Female social relationships are often shaped by the distribution of dietary resources. Socioecological models predict that females should form strict linear dominance hierarchies when resources are clumped and exhibit more egalitarian social structures when resources are evenly distributed. While many frugivores and omnivores indeed exhibit dominance hierarchies accompanied by differential resource access, many folivores deviate from the expected pattern and display dominance hierarchies despite evenly distributed resources. Among these outliers, geladas (Theropithecus gelada) present a conspicuous puzzle; females exhibit aggressive competition and strict dominance hierarchies despite feeding primarily on non-monopolizable grasses. However, these grasses become scarce in the dry season and geladas supplement their diet with underground storage organs that require relatively extensive energy to extract. We tested whether female dominance hierarchies provide differential access to underground storage organs by assessing how rank, season, and feeding context affect aggression in geladas under long-term study in the Simien Mountains National Park, Ethiopia. We found that the likelihood of receiving aggression was highest when feeding belowground and that the inverse relationship between rank and aggression was the most extreme while feeding belowground in the dry season. These results suggest that aggression in geladas revolves around belowground foods, which may mean that underground storage organs are an energetically central dietary component (despite being consumed less frequently than grasses), or that even “fallback” foods can influence feeding competition and social relationships. Further work should assess whether aggression in this context is directly associated with high-ranking usurpation of belowground foods from lower-ranking females following extraction.

中文翻译：

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主要食草灵长类动物的季节性开发资源的攻击率增加

女性的社会关系往往是由饮食资源的分配决定的。社会生态学模型预测，当资源聚集时，女性应该形成严格的线性统治等级制度；当资源均匀分配时，女性应该表现出更加平等的社会结构。虽然许多食果动物和杂食动物确实表现出优势等级，并伴随着不同的资源获取，但许多食叶动物却偏离了预期的模式，尽管资源分布均匀，但仍表现出优势等级。在这些异常值中，狒狒（Theropithecus gelada）呈现出一个明显的谜团。尽管雌性主要以不可垄断的草为食，但仍表现出积极的竞争和严格的统治等级。然而，这些草在旱季变得稀少，狒狒用地下储存器官来补充它们的饮食，而这些器官需要相对大量的能量来提取。我们在埃塞俄比亚塞门山国家公园进行了长期研究，通过评估等级、季节和进食环境如何影响狒狒的攻击性，测试了雌性统治等级是否提供了对地下储存器官的差异化访问。我们发现，在地下进食时受到攻击的可能性最高，并且在旱季在地下进食时，等级与攻击性之间的反比关系最为极端。这些结果表明，狒狒的攻击性主要围绕地下食物，这可能意味着地下储存器官是能量集中的饮食成分（尽管其消耗频率低于草），或者甚至“后备”食物也会影响进食竞争和社会关系。进一步的工作应该评估这种情况下的攻击行为是否与高级别的雌性在提取后从低级雌性那里夺取地下食物直接相关。