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I know about prisoners dilemma. I also know about trigger strategies and sustained collusion.
Meleerumble is not a prisoner's dilemma. It is a constant-sum game (all bots scores always sums 100%), while prisoner's dilemma is not. In constant-sum games, collusion is never sustained.
But 2 player sub-games do have prisoner's dilemma aspects. If 2 bots collude, they both increase their chances of survival against the other 8. If 2 bots shoot against each other, they both diminish their chances of survival. If one shoots but the other not, the one shooting gains an advantage (earning points and gaining energy).
Don't expect existing bots to learn collusion as most bots have no active developers looking after them. The rumble is full of hardcore defectors and hardcore cooperators, and in prisoner's dilemma, the best response against hardcore players who pick the same strategy regardless of what others are doing is to defect against them as well.
For collusion to work, even temporary, at least 2 bots need to go for it. If you upload a single trigger strategy bot, it won't work. If you upload a bunch of trigger strategy bots, like 300, then it might work.