Don't they want to be eaten??

August 29, 2009 9:36 PM

Why do plants make poisonous berries?

This question led me to the wikipedia entry for poisonous plants. For the moment, let's focus on berries.

The purpose of berries, from the plant's point of view, is to transplant seeds. here I'm using purpose in the general sense; obviously plants don't consciously do anything.

So wouldn't poisonous berries defeat the purpose? A sick animal is not going to spread the seeds.

A couple ideas...
1. Some berries are poisonous to humans but not birds. Okay, solid.
2. Maybe if the berries were delicious to humans, squirrels, whoever- as well as birds, then we would eat them all, leaving none for the birds. This would have an adverse effect on the seed distribution. plausible enough. any evidence of this?
3. In fact the purpose is not to spread the seeds; sometimes the berries only serve to ward off predators. kind of doubtful.
4. There must be some fascinating chemistry stuff going on here, with the poisons. I don't even know what to ask....is the poison a necessary byproduct of some other process that has some other purpose?
etc.

I'm interested to hear the hivemind's take on this!

posted by water bear to science & nature (15 answers total)

Well, it protects the plant... animals or people know they're poison and don't eat them, therefore leaving the plant/seeds alone to grow.
posted by IndigoRain at 9:44 PM on August 29, 2009

There are some plants for which only the unripe fruit is poisonous. The poison raises the odds that the fruit — and the seeds in it — will get to develop fully before they're eaten.
posted by nebulawindphone at 9:46 PM on August 29, 2009

Indeed, I expect that it is to keep critters from eating them until it's time. Also possible that the berries aren't poisonous to the right animals.

Look up how they have to get cashew nuts out. It's amazing.
posted by gjc at 9:54 PM on August 29, 2009

There are different kinds of fruits. Some use the fruit flesh as the food for the seed, some others use the fruit flesh as a lure for animals to eat and then poop out the seeds surrounded by nutrient poop.
posted by Ironmouth at 9:57 PM on August 29, 2009 [1 favorite]

Dead animal makes great fertiliser for plant seeds.
posted by fearthehat at 10:13 PM on August 29, 2009 [10 favorites]

My mycology prof said that since plants can't move to get rid of waste products, they put them into fruiting bodies so they will be carried away by animals. These 'waste products' might or might not be particularly harmful to whoever eats them; in the case of mushrooms the toxins that the mycelium is trying to get rid of can be psychotropic in humans, for instance.
posted by foobario at 10:20 PM on August 29, 2009

Maybe if the berries were delicious to humans, squirrels, whoever- as well as birds, then we would eat them all, leaving none for the birds. This would have an adverse effect on the seed distribution. plausible enough. any evidence of this?

This definitely happens, e.g., with capsaicin.
posted by Eamon at 10:38 PM on August 29, 2009

Why do plants make poisonous berries?

Because doing so worked for their parent(s).
posted by flabdablet at 11:23 PM on August 29, 2009 [4 favorites]

How the cashew propagates:

There's the cashew apple (mmmm, 'tis a juicy fruit), and attached to it, a cashew seed enclosed in a poisonous leathery shell. Birds and bats eat the apple, and the seed (happily enclosed in its shell) is dropped elsewhere, unwanted.

I'm glad I went and looked up how cashews work. Thanks, gjc and water bear, now it makes sense to me why some pits (such as peaches) and seeds (such as apples) are poisonous. I hadn't really thought about it before.
posted by aniola at 3:30 AM on August 30, 2009 [1 favorite]

There are a few things that can be said here.. First, some propagation methods can work in an illogical way, for instance there is a tropical tree (don't remember the name) which has a layer of nutrients originally meant for the seed to use when it begins to grow around it. There is a type of animal that found a way to strip the layer and bury the seed for future use. If the seed is forgotten, it sometimes does germinate, even without the nutrient layer. But, if the animal does not 'steal' the nutrient layer, there is a type of beetle larvae that will live in the layer and ruin the seed every time.

A tree may target its fruit to a certain type of animals. The key thing here, is it poisonous for everyone? For example, some animals may have digestion that destroys the seeds.

