A basic expectation of evolution by natural selection is that species morphologies will adapt to their ecological niche. In social organisms, this may include selective pressure from the social environment. Many non-ant parasites of ant colonies are known to mimic the morphology of their host species, often in striking fashion [1, 2], indicating there is selection on parasite morphology to match the host (Batesian and/or Wasmannian mimicry [3]). However, ants that parasitize other ant societies are usually closely related to their hosts (Emery’s rule) [4, 5, 6, 7, 8] and expected to be similar due to common ancestry, making any kind of mimicry difficult to detect [9]. Here, we investigate the diversification of the hyperdiverse ant genus Pheidole in Madagascar, including the evolution of 13 putative social parasite species within a broader radiation of over 100 ant species on the island. We find that the parasitic species are monophyletic and that their associated hosts are spread across the Malagasy Pheidole radiation. This provides an opportunity to test for selection on morphological similarity and divergence between parasites and hosts. Using X-ray microtomography and both linear measurements and three-dimensional (3D) geometric morphometrics, we show that ant social parasite worker morphologies feature a mix of “host-matching” and “parasitic” traits, where the former converge on the host phenotype and the latter diverge from typical Pheidole phenotypes to match a common parasitic syndrome. This finding highlights the role of social context in shaping the evolution of phenotypes and raises questions about the role of morphological sensing in nestmate recognition.

自然选择进化的一个基本预期是物种形态将适应它们的生态位。在社会有机体中，这可能包括来自社会环境的选择压力。众所周知，蚁群的许多非蚂蚁寄生虫会模仿其宿主物种的形态，通常以惊人的方式 [1, 2]，表明存在与宿主相匹配的寄生虫形态选择（Batesian 和/或 Wasmannian 拟态 [3] ）。然而，寄生于其他蚂蚁社会的蚂蚁通常与它们的宿主密切相关（埃默里法则）[4, 5, 6, 7, 8] 并且由于共同的祖先而被预期是相似的，使得任何种类的模仿都难以被发现[9 ]。在这里，我们调查了高度多样化的蚂蚁属Pheidole的多样化在马达加斯加，包括岛上 100 多种蚂蚁物种在更广泛的辐射范围内的 13 种推定的社会寄生虫物种的进化。我们发现寄生物种是单系的，它们的相关宿主分布在马达加斯加Pheidole辐射中。这提供了一个机会来测试对寄生虫和宿主之间形态相似性和差异的选择。使用 X 射线显微断层扫描以及线性测量和三维 (3D) 几何形态测量学，我们表明蚂蚁社会寄生虫工作者的形态具有“宿主匹配”和“寄生”特征的混合特征，前者集中在宿主表型上后者不同于典型的Pheidole表型与常见的寄生虫综合征相匹配。这一发现突出了社会背景在塑造表型进化中的作用，并提出了关于形态感知在巢穴识别中的作用的问题。