Changes in group size during resource shifts reveal drivers of sociality across the tree of life

Abstract

14From biofilms to whale pods, organisms have repeatedly converged on sociality as a strategy to improve individual fitness. Yet, it remains challenging to identify the most important drivers-and by extension, the evolutionary mechanisms-of sociality for particular species. Here, we present a conceptual framework, literature review, and model demonstrating that the direction and magnitude of the response of group size to sudden resource shifts provides a strong indication of the underlying drivers of sociality. We catalog six functionally distinct mechanisms related to the acquisition of resources, and we model these mechanisms' effects on the survival of individuals foraging in groups. We find that whether, and to what degree, optimal group size increases, decreases, or remains constant when resource abundance declines depends strongly on the dominant mechanism. Existing empirical data support our model predictions, and we demonstrate how our framework can be used to predict the dominant social benefit for particular species. Together, our framework and results show that a single easily measurable characteristic, namely, group size under different resource abundances, can illuminate the potential drivers of sociality across the tree of life. 15

Ecogeography of group size suggests differences in drivers of sociality among cooperatively breeding fairywrens

“…The patterns we document here are consistent with recent theoretical studies demonstrating that environmental variation can have different effects on cooperatively breeding social groups, depending on the different forms and intensities of helping behaviour [20,56,57]. Shen et al [20] used insider-outsider conflict theory to show that cooperatively breeding groups which gain collective social benefits may perform better in harsh environments, while conflict will be exacerbated in groups with helpers which do not increase group productivity.…”

“…Similarly, García-Ruiz et al [57] modelled coevolution of dispersal and helping behaviour under different ecological scenarios to show that group benefits drive evolution of cooperative breeding in harsh environments but not benign environments. Finally, Kao et al [56] used simulation models to show that groups in which additional members increase the per capita resources available to the group through their contributions to resource acquisition will increase in size under resource scarcity. Collectively, these theoretical studies pose the hypothesis that the specific forms of benefits accrued by cooperative breeding group members influence how those groups respond to differences in environmental conditions owing to their effects on within-group social dynamics.…”

Consequences of Single-Locus and Tightly Linked Genomic Architectures for Evolutionary Responses to Environmental Change

“…The connection between social traits and linked architectures has broad relevance for predicting responses to environmental change, as sociality itself is often under environmental selection. For example, thermal stress and resource scarcity select for more or less sociality in different species and contexts ( Doering et al 2018 ; Kao et al 2020 ). Further, social traits are often correlated with phenotypes that might be under selection from anthropogenic stressors such as climate change and harvesting (e.g.…”

Ecogeography of group size suggests differences in drivers of sociality among cooperatively breeding fairywrens

“…Supporting this theory, cooperative species are more common under harsh environments in an avian lineage with intrinsic benefits while the opposite is true in a lineage where groups do not provide intrinsic benefits (Lin et al 2019). Similarly, Kao et al (2020) suggest that groups in which additional members increase the per-capita resources available to the group through their contributions to resource acquisition (similar to the 'collective action' groups of Shen et al 2017) will increase in size under resource scarcity. Broader examination of group size variation along environmental gradients in larger sets of cooperative breeding species may provide further opportunity for empirical tests of these theories.…”