Proximity, Sociality, and Observation: The Definition of Social Groups

Social fluidity mobilizes contagion in human and animal populations

Humans and other group-living animals tend to distribute their social effort disproportionately. Individuals predominantly interact with a small number of close companions while maintaining weaker social bonds with less familiar group members. By incorporating this behavior into a mathematical model, we find that a single parameter, which we refer to as social fluidity, controls the rate of social mixing within the group. Large values of social fluidity correspond to gregarious behavior, whereas small values signify the existence of persistent bonds between individuals. We compare the social fluidity of 13 species by applying the model to empirical human and animal social interaction data. 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. For species that form more stable social bonds, the model describes frequency-dependent transmission that is sensitive to changes in social fluidity. As social fluidity increases, animal-disease systems become increasingly density-dependent. Finally, we demonstrate that social fluidity is a stronger predictor of disease outcomes than both group size and connectivity, and it provides an integrated framework for both density-dependent and frequency-dependent transmission.

A time-based method for defining associations using photo-identification

Photo-identification is an invaluable method for documenting associations. Based on the assumption that individuals photographed close together in time are physically close in space, the metadata associated with digital photography offers an opportunity to base association analyses on time between images. This was tested via analysis of associations within a population of bottlenose dolphins (Tursiops truncatus) in Doubtful Sound, New Zealand. We compared the widely used group-membership method and an alternative time-based method. Overall social structures between methods were similar; high degrees of association among all individuals and little support for sub-groups. Results also indicated an increase in the precision of pairwise indices for the time-based method. This study validated the approach of using time as a basis for analyses of associations. Importantly, this method can be retrospectively applied to any photo-ID data set in which images of uniquely identifiable individuals are time-stamped by the camera.

Coevolution of neocortical size, group size and language in humans

Group size covaries with relative neocortical volume in nonhuman primates. This regression equation predicts a group size for modern humans very similar to that for hunter-gatherer and traditional horticulturalist societies. Similar group sizes are found in other contemporary and historical societies. Nonhuman primates maintain group cohesion through social grooming; among the Old World monkeys and apes, social grooming time is linearly related to group size. Maintaining stability of human-sized groups by grooming alone would make intolerable time demands. It is therefore suggested (1) that the evolution of large groups in the human lineage depended on developing a more efficient method for time-sharing the processes of social bonding and (2) that language uniquely fulfills this requirement. Data on the size of conversational and other small interacting groups of humans accord with the predicted relative efficiency of conversation compared to grooming as a bonding process. In human conversations about 60% of time is spent gossiping about relationships and personal experiences. Language may accordingly have evolved to allow individuals to learn about the behavioural characteristics of other group members more rapidly than was feasible by direct observation alone.

Distribution of node characteristics in complex networks

Our enhanced ability to map the structure of various complex networks is increasingly accompanied by the possibility of independently identifying the functional characteristics of each node. Although this led to the observation that nodes with similar characteristics have a tendency to link to each other, in general we lack the tools to quantify the interplay between node properties and the structure of the underlying network. Here we show that when nodes in a network belong to two distinct classes, two independent parameters are needed to capture the detailed interplay between the network structure and node properties. We find that the network structure significantly limits the values of these parameters, requiring a phase diagram to uniquely characterize the configurations available to the system. The phase diagram shows a remarkable independence from the network size, a finding that, together with a proposed heuristic algorithm, allows us to determine its shape even for large networks. To test the usefulness of the developed methods, we apply them to biological and socioeconomic systems, finding that protein functions and mobile phone usage occupy distinct regions of the phase diagram, indicating that the proposed parameters have a strong discriminating power.

