Quorum sensing, recruitment, and collective decision-making during colony emigration by the ant Leptothorax albipennis

Abstract

When its nest is damaged, a colony of the ant Leptothorax albipennis skillfully emigrates to the best available new site. We investigated how this ability emerges from the behaviors used by ants to recruit nest- mates to potential homes. We found that, in a given emi- gration, only one-third of the colony's workers ever re- cruit. At first, they summon fellow recruiters via tandem runs, in which a single follower is physically led all the way to the new site. They later switch to recruiting the passive majority of the colony via transports, in which nestmates are simply carried to the site. After this switch, tandem runs continue sporadically but now run in the opposite direction, leading recruiters back to the old nest. Recruitment accelerates with the start of trans- port, which proceeds at a rate 3 times greater than that of tandem runs. The recruitment switch is triggered by pop- ulation increase at the new site, such that ants lead tan- dem runs when the site is relatively empty, but change to transport once a quorum of nestmates is present. A mod- el shows that the quorum requirement can help a colony choose the best available site, even when few ants have the opportunity to compare sites directly, because re- cruiters to a given site launch the rapid transport of the bulk of the colony only if enough active ants have been "convinced" of the worth of the site. This exemplifies how insect societies can achieve adaptive colony-level behaviors from the decentralized interactions of relative- ly poorly informed insects, each combining her own lim- ited direct information with indirect cues about the expe- rience of her nestmates.