From individuals to groups to societies
Interactions of individuals at the group or population level have important implications for the transmission of genes, diseases and information. More and more studies show that properties of complex systems may emerge from simple and local interactions, from schools of fishes to migration of ungulates, apparent social roles, and even traffic flow in human crowds. However, how simple rules at the individual level may explain both group processes and population structure remains unclear. However, some simple dynamic rules, based on physiological needs and social relationships, seem to explain either collective movements in cohesive groups, fission-fusion dynamics or irreversible fusions and eventually group size distribution and population structure. Needs of individuals and social relationships may influence the frequency of fissions but fissions also act as a feedback loop on social relationships. According to the strength of their social relationships and the differences in their physiological needs, the members of a group can maintain a cohesive structure or having a fission-fusion dynamic. However, the group will irreversibly split if physiological needs of individuals cannot be satisfied, giving two new groups and affecting the population structure. Many future researches need to be done for understanding this dynamical system. Bridging the gap between the individual and the population level is crucial if we want to deeply understand how simple, local interactions may drive ecological systems.
Animal population structures and dynamics have important implications for the transmission of genes, diseases and information, all of which impact the maintenance of or changes in genetic or cultural diversity. Animals inside these populations might interact differently, from a solitary way of life to sociality. Gregariousness, whatever the degree of sociality, can confer important advantages to individuals such as decreasing risk of predation, allowing better food search efficiency or better access to reproductive opportunities. Such benefits typically require organisms to synchronize their activities in a more or less pronounced way and under circumstances where individuals differ in age, sex, body weight, or other factors that affect physiological requirements, this may result in individuals paying significant costs in order to maintain this group membership.
In some species individuals do maintain high social and spatial cohesion and a certain group stability, such as some primate species, wolves and some species of mongoose. These cohesive groups should decide where to go and them move collectively and synchronise their activities during all the day. However for many species the dynamic nature of the costs of living in groups, such as when needs, the environment or the size of the group changes, results in individuals adopting strategies that may result in groups splitting. Splitting, or fissioning, is typically described as a response to within-group competition for food, but may relate to circumstances where heterogeneity in needs occurs among group members. Differences of sex, size or reproductive status between individuals may lead them to different behaviours such as being with individuals of same motivations or adopting a specific moving speed. As a consequence the group split in different groups sharing similar physiological and/or behavioural status.
The dispersal of some group members and the group fission are some strategies which decrease the high cost of living in a group of specific composition. Fissioning is described as a response to within-group competition for food, whereas dispersion is more attributed to maintenance of genetic diversity and competition for reproduction. Fissioning might be temporary or irreversible when group competition or heterogeneity between animals becomes too important. Temporary fission, called fission-fusion society or fission-fusion dynamics, exist in mammals, fishes, birds and insects. In fission-fusion dynamics, the group composition and size change more or less frequently, according to heterogeneity in ecological conditions, species biology or the heterogeneity in individuals’ needs. Then, these strategies show that, in species where group members maintain relatively stable relationships, changes affecting individual behaviours (to disperse, to fission or to fuse) may lead to changes in the structure and clustering at the group and population level.
To understand the evolution of diversity in social dynamics, we need to determine which mechanisms underlie group formation and structure. Such population-scale properties of social life can be understood as emerging from local rules of interactions between individuals. A same rule of thumb could lead to two apparently different complex phenomena according to the social environment in which individuals interact (i.e. the social network of the group and its composition). This social environment may act as a filter through which different patterns might emerge and even if the rule of interactions is the same between the different patterns. Indeed, more and more studies show that the observable patterns in collective phenomena can emerge from observed local and simple interactions among the components of the system, i.e., the group members, whatever the species and its cognitive abilities.
Whilst the consequences of these behaviors on the population structure are very different, irreversible and temporary fissions of groups seem to be affected by a trade-off between food competition between members and perceived predation risk, and then similar rules might underlie these two different phenomena. When some individuals cannot satisfy their own needs, they leave the original group and form a new one, leading to irreversible fission, as long as membership of this new group does not jeopardize their survival probability by exposing them to increased perceived predation risk. However, group members can manage between satisfying their own needs and decreased predation risks by forming temporal sub-groups in fission-fusion dynamics. In social species, investment in social relationships also influences how individuals split: individuals who are related or are closed by their dominance rank have more likely to be together. Indeed, this kinship or dominance closeness lead to a lower risk of aggressions and injuries in the new sub-groups than in the original group or in groups composed of non relatives for instance [37]. These factors, access to resources and social relationships, are also described as affecting leadership and organization (i.e., order and associations) of individuals during collective movements in cohesive groups. However, no study tries to compare if the mechanisms underlying organization of individuals during group movements and during fission are similar. We need to explain how a same simple individual rule can explain both collective movements in cohesive groups and temporary or irreversible fissions, fissions that are then affecting the structure of the population.