This mutualism is my favorite one, occurring between the bullthorn acacia (Acacia cornigera) and the ant Pseudomyrmex ferrugiea.
The bullthorn acacia has large hollow thorns where ants live, and it produces nectar (not associated with flowers) and protein-rich structures that the ants eat. The ants patrol the plant and sting anything that tries to nibble or land on it, and they clear the ground under and around the plant, killing any plants that might compete with the host acacia.
Not such a big deal, right? Wrong. The bullthorn acacia is unusual. Other acacias don't produce extrafloral nectaries; they don't produce the protein structures, which have no known function other than to feed the ants; and they're poisonous, producing chemicals to defend against herbivores. The bullthorn acacia doesn't have these characteristics because it doesn't need them; it has the ants instead.
And let me tell you, these ants really hurt when they sting. I know because I checked — personally. And I'm not the only person who thinks so. The Schmidt Sting Pain Index — yes, there really is one; you can see for yourself by going to http://scientiaestpotentia.blogspot.com/2 0 06/0 6/ schmidt-sting-pain-index.html — rates them at 1.8 (a bald-faced hornet gets a 2) and describes the pain as "A rare, piercing, elevated sort of pain. Someone has fired a staple into your cheek." Someone there must have been stung, too, which just goes to prove the lengths to which scientists will go to collect data.
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