Data from: 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，但学界对生态与遗传因素在塑造这种差异中的相对作用仍知之甚少。明确社会组织结构沿环境梯度的变化规律，可为解析偏好不同社会组织形式的生态条件提供研究视角。本研究以具有社会多态性的阿尔卑斯银蚁（Formica selysi）为研究对象，探究其种群内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低海拔与高海拔种群开展采样。研究发现，隶属于不同流域的种群存在遗传分化；而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作为独立的对比研究对象。无论海拔高低，所有采样充分的种群均表现出社会多态性，同时兼具单蚁后（monogynous，即1只蚁后）与多蚁后（polygynous，即多只蚁后）两种社群类型。但种群内单蚁后社群的占比随海拔升高而上升，而多蚁后社群的有效蚁后数量则随海拔升高而下降，且这一规律在所有流域中均一致存在。高海拔区域单蚁后社群占比的提升，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、不可预测且生境破碎化，这可能更青睐具备扩散能力并可独立建立初始单蚁后社群的大型蚁后；而低海拔区域生境稳定且连续，则更适合体型大、社群增长迅速的多蚁后社群。本研究明确了偏好单蚁后或多蚁后社群的环境条件差异，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见解。