3046 INTRODUCTION In large animal groups lacking global communication systems, the coordination of collective activities often relies on self-organised mechanisms, i.e. collective patterns emerge from interactions between individuals following simple behavioural rules in response to local information. Self-organisation was thus shown to underlie various natural collective processes in both invertebrates and vertebrates, including humans (Bonabeau et al., 1997; Camazine et al., 2001; Couzin and Krause, 2003; Sumpter, 2006; Moussaid et al., 2009). Self-organised biological processes are fundamentally distributed and do not require any global overview or central control. It has thus long been assumed that group members are interchangeable, following the same set of local behavioural rules and contributing equally to the collective process (Conradt and Roper, 2005; Dyer et al., 2008). This view was particularly widespread in studies on insect societies, where collective complexity and flexibility were often juxtaposed with individual simplicity and homogeneity (Deneubourg and Goss, 1989; Deneubourg, 1995; Bonabeau et al., 1997; Detrain and Deneubourg, 2002; Theraulaz et al., 2003; Detrain and Deneubourg, 2006). However, there is accumulating evidence that certain self-organised processes can be disproportionately influenced by key individuals playing the role of effective leaders. This has been mainly studied in synchronised movements and activity shifts by large groups of vertebrates (Couzin et al., 2005; Sumpter et al., 2008; Dyer et al., 2009; Lusseau and Conradt, 2009) and invertebrates (Schultz et al., 2008). In insect societies, such as ant and honeybee colonies, recent studies have also highlighted the key influence of specialised and/or experienced individuals on task performance (Robson and Traniello, 1999; Sendova-Franks et al., 2010), division of labour (Anderson and Ratnieks, 1999; Gordon, 2002) and mass recruitment (Collignon and Detrain, 2011). However, most studies of collective decision- making still consider that self-organised, multiple-choice decisions are organised in a ‘democratic’ way, i.e. evenly shared among decision-makers that follow similar rules (Conradt and Roper, 2005; Seeley, 2010). Although a recent study predicted the existence of leaders in cases where there are significant conflicts of interest among group members (Conradt et al., 2009), the existence of such leaders in natural self-organised decision-making systems has not been documented so far. Here we provide the first experimental evidence of the existence of such influential individuals in a multiple-choice self-organised decision process by an invertebrate group, namely nest site selection by the house-hunting ant Temnothorax albipennis. House-hunting social insects face the major challenge of selecting the best among several candidate new nest sites during emigrations The Journal of Experimental Biology 214, 3046-3054 © 2011. Published by The Company of Biologists Ltd doi:10.1242/jeb.059188 RESEARCH ARTICLE Knowledgeable individuals lead collective decisions in ants Nathalie Stroeymeyt 1,2,3, * ,† , Nigel R. Franks 3,‡ and Martin Giurfa 1,2,‡ 1 Université de Toulouse, UPS, Centre de Recherches sur la Cognition Animale, 118 route de Narbonne, F-31062 Toulouse Cedex 9, France, 2 CNRS, Centre de Recherches sur la Cognition Animale, 118 route de Narbonne, F-31062 Toulouse Cedex 9, France and 3 School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, United Kingdom *Present address: Département d’Ecologie et d’Evolution, Biophore, Quartier UNIL-Sorge, Université de Lausanne, CH-1015 Lausanne, Switzerland † Author for correspondence (Nathalie.Stroeymeyt@unil.ch) ‡ Both authors contributed equally to this work Accepted 29 June 2011 SUMMARY Self-organisation underlies many collective processes in large animal groups, where coordinated patterns and activities emerge at the group level from local interactions among its members. Although the importance of key individuals acting as effective leaders has recently been recognised in certain collective processes, it is widely believed that self-organised decisions are evenly shared among all or a subset of individuals acting as decision-makers, unless there are significant conflicts of interests among group members. Here, we show that certain individuals are disproportionately influential in self-organised decisions in a system where all individuals share the same interests: nest site selection by the ant Temnothorax albipennis. Workers that visited a good available nest site prior to emigration (the familiar nest) memorised its location, and later used this memory to navigate efficiently and find that nest faster than through random exploration. Additionally, these workers relied on their private information to expedite individual decisions about the familiar nest. This conferred a bias in favour of familiar nests over novel nests during emigrations. Informed workers were shown to have a significantly greater share in both recruitment and transport to the familiar nest than naïve workers. This suggests that they were the main determinants of the collective preference for familiar nests, and thus contributed greatly to enhance collective performance. Overall, these results indicate that self-organised decisions are not always evenly shared among decision-makers, even in systems where there are no conflicts of interest. Animal groups may instead benefit from well-informed, knowledgeable individuals acting as leaders in decisions. Supplementary material available online at http://jeb.biologists.org/cgi/content/full/214/18/3046/DC1 Key words: private information, social information, collective decision-making, leader, self-organisation, memory THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY