Manchineel -- I do not understand this fruit at all

The manchineel is considered to be the world’s most poisonous tree, with caustic sap that can blister the skin and take the paint off a car. Of course, it would have to Euphorbiaceae, wouldn’t it? Wikipedia describes what it is like to eat the fruit:

When ingested, the fruit is reportedly “pleasantly sweet” at first, with a subsequent “strange peppery feeling … gradually progress[ing] to a burning, tearing sensation and tightness of the throat.” Symptoms continue to worsen until the patient can “barely swallow solid food because of the excruciating pain and the feeling of a huge obstructing pharyngeal lump.”

The “pleasantly sweet” suggests a fruit meant for vertebrate dispersal; why would a plant expend the metabolic effort of putting sugar in the fruit if not to attract dispersing animals? Yet the same Wikipedia article goes on to say that the fruit is poisonous to many birds and other animals, which seems counterproductive to a vertebrate-dispersed fruit.

Interestingly, it also says that ctenosaurs (a type of iguana) can eat the fruit and even live among the limbs. How is that possible? Don’t lizards have a soft, fleshy inside of the mouth? Isn’t their esophagus made of the same kind of epithelial and mucous tissue as that of other terrestrial vertebrates? Wouldn’t the sap burn them just as badly as other animals?

I do not understand how a fruit like this “works” evolutionarily.

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It might be something similar to peppers, peppers evolved to be spicy because small mammals were opening their fruits and destroying their seeds by eating them directly. Mammals in general can’t stand the spiciness of peppers but birds can’t actually taste it, they are completely immune to pepper spice and so can still eat the fruit and disperse the seeds.

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If iguanas eat them fine, isn’t it the sign they coevolved? Some snakes are immune to all the frog poisons (that they get from plants), so lizards can get that ability too, this plant became so poisonous after very long time.

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I thought something along the same lines. Not knowing the tree or the fruit I immediately pictured one of those cases of co-evolution with the dispersal mechanism beeing very specialized to one or only few dispersers that are immune while dispersers that would not suit the goal of the plant beeing propelled by the toxin.

Another possibility that is circulating in the science world: (some) fruits might not generally have evolved to attract eaters, but rather to protect and nourish the seeds. In that case the toxic plants just try to protect their seeds and some animal might have overcome this defense, similar to animals eating other toxic parts of plants (e.g. eucalyptus and Koalas)

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A couple of things come to mind:

  1. This could simply be a holdover from a time when the plant had evolved a relationship with a specific dispersal host and needed protection from others. Often plants evolve traits in response to animals in their ecosystem, then those animals go extinct, but the plants retain the traits. Some plants in the Americas have oversized fruits and others are heavily armed, yet they currently have no reason for those, but prior to 10,000 years ago they had a dire need for them due to the abundance of browsing megafauna on the two continents. (This also happens with animals, as the North American Pronghorn Antelope demonstrates).

  2. At present the main dispersal method of the tree is water dispersal, and this requires the fruit to rot to release the seeds. Perhaps the sugar facilitates more rapid rotting of the fruit, and leads to greater seed survival, or maybe it attracts insects (I can’t find any information about its toxicity to insects), with eat the flesh and expose the seeds for dispersal).

It should be noted that option 1 and 2 are not mutually exclusive. The tree could have evolved a relationship with a specific dispersal animal in the past (even very distant past), then when that animal became extinct it relied more and more on water dispersal.

Sometimes evolved traits get kind of ‘locked in’, and are retained even if they are no longer needed because the process of ‘unevolving’ them is too complicated and requires too many mutations happening in exactly the right way and order.

Interestingly, the seeds have been found in Galápagos tortoise dung (see reference below). There are no giant tortoises in the Caribbean now, but they used to be widespread in the recent past (quoted text from the second linked paper):

Giant tortoises are absent from current-day insular Caribbean ecosystems, but tortoise remains from Quaternary deposits indicate the former widespread occurrence of these animals across the northern Caribbean.

Tortoises are well known to have a ‘sweet tooth’ as pet owners can attest to. Perhaps this tree evolved its fruit to be dispersed by giant tortoises that are now extinct in the region?

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Not at all familiar with the intricacies of this tree, but, my immediate thought is that like most Caribbean plants with absolutely absurd defenses, given they co-evolved with herbivorous reptiles and ground sloths, they’re adapted to both being spread by iguanas and now extinct tortoises, and defending against herbivory from also now extinct ground sloths

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In case this is relevant, I wanted to explain that the Manchineel fruit, which is green and looks approximately like a very small green apple, has only a very thin covering of soft flesh. Inside that thin soft layer is a very hard, large, woody, structure. The seeds are deep inside that woody core.

And yes the “poison apples” usually wash out to sea, and then at some point they get washed back in again and germinate high up on the beach.

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At some point I was convinced that this story might not be true, and that the actual target of Capsaicin might be fungal pathogens rather than mammals. Though I think it’s also true that mammals mind and birds don’t, and regardless, your general point stands that some clades might be immune.

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After hearing more about this species biology I’m now thinking it may be more like a coconut. Coconuts don’t actually want their nuts to be eaten the flesh inside is for the germinating seedling and they are so hard and thick to try and keep animals out. Perhaps the thin layer of toxic flesh acts like a coconuts shell trying to prevent animals from accessing the valuable insides