Complex Dynamics Based on a Quorum: Decision-Making Process by Cockroaches in a Patchy Environment

Aggregation occurs in many biological systems: from bacteria to vertebrates (Parrish & Edelstein-Keshet 1999; Krause & Ruxton 2002; Ben Jacob et al. 2004). Among ultimate causes of living in groups, costs and benefits of aggregating in space and time are extensively described in the literature (Allee 1926; Hamilton 1971; Parrish 1989; Krause 1994; Choe & Crespi 1997; Watt & Chapman 1998; Stephens & Sutherland 1999). Costs to group members involve a sharing of food resources (Giraldeau & Caraco 2000), a higher competition for sexual mates (Moller & Birkhead 1993), or an increased parasitic burden (Van Vuren 1996). However, it also brings several advantages by allowing information transfer between individuals (Dall et al. 2005), promoting cooperation in foraging (Creel & Creel 1995; Vasquez & Kacelnik 2000) or in parental care (Choe & Crespi 1997; Sempo et al. 2006a) and facilitates thermoand hygroregulation (Heinrich 1981; Ancel et al. 1997; Dambach & Goehlen 1999) or protection against predators (Bertram 1978; Treherne & Foster 1980). In a patchy environment, the habitat at which aggregation takes place will deeply influence the fitness of the inhabiting species. As the intrinsic quality of the habitat varies in time and space (Orians & Wittenberger 1991), animal species have to select the optimal site and could notably rely on public information that provides a more accurate estimate of habitat quality (Doligez et al. 2004). In this respect, the presence of conspecifics provides a local cue (Boulinier & Danchin 1997; Detrain & Deneubourg 2009) that can be used by individuals in their ‘shared information’ strategy (i.e. social attraction: Stamps 1988; Reed & Dobson 1993; Muller 1998; Conradt & Roper 2005). The rate of encounters or any activity by-product testifying the presence of Correspondence Sempo Grégory, Unit of Social Ecology (CP231), Université libre de Bruxelles, 50 Avenue F.D. Roosevelt, 1050 Bruxelles, Belgium. E-mail: [email protected]