Measuring opportunity for sociality: quantifying social stability in a colonially breeding phocid

The measurement of sociality has been primarily concerned either with the quantification of pairwise associations between individuals or using network approaches to describe multiple relationships between individuals in a group. However, these approaches may not be appropriate or necessary for all studies, for instance, where group dynamics are more biologically relevant than individual associations or interactions. Such a case occurs when the changes to a number of different social groups are to be compared over varying temporal and spatial scales. Methods for assessing comparative group dynamics have been conspicuously lacking from the biologist's toolbox. Here, we investigated the suitability of indexes derived from quantification of diversity as methods for direct quantification of changes in social group composition. First, we specified the requirements of a suitable index for social stability, then we reviewed indexes available for such a task, examining their suitability, through tests of their mathematical properties, in this new context. Finally, we used the specific model of the breeding female grey seal, Halichoerus grypus, on North Rona, Scotland to conclude that the Jaccard index is the most suitable tool for quantifying social stability in this system. We advocated that this approach can offer advantages in any study where social stability rather than any specific interaction is the principal focus.