Data from: Experimental evidence that density mediates negative frequency-dependent selection on aggression

1. Aggression can be beneficial in competitive environments if aggressive individuals are more likely to access resources than non-aggressive individuals. However, variation in aggressive behaviour persists within populations, suggesting that high levels of aggression might not always be favoured. 2. The goal of this study was to experimentally assess the effects of population density and phenotypic frequency on selection on aggression in a competitive environment. 3. We compared survival of two strains of Drosophila melanogaster that differ in aggression across three density treatments and five frequency treatments (single strain groups, equal numbers of each strain, and strains mixed at 3:1 and 1:3 ratios) during a period of limited resources. 4. While there was no difference in survival across single-strain treatments, survival was strongly density-dependent, with declining survival as density increased. Furthermore, at medium and high densities, there was evidence of negative frequency-dependent selection, where rare strains experienced greater survival than common strains. However, there was no evidence of negative frequency-dependent selection at low density. 5. Our results indicate that the benefits of aggression during periods of limited resources can depend on the interaction between the phenotypic composition of populations and population density, both of which are mechanisms that could maintain variation in aggressive behaviours within natural populations.

1. 在竞争环境中，若攻击性个体相较于非攻击性个体更易获取资源，则攻击行为可带来适应性收益。然而种群内部始终存在攻击行为的表型变异，这提示高攻击性并非总能受到自然选择的青睐。
2. 本研究旨在通过实验，评估竞争环境下种群密度与表型频率对攻击行为相关选择的调控效应。
3. 本研究在资源受限的实验周期内，对比了两种攻击行为存在差异的黑腹果蝇（Drosophila melanogaster）品系，在3种密度处理与5种频率处理（单一品系组、两品系数量均等组以及品系以3:1与1:3比例混合组）下的存活率差异。
4. 尽管各单一品系处理组间的存活率无显著差异，但存活率整体表现出强烈的密度依赖性：随种群密度升高，存活率逐步降低。进一步分析显示，在中、高密度处理条件下，存在负频率依赖选择（negative frequency-dependent selection）的证据——稀有品系的存活率显著高于常见品系；但在低密度处理组中未观察到该选择效应。
5. 本研究结果表明，资源受限时期攻击行为的适应性收益，取决于种群表型组成与种群密度之间的交互作用；而这两个因素均为维持自然种群内攻击行为表型变异的潜在机制。