Emigrating colonies of the ant Temnothorax (formerly Leptothorax) albipennis can choose the best of several nest sites, even when the active ants organizing the move do not compare sites. This collective ability depends on a quorum rule used by ants assessing a candidate site. Only when the site's population has surpassed a threshold do they switch from slow recruitment of fellow active ants by tandem runs to rapid transport of the majority of the colony. Here, I show that ants perceive the achievement of a quorum through their rate of direct encounters with nest mates at the site. When ants in a crowded site were prevented from tactile contact with nest mates, they recruited by tandem runs, as though to an empty nest. Furthermore, when the encounter rate was raised independent of population, by reducing the size of the candidate nest, ants started to transport at a significantly lower population. The switch occurred at the same encounter rate regardless of nest size, whether the rate was measured as the mean over the entire visit or as the inverse of the latency until the first encounter. Because encounter rate reflects the density of nest mates and thus varies with nest size as well as population, the ants' collective decision-making algorithm may be robust to the exact population at which the switch to transport occurs. Ants cease monitoring quorum presence after switching to transport, coincident with an abrupt shortening of visit duration by approximately 2 min, which may be interpreted as the time required for quorum detection.
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