Mixed-species groups describe active associations among individuals of 2 or more species at the same trophic level. Mixed-species groups are important to key ecological and evolutionary processes such as competition and predation, and research that ignores the presence of other species risks ignoring a key aspect of the environment in which social behavior is expressed and selected. Despite the defining emphasis of active formation for mixed-species groups, surprisingly little is known about the mechanisms by which mixed-species groups form. Furthermore, insects have been almost completely ignored in the study of mixed-species groups, despite their taxonomic importance and relative prominence in the study of single-species groups. Here, we measured group formation processes in Drosophila melanogaster and its sister species, Drosophila simulans. Each species was studied alone, and together, and one population of D. melanogaster was also studied both alone and with another, phenotypically distinct D. melanogaster population, in a nested-factorial design. This approach differs from typical methods of studying mixed-species groups in that we could quantitatively compare group formation between single-population, mixed-population, and mixed-species treatments. Surprisingly, we found no differences between treatments in the number, size, or composition of groups that formed, suggesting that single- and mixed-species groups form through similar mechanisms of active attraction. However, we found that mixed-species groups showed elevated interspecies male–male interactions, relative to interpopulation or intergenotype interactions in single-species groups. Our findings expand the conceptual and taxonomic study of mixed-species groups while raising new questions about the mechanisms of group formation broadly.

中文翻译：

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比较果蝇中的单一物种和混合物种群体：群体动力学的差异，但不是群体形成

混合物种群描述了同一营养级别的 2 个或更多物种的个体之间的活跃关联。混合物种群体对于关键的生态和进化过程（如竞争和捕食）很重要，而忽视其他物种存在的研究可能会忽视表达和选择社会行为的环境的一个关键方面。尽管混合物种群体的活跃形成具有明确的重点，但令人惊讶的是，人们对混合物种群体形成的机制知之甚少。此外，尽管昆虫在分类学上的重要性和在单一物种群的研究中相对突出，但在混合物种群的研究中几乎完全被忽略了。在这里，我们测量了黑腹果蝇及其姐妹物种模拟果蝇的群体形成过程。每个物种都被单独研究，并且一起研究，并且在嵌套因子设计中，一个黑腹果蝇种群也被单独研究和与另一个表型不同的黑腹果蝇种群一起研究。这种方法不同于研究混合物种群体的典型方法，因为我们可以定量比较单一群体、混合群体和混合物种处理之间的群体形成。令人惊讶的是，我们发现处理组的数量、大小或组成的处理之间没有差异，这表明单一物种和混合物种的群体是通过类似的主动吸引机制形成的。然而，我们发现混合物种群体表现出更高的种间雄性 - 雄性相互作用，相对于单一物种群体的种群间或基因型间相互作用。