there is no intelligence. every ant follows very simple instructions, even the queen. It’s just that after millions of years of evolution, these instructions got refined into something that looks like intelligent behavior.
E.g. instructions for food gathering:
1. Go exploring, and leave a trail of “chemicals”.
2. If tired (or it is getting dark), follow your own trail back to home.
3. If found food, take a piece of it back home, and mark trail with “food!!!” chemicals
4. If found somebody else’s trail of “food!!!” chemicals, follow it.
But ants cannot change their instructions , and they can look very dumb in un-natural circumstances. E.g. ants will continue to bring poisoned food into the nest, because they need food to replace the dying ants, and they do not realize that ants started dying after they discovered this new food source.
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