Ants distinguish neighbors from strangers.

Ants are known to distinguish their own nests and nestmates from all others, using colony-specific odors. Here I show that harvester ants (Pogonomyrmex barbatus) can further distinguish between two kinds of non-nestmates of the same species: neighbors and strangers. Interactions between colonies were thought to depend on the numbers of alien ants that each colony encounters on its territory. The results described here show that such interactions also depend on information about colony identity. Encounters on foraging trails with ants from neighboring colonies, deter foraging more than encounters with ants from distant ones. The history of interactions between particular pairs of colonies may have important effects on intraspecific competition for food.