This study explored how ants detect a crucial cue used to coordinate colony members performing a complex collective task. During emigrations by the ant Leptothorax albipennis, scouts recruiting to a potential nest cavity switch from tandem runs to transports once the siteís population reaches a quorum, a rule that may enhance the colonyís ability to choose the best among several sites. Quorum detection requires direct contact with nestmates, as shown by the behavior of naÔve scouts discovering a site that had recently been the target of transports by nestmates. Just prior to each scoutís entry, the nestmates were moved behind a mesh partition. The scouts consistently led a tandem run, suggesting that they judged the nest empty, despite any pheromones left by their nestmates or emitted by them through the permeable screen. Quorum detection may instead depend on the scoutís encounter rate with nestmates. Consistent with this, ants moving into a large nest, where encounter rates were low, switched to transport at a higher population than they did in a small nest, where encounter rates were high. Quorum detection is also time-consuming. After their first transport, scouts simultaneously reduced the length of their visits to the new site, and ceased to monitor site population, as shown by their continuing to transport even after the site was experimentally depopulated. These results are consistent with a Bayesian model of decision-making in which a scout gradually accumulates information by using each encounter to update her estimate of whether the quorum has been met.
Species 1: Hymenoptera Formicidae Leptothorax albipennis
Keywords: self-organization, collective decision-making
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