According to this article, the Acacia tree has a chemical defence system which leads to the release of ethylene in the surroundings when a herbivore grazes on it. This leads to an increase in tannin concentration in the leaves not only in the plant where the herbivore is grazing but also in the acacias surrounding 45m.

The question is, what prevents acacias to store tannins beforehand? Wouldn't the giraffes totally avoid them then? Tannins aren't poisonous by themselves (correct me if wrong).

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    $\begingroup$ I was unable to find the original research article. A link would be really appreciated $\endgroup$ – Polisetty Jul 26 '18 at 10:12
  • $\begingroup$ The original article doesn't seem to be online. A summary of the findings was in the Scientific American in 1990. A thesis by a student of van Hoven's seems to cover it (THE INFLUENCE OF PLANT TANNIN ON THE FEEDING ECOLOGY OF GIRAFFE (GIRAFFA CAMELOPARDALIS)) but only the abstract seems to be online and the thesis is in Afrikaans. $\endgroup$ – iayork Jul 26 '18 at 11:56
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    $\begingroup$ Usually the answer to these sorts of questions is that generating the toxic substance (tannins) is metabolically costly, so it's bad for the plant to produce the toxin when it's not needed. When the plant is threatened by browsers, it's beneficial to turn on the production until the threat goes away. $\endgroup$ – iayork Jul 26 '18 at 11:57

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