Social structure varies with elevation in an alpine ant

Insect societies vary greatly in social organization, yet the relative roles of ecological and genetic factors in driving this variation remain poorly understood. Identifying how social structure varies along environmental gradients can provide insights into the ecological conditions favouring alternative social organizations. Here, we investigate how queen number variation is distributed along elevation gradients within a socially polymorphic ant, the Alpine silver ant Formica selysi. We sampled low and high elevation populations in multiple Alpine valleys. We show that populations belonging to different drainage basins are genetically differentiated. In contrast, there is little genetic divergence between low and high elevation populations within the same drainage basin. Thus, elevation gradients in each of the drainage basins represent independent contrasts. Whatever the elevation, all well-sampled populations are socially polymorphic, containing both monogynous (= one queen) and polygynous (= multiple queen) colonies. However, the proportion of monogynous colonies per population increases at higher elevation, while the effective number of queens in polygynous colonies decreases, and this pattern is replicated in each drainage basin. The increased prevalence of colonies with a single queen at high elevation is correlated with summer and winter average temperature, but not with precipitation. The colder, unpredictable, and patchy environment encountered at higher elevations may favour larger queens with the ability to disperse and establish incipient monogynous colonies independently, while the stable and continuous habitat in the lowlands may favour large, fast-growing polygynous colonies. By highlighting differences in the environmental conditions favouring monogynous or polygynous colonies, this study sheds light on the ecological factors influencing the distribution and maintenance of social polymorphism.

昆虫社会的社会组织形式存在极大差异，但生态与遗传因素在塑造这种差异中各自发挥的相对作用仍不甚明晰。解析社会组织结构沿环境梯度的变化规律，可为探究偏好不同社会组织形式的生态条件提供关键洞见。本研究以具有社会多态性（socially polymorphic）的阿尔卑斯银蚁（Formica selysi）为研究对象，探究其蚁后数量变异沿海拔梯度的分布模式。我们在多个阿尔卑斯山谷中选取了低海拔与高海拔种群作为研究样本。研究结果显示，隶属于不同流域的种群在遗传上存在显著分化；与之相对，同一流域内的低海拔与高海拔种群之间遗传分化程度极低。因此，每个流域内的海拔梯度均可视为独立的对比研究体系。无论海拔高低，所有采样充分的种群均表现出社会多态性，同时兼具单后制（monogynous，即1只蚁后）与多后制（polygynous，即多只蚁后）的蚁群。不过，种群内单后制蚁群的占比随海拔升高而上升，而多后制蚁群的有效蚁后数量则随海拔升高下降，且这一规律在所有流域中均得到重复验证。高海拔区域单后制蚁群占比提升的现象，与夏季、冬季的平均气温显著相关，但与降水量无明显关联。高海拔区域环境更为寒冷、不可预测且生境斑块化，这或许更倾向于选择具备扩散能力、可独立建立初期单后制蚁群的大型蚁后；而低地生境稳定且连续，则更有利于大型、生长迅速的多后制蚁群存续。本研究揭示了偏好单后制或多后制蚁群的环境条件差异，为解析影响社会多态性分布与维持的生态因素提供了新的视角。