What makes the ants intelligent?

The Brazilian official who comes to see Leiningen makes it clear that he believes the ants are more than a match for Leiningen. He describes them as "elemental," each one of them "a fiend from hell," suggesting that they are driven by pure devilry and will set upon devouring not only the plantation, but Leiningen himself. Initially, Leiningen dismisses this, believing that his brain will be enough to see off the ants.
Of course, he is proven wrong. Leiningen builds a "girdle of water" and thinks the ants will be unable to get through it unless they are able to build rafts. However, when he sees the ants approach in perfect formation, he begins to see that he may be wrong. The ant scouts report back to the others what is in their way—the water. The watchers imagine that there is some kind of "telepathy" at work among the ants, as they become more and more aware of the true situation and the shape of the horseshoe ditch. Every time the ants become more aware, they regroup. The ants dying in the water serve as stepping stones for others. They advance across the ditch like an army making a tactical advance.
At one point, Leiningen thinks he has won, but no—it seems that the ants actually can build their own rafts out of leaves. They are intelligent enough to learn from their initial failure and to think tactically of a different strategy. This is the sort of intelligence Leiningen had thought impossible from such creatures.