Dynamic foraging strategy adapted to heterogeneous environment contributes to social aggregation in snub-nosed monkeys

Patterns of competition and cooperation in environment affect animal societies. In folivorous colobine primates, group size is predicted to increase in year-round food abundant rainforests due to less feeding competition. However, the observed pattern shows larger groups in seasonally leaf-deprived high-altitude/latitude montane ecosystems. This paradox is hypothesized to arise from cooperative benefits in heterogeneous environments. To test this, we first performed 6-year fieldwork on two neighboring groups of the golden snub-nosed monkey (Rhinopithecus roxellana), which is the northernmost distributed colobine primate. We found the group adjusted movement and selected high-quality habitats in response to fluctuating climate and spatial-temporally heterogeneous resources indicating a dynamic foraging strategy. In winter with cold temperatures and scarce resources, the large group inhabited food-rich habitats but didn't show significantly longer daily travel distances than its neighboring small group. We then compiled an eco-behavioral dataset of 52 colobine species to determine the evolutionary path. One path suggests that an increase in group size may have resulted from an enlargement of the home range in response to the cold and heterogeneous climates in high altitudes or latitudes. Hence, we introduce a multi-benefits framework to interpret large group formation integrating the environment heterogeneity. In cold, heterogeneous environments, even small groups require extra-large home ranges for dynamic survival needs. The spatiotemporal specificity of high-quality resources within the enlarged home range allows for frequent group encounters, which promotes social aggregation into larger groups by increasing the benefits of collective action and reproduction while constraining travel costs through a dynamic foraging strategy.