Group augmentation underlies the evolution of complex sociality in the face of environmental instability

Although kin selection is assumed to underlie the evolution of sociality, many vertebrates-including nearly half of all cooperatively breeding birds-form groups that also include unrelated individuals. Theory predicts that despite reducing kin structure, immigration of unrelated individuals into groups can provide direct, group augmentation benefits, particularly when offspring recruitment is insufficient for group persistence. Using population dynamic modeling and analysis of long-term data, we provide clear empirical evidence of group augmentation benefits favoring the evolution and maintenance of complex societies with low kin structure and multiple reproductives. We show that in the superb starling (Lamprotornis superbus)-a plural cooperative breeder that forms large groups with multiple breeding pairs, and related and unrelated nonbreeders of both sexes-offspring recruitment alone cannot prevent group extinction, especially in smaller groups. Further, smaller groups, which stand to benefit more from immigration, exhibit lower reproductive skew for immigrants, suggesting that reproductive opportunities as joining incentives lead to plural breeding. Yet, despite a greater likelihood of becoming a breeder in smaller groups, immigrants are more likely to join larger groups where they experience increased survivorship and greater reproductive success as breeders. Moreover, immigrants form additional breeding pairs, increasing future offspring recruitment into the group and guarding against complete reproductive failure in the face of environmental instability and high nest predation. Thus, plural breeding likely evolves because the benefits of group augmentation by immigrants generate a positive feedback loop that maintains societies with low and mixed kinship, large group sizes, and multiple reproductives.

Environmental quality mediates the ecological dominance of cooperatively breeding birds

Although social species as diverse as humans and ants are among the most abundant organisms on Earth, animals cooperate and form groups for many reasons. How these different reasons for grouping affect a species' ecological dominance remains unknown. Here we use a theoretical model to demonstrate that the different fitness benefits that animals receive by forming groups depend on the quality of their environment, which in turn impacts their ecological dominance and resilience to global change. We then test the model's key predictions using phylogenetic comparative analysis of >6500 bird species. As predicted, we find that cooperative breeders occurring in harsh and fluctuating environments have larger ranges and greater abundances than non-cooperative breeders, but cooperative breeders occurring in benign and stable environments do not. Using our model, we further show that social species living in harsh and fluctuating environments will be less vulnerable to climate change than non-social species.

Demographic inference provides insights into the extirpation and ecological dominance of eusocial snapping shrimps

Although eusocial animals often achieve ecological dominance in the ecosystems where they occur, many populations are unstable, resulting in local extinction. Both patterns may be linked to the characteristic demography of eusocial species-high reproductive skew and reproductive division of labor support stable effective population sizes that make eusocial groups more competitive in some species, but also lower effective population sizes that increase susceptibility to population collapse in others. Here, we examine the relationship between demography and social organization in Synalpheus snapping shrimps, a group in which eusociality has evolved recently and repeatedly. We show using coalescent demographic modelling that eusocial species have had lower but more stable effective population sizes across 100,000 generations. Our results are consistent with the idea that stable population sizes may enable competitive dominance in eusocial shrimps, but they also suggest that recent population declines are likely caused by eusocial shrimps' heightened sensitivity to environmental changes, perhaps as a result of their low effective population sizes and localized dispersal. Thus, although the unique life histories and demography of eusocial shrimps have likely contributed to their persistence and ecological dominance over evolutionary timescales, these social traits may also make them vulnerable to contemporary environmental change.

Ten recent insights for our understanding of cooperation

Since Hamilton published his seminal papers in 1964, our understanding of the importance of cooperation for life on Earth has evolved beyond recognition. Early research was focused on altruism in the social insects, where the problem of cooperation was easy to see. In more recent years, research into cooperation has expanded across the entire tree of life, and has been revolutionized by advances in genetic, microbiological and analytical techniques. We highlight ten insights that have arisen from these advances, which have illuminated generalizations across different taxa, making the world simpler to explain. Furthermore, progress in these areas has opened up numerous new problems to solve, suggesting exciting directions for future research.

Social transitions in sponge-dwelling snapping shrimp

Sociality is exceedingly rare in the marine environment, with true eusociality found only within a single genus of sponge-dwelling snapping shrimp. This genus is socially diverse and exhibits multiple independent evolutionary origins of both eusociality and communal breeding from pair-forming ancestors. Ecology was critical to the evolution of shrimp sociality, as the transition from host specialization to generalism preceded the evolution of eusociality, and the transition from small to large host sponges favored the evolution of communal breeding. Moreover, a change in life history from planktonic to non-dispersing, crawling larvae only occurred in eusocial species. Here, we present a hypothesis describing the evolutionary transitions toward sociality in shrimp that serves to illustrate how ecology and life history interact to shape social evolution more broadly.

Resolving the Paradox of Environmental Quality and Sociality: The Ecological Causes and Consequences of Cooperative Breeding in Two Lineages of Birds

Cooperatively breeding animals occur in virtually every ecosystem on earth. Comparative and biogeographic studies suggest that both benign and harsh-as well as stable and fluctuating-environments can favor the evolution of cooperative breeding behavior. The fact that cooperative societies occur in environments of such contrasting quality creates a paradox of environmental quality and sociality. The dual benefits framework-which leads to the prediction that the ecological consequences of sociality (e.g., range size) vary depending on the benefits that individuals of each species receive by forming social groups-offers a potential resolution to this paradox. Here we use a case study of two avian lineages, starlings (Sturnidae) and hornbills (Bucerotidae), in which environmental unpredictability appears to have opposite effects on the evolution of cooperation to test the dual benefits framework. Consistent with previous work, harsh and unpredictable environments promote cooperative breeding behavior in starlings, which in turn leads to larger geographic ranges. However, cooperatively breeding hornbills occur in benign and stable environments, but sociality does not influence range size. Our study suggests that the paradox of environmental quality and sociality arises largely because cooperative breeding is an umbrella term encompassing social species that form groups for different reasons. We demonstrate that differentiating among the functional causes of social group formation is critical for developing a predictive framework for understanding the evolution of cooperative breeding behavior.

Drivers of sociality in Gobiodon fishes: An assessment of phylogeny, ecology and life-history

What drives the evolution of sociality in animals? Many robust studies in terrestrial organisms have pointed toward various kinship-based, ecological and life-history traits or phylogenetic constraint which have played a role in the evolution of sociality. These traits are not mutually exclusive and the exact combination of traits is likely taxon-specific. Phylogenetic comparative analyses have been instrumental in identifying social lineages and comparing various traits with non-social lineages to give broad evolutionary perspectives on the development of sociality. Few studies have attempted this approach in marine vertebrate systems. Social marine fishes are particularly interesting because many have a pelagic larval phase and non-conventional life-history strategies (e.g. bi-directional sex-change) not often observed in terrestrial animals. Such strategies provide novel insights into terrestrially-derived theories of social evolution. Here, we assess the strength of the phylogenetic signal of sociality in the Gobiodon genus with Pagel's lambda and Blomberg's K parameters. We found some evidence of a phylogenetic signal of sociality, but factors other than phylogenetic constraint also have a strong influence on the extant social state of each species. We then use phylogenetic generalized least squares analyses to examine several ecological and life-history traits that may have influenced the evolution of sociality in the genus. We found an interaction of habitat size and fish length was the strongest predictor of sociality. Sociality in larger species was more dependent on coral size than in smaller species, but smaller species were more social overall, regardless of coral size. Finally, we comment on findings regarding the validity of the species G. spilophthalmus which arose during the course of our research. These findings in a group of marine fishes add a unique perspective on the evolution of sociality to the excellent terrestrial work conducted in this field.