, CO2, and the sunlight to process them.)

If your plant is in a region where the soil lacks key micronutrients, carnivorous plants may have a poison that strengthens or weakens over time, so that animals may get addicted to side effects and harvest regularly, then when the plants needs that nutrient, it could increase the potency of its toxin, hoping that some animals die above its root system.

If your village has been eating the same berries frequently, you might not suspect a plant when it only rarely has a dangerous phase. It would take most people several cycles before they figure it out.

And, of course, the mutation that causes the slow acting toxin could be completely unrelated to either the survival or virility of the plant... Prions are a class of proteins found in all mammals, usually specific to certain families of species (primates, canines, felines, rodents, etc.) A prion that mis-folds can start causing damage to various cells, and would cause other related prions to also mis-fold -- essentially a viral infection without a virus, and without an immune response. Prion diseases can be dormant for decades and, besides getting it from just bad luck, prion diseases can also be transmitted by eating food contaminated with that prion. (Cannibalism is bad. Bury your dead away from your farms.)

It's possible (unlikely, but most certainly in the realm of believable Sci-Fi) that a plant can start producing the dangerous form of a prion that humans use. In which case, in a few years, the people who eat from it will start showing signs of prion diseases. This mutation wouldn't help or hurt the plant at all, it would just make sure that any primates that eat from it will die between 2 and 20 years from that day.

Ghedipunk
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It bears remembering that not everything that evolves provides an advantage. Most changes are neutral, or only mildly deleterious so there's insufficient pressure to weed them out of the gene pool.

The fact that the berries are poisonous to some species that eat them might be entirely co-incidental... for some reason, some random structural protein is kinked in just the right way to cause hideous liver failure, but that's just one of those things.

Starfish Prime
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    An example of this in nature is Poison Oak/Ivy/Summac, which are irritating to humans by accident. The compound responsible is probably antimicrobial, as it doesn't affect the birds or most mammals. – Morgen Nov 23 '19 at 03:27
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The berry is poisonous because it evolved to be eaten by a different animal. Plants have fruit to trick animals into distributing their seeds. Any plant whose berries are eaten by an animal whose digestive tract will destroy the seeds will fail to reproduce. So there is evolutionary pressure to evolve berries that are only eaten by useful animals. Maybe this is through taste, or smell, or poison.

But it's slow-acting because the people in question have evolved alongside it. They eat it during famines because the risk of poison is better than the certain death from starvation. So over generations, people become more tolerant of the poison. But the poison is still relatively young, so nobody is totally immune to it yet.

Ryan_L
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It merely inebriates the target consumer but kills humans

The substance that is poisonous to humans causes only mild intoxication in the species of birds that usually consume the berries and then spread the seeds through their feces. This is beneficial to the plant, so it produces the substance the birds crave.

Humans are much larger than those birds and would need a much higher dosage to get inebriated from the substance, but they lack an enzyme needed to break it down. So even small dosages will build up in the liver and inhibit its normal functions. Which causes other toxins to build up and slowly kill the human.

Depending on how much exactly the substance inhibits liver functions it could take anything from days to months or even longer. The symptoms would at first include difficulty to concentrate and fatigue. The person would sleep for progressively longer intervals, leading up to a coma and eventual death. Also the skin and the white parts of the eye would turn yellow. (The usual symptoms of liver failure)

Lukas G
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It did not evolve to be eaten by humans! Or humans did not co-evolve with it.

A berry is a bribe, to get an animal to eat the berry and to transport the seed within in its gut. A slow poison is pointless, it lacks even deterrent value. But if it evolved to be harmless to some common seed-distributing animals, it may still express toxins that poison (typically) insects that are eating the plant. And when humans arrive, those toxins may be slow-acting poisons to them, because they did not co-evolve with that plant and thereby evolve digestive enzymes to detoxify it.

I can't think of a slow-but-deadly example of fruit or berries. Deadly nightshade is fairly fast and fairly deadly. Birds eat those berries with impunity. (Most but not all humans taste them as bitter -- our poison-warning taste -- and don't eat a lethal dose).

The death cap/ destroying angel fungus is slowly deadly to humans, but not to rabbits. It's also reported by victims to be tasty! Yes, I know it's not a fruit, and the rabbits are not helping to spread its seeds. They've just evolved something to detoxify this fungus, which humans have not. Possibly because that fungus does not grow in Africa where we evolved?

