The extent of male mate choosiness is driven by a trade-off between various environmental factors associated with the costs of mate acquisition, quality assessment and opportunity costs. Our knowledge about natural variation in male mate choosiness across different populations of the same species, however, remains limited. In this study, we compared male mate choosiness across 10 natural populations of the freshwater amphipod Gammarus roeselii (Gervais 1835), a species with overall high male mating investments, and evaluated the relative influence of population density and sex ratio (both affecting mate availability) on male mate choosiness. We investigated amplexus establishment after separating mating pairs and presenting focal males with a novel, size-matched female from the same population. Our analysis revealed considerable effects of sex ratio and (to a lesser extent) population density on time until amplexus establishment (choosiness). Male amphipods are able to perceive variable social conditions (e.g., sex ratio) and modify their mating strategy accordingly: We found choosiness to be reduced in increasingly male-biased populations, whereas selectivity increases when sex ratio becomes female biased. With this, our study expands our limited knowledge on natural variations in male mate choosiness and illustrates the importance of sex ratio (i.e., level of competition) for male mating decisions in natural environments. Accounting for variation in sex ratios, therefore, allows envisioning a distinctive variation of choosiness in natural populations and highlights the importance of considering social background information in future behavioral studies.

中文翻译：

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社会条件的自然变化影响片脚类动物 Gammarus roeselii 的雄性择偶

男性择偶的程度是由与获得配偶成本、质量评估和机会成本相关的各种环境因素之间的权衡驱动的。然而，我们对同一物种不同种群的雄性择偶自然变异的了解仍然有限。在这项研究中，我们比较了淡水片脚类动物 Gammarus roeselii (Gervais 1835) 的 10 个自然种群的雄性交配选择，这是一种雄性交配投资总体较高的物种，并评估了种群密度和性别比的相对影响（两者都会影响交配可用性）关于男性择偶。我们在分离交配对并将焦点雄性与来自同一种群的新的、大小匹配的雌性呈现后调查了丛的建立。我们的分析揭示了性别比例和（在较小程度上）人口密度对时间的相当大的影响，直到复合体建立（选择）。雄性端足类动物能够感知可变的社会条件（例如，性别比例）并相应地修改它们的交配策略：我们发现，在越来越偏向男性的人群中，选择性会减少，而当性别比例变得偏向女性时，选择性会增加。有了这个，我们的研究扩展了我们对男性择偶自然变化的有限知识，并说明了性别比例（即竞争水平）对于自然环境中男性交配决策的重要性。因此，考虑到性别比例的变化，