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

Abstract

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.

Harsh environments promote alloparental care across human societies

“…Our results, therefore, provide evidence that human alloparenting is responsive to the effects of ecological conditions on both the costs and benefits of cooperation. While our findings are broadly consistent with previous comparative research linking cooperative breeding to harsh environments [36][37][38][39][40], the specific climatic effects identified across studies have been notably heterogeneous. For example, cooperative breeding is more likely in regions of high rather than low annual temperatures and precipitation among birds [39], in contrast to the findings of the present study as well as previous work linking cooperative breeding to lower temperatures and precipitation across other mammals [40].…”

“…[35]). However, alloparental care is also frequently observed among species inhabiting harsher environments [36][37][38][39][40]. This indicates that cooperative breeding can provide fitness benefits in less predictable and more extreme ecological contexts, where individuals face common challenges that prevent them from successfully surviving and reproducing alone.…”

High temperatures drive offspring mortality in a cooperatively breeding bird

“…An improved understanding of life history responses to current environmental variability is required to predict species-specific responses to climate change [4]. While cooperative breeders occur in diverse habitats [5,6], comparative research has demonstrated that both cooperatively-breeding birds [7] and mammals [8] occur with disproportionate frequency in regions characterised by high spatial and temporal variability in environmental conditions. This implies that group living enhances the ability to persist in challenging environments [9].…”

Cooperative breeding behaviors in the Hawaiian Stilt (Himantopus mexicanus knudseni)

“…Drivers of cooperative breeding are often paradoxical. For example, both benign and harsh, as well as stable and fluctuating, environments can favor the evolution of cooperative breeding behavior (Lin et al, 2019).…”