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The machineel tree is incredibly toxic to humans. What advantage could Hippomane mancinella have for being so poisonous? It's name even apparently translates to “little apple that makes horses mad”.

Is it an incidental toxin, or an evolutionary hangover from a previous predator, or is it still actively predated by a species with high immunity?

I ask because this seems somewhat paradoxical; the tree has fruit, yet is toxic to animals.

Fruit of the tree

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  • $\begingroup$ I found an answer at atlasobscura.com/articles/…. $\endgroup$ – Timothy Jun 20 '16 at 2:41
  • $\begingroup$ I answered a catch-all version of this: biology.stackexchange.com/questions/36977/… The same applies to trees. $\endgroup$ – James Jun 20 '16 at 3:07
  • $\begingroup$ I've made some drastic edits to the post to make it answerable (it was a bit rambly)! Feel free to roll back; I may have changed the question in some way unwittingly. Also, your linked article doesn't answer the question you asked. It unverifiably says iguanas have been seen eating it in one continent, and that nobody knows why it's toxic to humans. $\endgroup$ – James Jun 20 '16 at 3:28
  • $\begingroup$ The sap is full of Phorbols. Phorbol esters are so carcinogenic, labs use them to cause cancer in animal models. $\endgroup$ – user137 Jun 20 '16 at 4:10
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    $\begingroup$ @James The question you linked is asking about animal venom, which may not be completely relevant to this question. $\endgroup$ – March Ho Jun 20 '16 at 13:54
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According to http://www.atlasobscura.com/articles/whatever-you-do-do-not-eat-touch-or-even-inhale-the-air-around-the-manchineel-tree, it evolved to be poisonous to discourage animals from digging into it. Here's one possible way that could have happened. It was once only a tiny speck more poisonous than other trees by random fluctuations in toxicity. By natural selection, animals evolved to prefer it less. They were more likely to dislike the taste of the more poisonous manchineel trees, which in turn caused it to evolve to be more poisonous. Maybe it was such an advantage to get more energy by eating some of it that some species evolved to be more resistant to the poison so that they could eat some of it. That caused the manchineel tree to evolve to be even more poisonous to discourage animals from eating it. It was an evolutionary disadvantage for the apples to be poisonous. That was just an accidental result of the rest of the tree being so poisonous. Indeed, they did evolve to be much less poisonous than the sap for that reason. Animals evolved to dislike the taste of the apples because they're poisonous. The less poisonous ones were more likely to get eaten and have their seeds spread. Since they couldn't evolve to be nonpoisonous, they instead evolved to taste like there's nothing wrong with them so that they could get eaten. Since iguanas did evolve to be immune to the poison apples, there wasn't much of an advantage in them evolving to be even less poisonous.

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