Self-organised choice based on inter-attraction: the example of gregarious animals

The spatial distribution of individuals is an important subject in many fields because it conditions the levels of interactions among individuals, and more generally the structuring as well as the organization of populations. Increase in density of individuals in a given area can be induced by environmental stimuli and/or by interactions among individuals (1–3). Thus, various definitions of aggregation have been given, ecologists privilege the importance of environmental stimuli, while others privilege existing relationships between group members. Aggregation is one of the most widespread social phenomena and occurs at all biological levels, from bacteria to mammals including humans (4, 5). If sometimes, aggregation is associated to non-adaptive, often it is the ground on which more complex social structures are built such as synchronization or division of labour (6). However, knowledge of the mechanisms implied in the formation of aggregates remains fragmentary. The study of the proximal causes, i.e. mechanisms involved in group formation, can benefit from concepts of self-organization (5, 7). These groups find their origin and their cohesion in the inter-attraction among individuals: group members are then the source of attraction. However, in most of the situations, patterns of aggregation, resulting from individual responses to conspecifics are modulated by environmental heterogeneity (5). Previous studies on cockroaches have already described their aggregative distribution in a natural environment where different age-classes share the resources that are present in their home range. They exhibit a strong tendency to gather during their resting period in safe shelters. Therefore, shelters are important, but also limited environmental resources for these insects. The basic mechanisms underlying group formation is the modulation of the individual resting time as a function of the number of conspecifics on a site. In insects cuticular hydrocarbons act as a recognition signal allowing attraction between individuals (8). Cockroaches prefer their own strain odour to another strain (9). Nevertheless, when groups in tests came from two different strains, they aggregated on one site only and did not show any difference from group coming from one strain. We used this insect as an example to show that a self-organized process leads to a diversity of optimal patterns without modification of the individual behaviours and any general knowledge of the available resources. These experimental and theoretical results point to a generic self-organized pattern-formation process independent of the level of animal sociability that should be found in other group-living organisms that present inter-attraction.