Social fluidity mobilizes contagion in human and animal populations

Abstract

Humans and other group-living animals tend to distribute their social effort heterogeneously; individuals predominantly interact with their closest companions while maintaining weaker social bonds with less familiar group members. By incorporating this heterogeneity into a mathematical model we find that a single parameter, which we refer to as social fluidity, controls the level of social mixing in the population. Large values of social fluidity correspond to gregarious behavior whereas small values signify the existence of persistent bonds between individuals. To investigate how social behavior influences the likelihood of an epidemic outbreak we derive an analytical expression of the relationship between social fluidity and the basic reproductive number of an infectious disease. We compare social behavior across 12 species by applying the model to empirical human and animal social interaction data. For species that form strong social bonds, the model describes frequency-dependent transmission that is highly sensitive to changes in social fluidity. As social fluidity increases, animal-disease systems become increasingly density-dependent. Finally, based on a computational disease spread model on empirical social data, we find that social fluidity is a stronger predictor of disease outcomes than both group size and connectivity. S ocial behavior is fundamental to the survival of many species such as ants, humans, and dolphins. It allows the formation of social groups providing fitness advantages from greater access to resources and better protection from predators [1]. Within these groups structure emerges from the interactions that occur when individuals communicate across space, cooperate in sexual or parental behavior, or clash in territorial or mating conflicts [2]. While many animal societies have been studied independently [3], some questions about the nature of social living can only be answered by comparing social behavior across a range of species [4]. This motivates the question: how can we compare the social behavior of one species to that of another?Social animals typically have limited time and resources to invest in their relationships [5]. To compensate, the social effort of an individual tends to be distributed heterogeneously among the members of their group. Despite the growing evidence for this in human communication [6,7,8], attempts to quantify this aspect of sociality in animal systems are often challenged by unavoidable sampling biases [9]. Furthermore, while heterogeneous interaction frequencies and temporal dynamics such as circadian rhythms and bursty activity patterns have become common in social network models [10], realistic assumptions about how individuals distribute their social effort are rarely incorporated.When social interaction requires shared physical space it can also be a conduit for the transmission of infectious disease [11]. It is generally thought that if transmission occurs * ec975@georgetown.edu through the environment then the risk of epidemic is driven by group si...

Disease implications of animal social network structure: A synthesis across social systems

“…The reason why filtering out weak weighted edges removes most structural differences between social systems lies in their organization of weak ties. Individuals of social species disproportionately allocate effort among their social connections in order to maintain overall group connectivity (Figure S1) and are also known to have high social fluidity (Colman et al., ). Removing weak ties from networks of social species therefore increases variation in individual connectivity (degree heterogeneity), with a relatively minor decrease in their global connectivity (average betweenness centrality).…”

Disease implications of animal social network structure: a synthesis across social systems

“…Individuals of social species disproportionately allocate effort among their social connections in order to maintain overall group connectivity (Fig. S1, Supporting information) and are also known to have high social fluidity (Colman & Bansal, 2017). Re-moving weak ties from networks of social species therefore increases variation in individual connectivity (degree heterogeneity), with a relatively minor decrease in their global connectivity (average betweenness centrality).…”

Social Fluidity in Children's Face-to-Face Interaction Networks

“…A candidate measure to compare group behaviors is social fluidity, a measure that was recently introduced by Colman et al (2020) [2]. The authors state that social fluidity can be used to compare groups of human and non-human animals (also called systems) on how individuals distribute their attention amongst other individuals.…”

“…Colman et al [2] define social fluidity as "variability in the amount of social effort the individual invests in each member of their social group" and estimate it for more than 50 different animal systems, including several human face-toface interaction networks. Face-to-face interaction data can be represented as a temporal network, with people or animals as nodes and their interactions as edges.…”