Weaver ants Oecophylla smaragdina encounter nasty neighbors rather than dear enemies.

The evolution of territorial behavior requires that the benefits of territoriality outweigh the costs. The costs are primarily those of territorial defense against encroaching neighbors or against floaters seeking to establish their own territory. One way to reduce the cost of defense might be to restrict serious conflict to encounters with those posing the greatest threat. Studies of many animals have found that less aggression is shown toward neighbors than toward, strangers, a phenomenon known as the "dear enemy" effect. However, the opposite can also be true, namely, that more aggression is shown toward neighbors than strangers: the "nasty neighbor" effect. This may be particularly true of group-living species that defend a resource-based territory. Here we show that (1) colonies of the weaver ant Oecophylla smaragdina were able to recognize a greater proportion of workers from neighboring colonies as non-colony members; and (2) when recognized as non-colony members, more aggression was exhibited toward neighbors than non-neighbors. We present for the first time evidence that differential levels of aggression involve both a perceptual and behavioral component. On the other hand, we found no evidence that weaver ant workers were better able to recognize workers from previously unknown colonies or responded more aggressively to them, even after a 10-day period of contact. This contrasts with other species in which rapid learning of the identity of new potential enemies has been demonstrated. We suggest that such a response is unnecessary for weaver ants, as encounters with intruders from non-neighboring colonies are probably rare and of little consequence. This study adds to the growing body of evidence that the nasty neighbor effect may be much more common than the dear enemy effect among group-living species.