The dynamics of animal social structures are heavily influenced by environmental patterns of competition and cooperation. In folivorous colobine primates, prevailing theories suggest that larger group sizes should be favored in rainforests with a year-round abundance of food, thereby reducing feeding competition. Yet, paradoxically, larger groups are frequently found in high-altitude or high-latitude montane ecosystems characterized by a seasonal scarcity of leaves. This contradiction is posited to arise from cooperative benefits in heterogeneous environments. To investigate this hypothesis, we carried out a six-year field study on two neighboring groups of golden snub-nosed monkey ( Rhinopithecus roxellana), a species representing the northernmost distribution of colobine primates. Results showed that the groups adjusted their movement and habitat selection in response to fluctuating climates and spatiotemporal variability of resources, indicative of a dynamic foraging strategy. Notably, during the cold, resource-scarce conditions in winter, the large group occupied food-rich habitats but did not exhibit significantly longer daily travel distances than the smaller neighboring group. Subsequently, we compiled an eco-behavioral dataset of 52 colobine species to explore their evolutionary trajectories. Analysis of this dataset suggested that the increase in group size may have evolved via home range expansion in response to the cold and heterogeneous climates found at higher altitudes or latitudes. Hence, we developed a multi-benefits framework to interpret the formation of larger groups by integrating environmental heterogeneity. In cold and diverse environments, even smaller groups require larger home ranges to meet their dynamic survival needs. The spatiotemporal distribution of high-quality resources within these expanded home ranges facilitates more frequent interactions between groups, thereby encouraging social aggregation into larger groups. This process enhances the benefits of collaborative actions and reproductive opportunities, while simultaneously optimizing travel costs through a dynamic foraging strategy.
环境中动物的竞争与合作模式影响其社会形态。经典模型预测植食性灵长类在植物资源丰富的地区如热带雨林，食物竞争较小，倾向于形成大群。实际观察模式却显示大群常出现在植物季节性凋落的高海拔或高纬度山地生态系统。栖息地异质性假说认为该矛盾可能来源于异质性环境中的集体合作收益。为检验该假说，我们首先以植食性灵长类中分布最北的川金丝猴( Rhinopithecus roxellana)为对象，对两个不同规模的邻域猴群进行了连续6年的野外追踪。家域分析发现两个猴群均采取动态觅食策略，即根据季节性气候变化和时空异质的食物资源实时调整移动行为，选择食物丰富多样的高质量栖息地作为核心家域。群间比较显示，大群中有显著更多的性成熟个体，栖息于食物更丰富的斑块，且冬季未发生显著更长距离的觅食性移动。我们进一步构建含52个物种的疣猴亚科行为-生态数据集以确定演化路径。其中一种路径显示高海拔高纬度环境中的寒冷及季节性气候可促进家域面积增大，从而间接导致群体规模增加。因此，我们引入一个综合栖息地异质性假说的多重利益框架来解释金丝猴大型群体的形成。在寒冷、资源异质的环境中，即使小群也需要超大家域满足动态生存需求。在这样的超大家域中，高质量食物的时空特异性使得原始小群间相遇概率增加，集体行动可获得较高的食物和繁殖机会等多重收益，并且可通过动态觅食策略控制移动成本，从而促进社会聚集为大型群体。.
Keywords: Folivore paradox; MLS; Multi-benefits framework; Rhinopithecus roxellana; Social evolution.