Personal relationships are the cornerstone of vertebrate societies, but insect societies are either too large for individual recognition, or their members were assumed to lack the necessary cognitive abilities 1, 2. This paradigm has been challenged by the recent discovery that paper wasps recognize each other's unique facial color patterns [3]. Individual recognition is advantageous when dominance hierarchies control the partitioning of work and reproduction 2, 4. Here, we show that unrelated founding queens of the ant Pachycondyla villosa use chemical cues to recognize each other individually. Aggression was significantly lower in pairs of queens that had previously interacted than in pairs with similar social history but no experience with one another. Moreover, subordinates discriminated familiar and unfamiliar dominants in choice experiments in which physical contact, but not odor perception, was prevented and in tests with anaesthetized queens. The cuticular chemical profiles of queens were neither associated with dominance nor fertility and, therefore, do not represent status badges 5, 6, and nestmate queens did not share a common odor. Personal recognition facilitates the maintenance of stable dominance hierarchies in these small societies. This suggests that the ability to discriminate between individual traits is selected for when it incurs net benefits for the resolution of conflict.