The social structure and strategies of delphinids: predictions based on an ecological framework

Dolphins live in complex social groupings with a wide variety of social strategies. In this chapter we investigate the role that differing habitats and ecological conditions have played in the evolution of delphinid social strategies. We propose a conceptual framework for understanding natural patterns of delphinid social structure in which the spatial and temporal predictability of resources influences the ranging patterns of individuals and communities. The framework predicts that when resources are spatially and temporally predictable, dolphins should remain resident in relatively small areas. Predictable resources are often found in complex inshore environments where dolphins may hide from predators or avoid areas with high predator density. Additionally, available food resources may limit group size. Thus, we predict that there are few benefits to forming large groups and potentially many benefits to being solitary or in small groups. Males may be able to sequester solitary females, controlling mating opportunities. Observations of inshore populations of bottlenose dolphins (Tursiops sp.) and island-associated spinner dolphins (Stenella longirostris) seem to fit this pattern well, along with forest-dwelling African antelope and primates such as vervets (Cercopithicus aethiops), baboons (Papio sp.), macaques (Macaca sp.) and chimpanzees (Pan troglodytes). In contrast, the framework predicts that when resources such as food are unpredictable, individuals must range further to find the necessary resources. Forming groups may be the only strategy available to avoid predation, especially in the open ocean. Larger home ranges are likely to support a greater number of individuals; however, prey is often sparsely distributed, which may act to reduce foraging competition. Cooperative foraging and herding of prey schools may be advantageous, potentially facilitating the formation of long-term bonds. Alternately, individuals may display many short-term affiliations. These large groups make it difficult for a male or a small group of males to sequester a female, and polygynandry is the most likely mating strategy. While it is difficult to study wide-ranging delphinids to examine these predictions, this ranging and behavioural pattern has been suggested for dusky dolphins (Lagenorhynchus obscurus), coastal bottlenose dolphins (Tursiops sp.) and mixed species of dolphins in the Eastern Tropical Pacific. These patterns also resemble the ranging and social strategies of open savannah African antelopes and desert-dwelling macropods. Resource availability exists in a range of complex distributions and we predict that delphinid ranging patterns will also vary. At intermediate-ranging patterns, the framework predicts that individuals should form mid-sized groups balancing intra-group competition with predation protection. Humpback dolphins (Sousa sp.) appear to fit this pattern, with some site fidelity over relatively large ranges. They display fluid associations with other individuals. Predation pressure is not sufficiently high to cause large groups to form, and individuals probably reduce predation pressure more by hiding whenever possible. This pattern is likely to prevent the formation of long-term complex bonds. In contrast, killer whales (Orcinus orca) also display intermediate-ranging patterns, but have extremely strong social bonds within familial groups. Cooperative and altruistic behaviour in killer whales facilitate the formation of life-long bonds, similar to those observations in sperm whales (Physeter macrocephalus) and elephants (Loxodonta africana). This conceptual framework remains largely untested, and for many species it is not currently possible to describe ranging behaviours, anti-predator tactics or social behaviour in sufficient detail for appropriate examination of these ideas. Few studies on dolphins have been conducted to explicitly test this type of framework; however, existing observations of delphinid social strategies and communities are used throughout this chapter to examine this framework. Additionally, we anticipate that the present framework may provide a starting point to test hypotheses regarding the evolution of social strategies of delphinids.

A method for testing association patterns of social animals

Association indices were originally developed to describe species co-occurrences, but have been used increasingly to measure associations between individuals. However, no statistical method has been published that allows one to test the extent to which the observed association index values differ from those of a randomly associating population. Here, we describe an adaptation of a test developed by Manly (1995, Ecology, 76, 1109-1115), which uses the observed association data as a basis for a computer-generated randomization. The observed pattern of association is tested against a randomly created one while retaining important features of the original data, for example group size and sighting frequency. We applied this new method to test four data sets of associations from two populations of Hector's dolphin, Cephalorhynchus hectori, using the Half-Weight Index (HWI) as an example of a measure of association. The test demonstrated that populations with similar median HWI values showed clear differences in association patterns, that is, some were associating nonrandomly whereas others were not. These results highlight the benefits of using this new testing method in order to validate the analysis of association indices. Copyright 1998 The Association for the Study of Animal Behaviour.