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. Keywords Decentralized control · Emigration · Recruitment · Transport · Quorum sensing Introduction The emigration of an insect society demands an excep- tional degree of coordination among colony members. At a minimum, the colony must find a suitable nest site and safely bring its entire population there. Typically, it must also evaluate several potential sites, compare them, and choose the best one. Furthermore, the colony must reach consensus on a single site, rather than splitting its mem- bers among several. Recent work has shown that honey bees and ants can achieve these ends without any form of central control (Camazine et al. 1999; Seeley and Buhr- man 1999; Visscher and Camazine 1999; Mallon et al. 2001). The society instead functions as a single informa- tion-processing unit, distributing its cognitive tasks across a multitude of workers. This ability, evident in many aspects of social insect behavior, makes them lead- ing examples of decentralized control in biology (Tofts and Franks 1992; Seeley 1995; Bonabeau et al. 1997; Pratt 1998; Gordon and Hirsh 2001). The challenge in understanding these systems is to trace the links between the behavioral rules and information sources guiding in- dividual insects, and the cognitive abilities of the colony as a whole. Here we address this problem in the context of emi- grations by the ant Leptothorax albipennis. These ants form small colonies (fewer than 500 workers) that typi- cally nest in thin rock crevices whose fragility is likely to require frequent emigrations (Partridge et al. 1997). In the laboratory, ants can readily be induced to move from one artificial nest to another by opening the old nest and providing an intact new one nearby (Sendova-Franks and Franks 1995). A curious aspect of the ants’ behavior dur- Communicated by L. Sundström S.C. Pratt ( ✉ ) · E.B. Mallon · N.R. Franks University of Bath, Department of Biology and Biochemistry, Bath BA2 7AY, UK D.J.T. Sumpter Centre for Mathematical Biology, Mathematical Institute, Oxford University, 24–29 St Giles’, Oxford OX1 3LB, UK Present addresses: S.C. Pratt, Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA, e-mail: spratt@princeton.edu, Fax: +1-609-2581334 N.R. Franks, School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK Behav Ecol Sociobiol (2002) 52:117–127 DOI 10.1007/s00265-002-0487-x ORIGINAL ARTICLE Stephen C. Pratt · Eamonn B. Mallon David J. T. Sumpter · Nigel R. Franks Quorum sensing, recruitment, and collective decision-making during colony emigration by the ant Leptothorax albipennis Received: 28 August 2001 / Revised: 18 February 2002 / Accepted: 11 March 2002 / Published online: 9 May 2002 © Springer-Verlag 2002