Grapefruit are not toxic to humans. I don't know that they are toxic to any animal these days. But they once evolved a poison that attacked an important digestive enzyme in some creature, to make it sick or worse. These creatures, in turn, evolved other digestive enzymes that are not affected, thereby working around the problem. The grapefruit plant has no evolutionary pressure to stop expressing this now-non-toxin. However, this is of considerable interest to you if you take certain drugs, which are acted on by that enzyme. You have alternatives for processing food, but eating grapefruit will result in the dose of that drug being either too high (because the enzyme mormally destroys a lot of the drug before you absorb it) or too low (bcause that enzyme converts it into an active form). Which is why if the drug information leaflet or your doctor tells you not to eat grapefruit, don't!

nigel222
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Evolution is messy and slow

Based on my understanding, evolution comes about from mutations in individual members of the species which makes them more or less likely to produce offspring and pass on that mutation.

This means evolution would favour traits that provide an evolutionary benefit, but not providing an evolutionary benefit doesn't mean such traits would necessarily be avoided.

An ancestor species needed (fast-acting) poison, this one doesn't need poison (at all).

There are plenty of instances in nature of body parts or features might've been useful in the distant past, but don't seem to serve any purpose any longer.

That's not particularly extraordinary.

The strength of the poison may just have been decreasing for the last few generations as it's no longer providing a benefit.

It's a side effect or coincidence.

Maybe the plant needs to have poison running through it to not get eaten and destroyed by bugs and the poison ending up in the berry is just a coincidence.

A less direct alternative might be that the poison is a byproduct of something else: maybe it needed a thick skin to survive in a harsh climate and the first and easiest way evolution found to generate that skin was by also producing poison in the process.

Or it may just have evolved it because doing so didn't do any harm (nor provide any benefit).

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NotThatGuy
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The seeds of this plant require darkness and a lot of proteins to develop. When an animal eats it, the seed implants itself in the digestive tract of the animal and slowly releases its poison. After several days, the animal dies and the seed digests its dead body from the inside in order to grow. As the body decays, the new plant emerges and grows to maturity. A plant of this kind needs animals to eat its berries without seeing the cause and effect relationship between the berry and the poisoning. It is also beneficial for the animal to move far away from the parent plant so that the seedling will not be in direct competition.

IAntoniazzi
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    Heh. Maybe it's the final stage of Kopi Luwak coffee. https://en.wikipedia.org/wiki/Kopi_luwak Also, the poison should cause the animal to develop a desire to go to a place that would be good for the plant to grow, and to want to be alone. Drinking coffee makes you grumpy, and needing to pee makes you want to go into the woods and find a tree to go behind. – puppetsock Nov 20 '19 at 21:24
  • Much like "zombia ant fungus." https://en.wikipedia.org/wiki/Ophiocordyceps_unilateralis – puppetsock Nov 20 '19 at 21:29
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    I have heard of a WW2 soldier who was left stuck in a cave with two of mates. Eventually when his body was found a couple of years ago, his last meal of a guava was growing out of his thoroughly rotted and decomposed body. Not that guavas are poisonous.... – Varad Mahashabde Nov 22 '19 at 17:48
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The plant was too popular a meal

Berries are used to spread the plant in animal spoor, but there are simply too many animals eating these berries. They even eat the berry bushes, because there's just too many and they're all hungry. Even though berries are spreading the seeds, the plants are just being killed too quickly by predation.

Slow acting deadly berries accomplish two things. One, the berries will still spread the plant. Animals will eat the berries and spread the seeds in their spoor. Because the poison is slow, they never learn not to eat the berries. Two, they then die. The more berries are eaten, the more of them die, which stops them decimating the plant's population.

By parasitizing the animal population this way, the plant still spreads itself, and it also defends itself from predation.

As the plant becomes more successful, it kills more animals, so its rate of growth will slow. As its numbers dwindle, there will be more animals to spread them. The end result is an equilibrium, where the plant survives in its niche. It will be more common if the berries only sometimes make the slow-acting poison.

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Ton Day
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Non-deterrence. The substances humans use as rat poison act with a large delay in order to keep the rats from learning to avoid the poison.

A slow-acting deadly poison is developed by the "death cap" mushroom, Amanita phalloides. Its mycelium lives in symbiosis with hardwood tree roots. Hardwood trees grow slowly and are endangered by plant-eating animals that tend to debark them, like deer.

The very powerful and rather universal poison of the death cap enzymatically blocks RNA polymerase and consequently protein synthesis in the cell metabolism. First symptoms appear with a day of delay and abate. After about a week, the victims' health rapidly deteriorates and they die from liver failure.

The mushroom does not show typical warning signals of poisonous mushrooms and is described to have a pleasant, nutty taste.

The actual organism is the mycelium which is not harmed by the mushroom getting eaten: the mushroom is just the means for proliferation.

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Leave no survivors

This is one of those environmental factors that are a bit specific to evolving alongside humans. When you poison most animals, they respond by not eating you anymore, but when you poison a human, we respond by going out of our way to destroy your species to make sure we never get poisoned again.

When a tribe of humans wanders into your area and eats one of your berries, if they suddenly fall over dead, the remaining humans will start ripping up every one of those plants they find to make sure that none of their defenseless little offspring eat it by accident.

If instead a human eats your berries, likes it, then brings a whole bunch back to the rest of the tribe to share, then your berries can poison everyone before the humans realize that you are a dangerous plant. All the humans die, and your species is safe minus a few handfuls of berries.

Nosajimiki
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    you'd think so but there are lots of things poisonous to humans, usually we just learn not to eat that, only with the rise of tourism is ingesting unknown plants common. – John Nov 20 '19 at 23:47
  • @John Not that I doubt you, but do you have some source for that? I'd be curious to read about the connection with tourism. – thanby Nov 21 '19 at 13:14
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    The problem with this answer is the implied "directed evolution". It seems there's not enough evolutionary pressure to select this feature. Remembers, species are inexact. Reality has only individual plants. There's not really that much evolutionary pressure on each individual plant, because their survival rates don't differ that much. And the production of the poison has an evolutionary disadvantage; it takes chemicals and energy. This gives a free rider effect. – MSalters Nov 21 '19 at 13:29
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    @thanby-reinstateMonica native populations have detailed knowledge of their environment, my example is the Manchineel which is widely known to the natives, but with the rise of tourism (including intra-national tourism) they have started putting signs on the trees to keep tourists away from them before that they either used simple markers like a red X or just knew which trees to avoid. the best source Ican think of is studies on local medical plant knowledge, but it should be similiar. https://www.sciencedirect.com/science/article/pii/S0378874114001536 – John Nov 21 '19 at 14:17
  • https://en.wikipedia.org/wiki/Marsilea_drummondii is an example of something that would be edible by natives but not "tourists" - proper preparation prevents slow and painful death... oh wait... – Journeyman Geek Nov 24 '19 at 03:22
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The berry itself does not contain a toxin, but the body of certain animals will change it into a poisonous chemical. For example, take the chemical methanol. Methanol is poisonous because human livers convert it to Formaldehyde, which then poisons them. Methanol has the potential to blind, as well as kill. https://www.sciencedirect.com/topics/chemical-engineering/formaldehyde

Czernina
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I would have to say that the berries are mildly poisonous to cause the animal that ingests them to have a mild diarrhea. Not bad enough to kill the animal or make it sick enough that the animal never wants to eat the berry again. Just enough to trigger diarrhea which can spread the seeds in the stool and not be digested by the acids.

KillingTime
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Tom
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It just happens to be lethal prepared improperly and just happens to be that way. Quite a few plants have evolved to be edible by specific species - capsain just so happens to have little to no effect on birds for example, and there are stories of plants that needed specific species to germinate

The nardoo fern of australia seems a perfect analog for this. Wet ground or whole, the sporocarps are harmless. Dry ground they cause a slow, lingering death.

Likewise, quite a few edible fruit with stones can have tasty fruit and slightly poisonous seeds.

So evolutionarily? If you were a plant, the berry is expendable, the seeds are not. A human digestive system might be a little strong for the seeds and they may slowly poison the eater. An animal evolved to eat the berries would just let it pass through

Journeyman Geek
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You need a lot of this poison to make a difference.

Some animals will eat just a little of a thing, to make sure it is OK. Mice and rats are great example. If they feel sick, they will stop eating that thing.

This is why rat poisons like warfarin are slow acting. They take days to kill, because the rat does not feel them acting and so goes back and eats more.

The things that eat these slow-poison plants are big, durable animals. It will take a lot of eating to get enough poison into such an animal to make it sick. The delayed effect means that the animal keeps feeling good, decides that the plant is good food and is eating lots of it. When the first-eaten poison starts kicking in, there is lots more behind it and this animal is in for a rough time.

Willk
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