Self-organized aggregation in cockroaches RAPHAEL JEANSON * , COLETTE RIVAULT †, JEAN-LOUIS DENEUBOURG ‡, STEPHANE BLANCO§, RICHARD FOURNIER §, CHRISTIAN JOST * & GUY THERAULAZ * *Centre de Recherches sur la Cognition Animale, CNRS, Universite ´ Paul Sabatier yLaboratoire d’Ethologie, Ecologie, Evolution, CNRS, Universite ´ de Rennes I zCenter for Nonlinear Phenomena and Complex Systems, Universite ´ Libre de Bruxelles xLaboratoire d’Energe ´tique, Universite ´ Paul Sabatier (Received 2 October 2003; initial acceptance 28 November 2003; final acceptance 22 February 2004; published online 23 November 2004; MS. number: 7871) Aggregation is widespread in invertebrate societies and can appear in response to environmental heterogeneities or by attraction between individuals. We performed experiments with cockroach, Blattella germanica, larvae in a homogeneous environment to investigate the influence of interactions between individuals on aggregations. Different densities were tested. A first phase led to radial dispersion of larvae in relation to wall-following behaviours; the consequence of this process was a homogeneous distribution of larvae around the periphery of the arena. A second phase corresponded to angular reorganization of larvae leading to the formation of aggregates. The phenomenon was analysed both at the individual and collective levels. Individual cockroaches modulated their behaviour depending on the presence of other larvae in their vicinity: probabilities of stopping and resting times were both higher when the numbers of larvae were greater. We then developed an agent-based model implementing individual behavioural rules, all derived from experiments, to explain the aggregation dynamics at the collective level. This study supports evidence that aggregation relies on mechanisms of amplification, supported by interactions between individuals that follow simple rules based on local information and without knowledge of the global structure. Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. The most common collective behaviour among living organisms is probably grouping, which occurs in a wide range of taxa, including bacteria, arthropods, fish, birds and mammals (Parrish & Hamner 1997; Parrish & Edel- stein-Keshet 1999; Parrish et al. 2002). Depending on the species, these assemblages may be labelled as herds, shoals, flocks, schools or swarms and are more broadly denoted as aggregations (Allee 1931). In reference to the spatial distribution of organisms, an aggregation could be defined as any assemblage of individuals that results in a higher density of individuals than in the surrounding area (Camazine et al. 2001). Studies on aggregation have investigated mainly the benefits from association with conspecifics or differences in fitness related to the spatial position of individuals in groups (Hamilton 1971; Parrish 1989; Krebs & Davis 1993; Krause 1994; Romey 1995; Watt & Chapman 1998; Krause & Ruxton 2002). Much less attention has been paid to proximal causes, addressing the question of the underly- ing mechanisms, with the exception of studies on fish (Krause & Tegeder 1994; Parrish & Hamner 1997; Croft et al. 2003) and social amoebae (Raper 1984). Attempts have been made to classify aggregations according to the cues leading to these assemblages and the relations between their components (Allee 1931 and references therein; Parrish & Hamner 1997). Aggregates could be formed incidentally by passive collection of organisms resulting from abiotic factors. For example, zooplankton could aggregate in response to physical constraints such as marine currents (Hamner & Schneider 1986). Aggregation Correspondence: R. Jeanson, Centre de Recherches sur la Cognition Animale, CNRS UMR 5169, Universite ´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Ce ´dex 4, France (email: jeanson@ cict.fr). C. Rivault is at the Laboratoire d’Ethologie, Ecologie, Evolution, CNRS UMR 6552, Universite ´ de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France. J.-L. Deneubourg is at the Center for Nonlinear Phenomena and Complex Systems, Universite ´ Libre de Bruxelles, C.P. 231, Campus Plaine, 1050 Brussels, Belgium. S. Blanco and R. Fournier are at Laboratoire d’Energe ´tique, Universite ´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Ce ´dex 4, France. 169 0003–3472/04/$30.00/0 Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2005, 69, 169–180 doi:10.1016/j.anbehav.2004.02.009