But the most likely explanation, in my view, is protection from insects that would eat the fruit and seeds together but can be repulsed by poisoned fruit while there are some animals that will tolerate poison.

It may also be that what is poisonous to some animals may be attractive to other animals? Maybe the poison can have medicinal value in small concentrations, get rid of an animal's internal parasites?
posted by rainy at 4:57 AM on August 30, 2009

Actually I might be a bit off on the details about the tropical tree in my first example. That's the gist of it but I've read about it a while ago and maybe someone can correct me if I got some details wrong?
posted by rainy at 5:07 AM on August 30, 2009

From http://www.cnr.uidaho.edu/range556/Appl_BEHAVE/projects/seed_dispersal.html:

Not all animals that eat fruit are appropriate dispersers. The effectiveness of an animal as a seed dispersal agent depends on how many seeds it disperses, and how it treats these seeds. Animals that visit a plant for fruit more frequently may be more reliable than those animals that eat fruit more rarely. Some fruit-eating animals are considered seed "predators" if they kill seeds by digesting the seed along with the fruit pulp. On the other hand, seeds can sprout better after passing through the digestive tract of certain frugivores. In addition to enhancing sprouting, an effective dispersal agent deposits seeds in appropriate habitats for their survival to reproductive adulthood. The survival of a seed greatly depends on where it lands. Seeds that move farther away from other seeds have greater success because they can better escape resource competition, interbreeding with parents, and post-dispersal mortality. Because not all animals that eat fruit are equally good at dispersing seeds, plants should change fruit characteristics to discourage frugivores that are seed predators, and encourage frugivores that are effective dispersers.

Also:

Some fruits contain toxins to deter seed pathogens or predators. Animals can learn which fruits are toxic, and thus learn to avoid them. Toxins can also limit the amount of time a more toxin-tolerant frugivore spends feeding at the plant. Because animals are limited to the amount of toxins they can handle over a period of time, a forager must stop eating a food item when maximum toxin load is reached. The foraging animal may then leave to find an alternative source of food or to seek antidotes. By forcing an animal to leave early, the plant may be able to ensure that its seeds will be deposited far enough away to avoid inbreeding, competition, and pathogens as mentioned above.

Ever get a stomachache after eating too much of one kind of fruit? Now you know...
posted by ayerarcturus at 10:57 AM on August 30, 2009

Dead animal makes great fertiliser for plant seeds.

fearthehat, unless you can provide evidence to the contrary, I think you have just invented a new urban myth. The vast majority cases of poisoning do not result in death, but discomfort; the next step is learning to never ever eat that berry again!
posted by IAmBroom at 7:31 AM on August 31, 2009

Eamon, capsaicin is an interesting case. What I've read suggests that the capsaicin evolved as a anti-fungal agent (eg. this paper).

The wikipedia article you linked to also suggests that natural selection may have led to increasing capsaicin production because it makes the plant less likely to be eaten by animals that do not help it reproduce [animals would digest the seeds after grinding them with molars]'. This is a somewhat problematic hypothesis, as some think human's taste for spicy foods developed so they could self-medicate with the anti-fungal capsaicin; hence, increasing the capsaicin content of a fruit could make it more likely its to be consumed by animals.
posted by James Scott-Brown at 11:22 AM on August 31, 2009

The toxicity of a given chemical can vary quite a bit between species. Many of the plant-derived alkaloids that humans use (caffeine, nicotine, cocaine, morphine, etc) are toxic to insects. They're toxic to humans as well, but only in much higher dosages; insects need to ingest far less to be seriously affected (remember that dosages are often measured in terms of body weight—mg/kg). These chemicals play no other role in the plant—they are specifically designed as a deterrent to herbivores, which is why they are more often present in the leaves and roots of plants. But they may not serve (or be intended to serve) as an equal deterrent to all species, as is the case with the above-noted alkaloids.

It’s also worth noting that alkaloid levels in plants are not usually constant. They can vary quite a bit depending on the growth stage of the plant, the environmental conditions, and the amount of damage it’s previously sustained. Plants which are under stress, particularly by insects, will tend to produce more alkaloids.
posted by dephlogisticated at 3:01 PM on August 31, 2009