Most experimental tests of mating systems theory have been conducted in the laboratory, using operational sex ratios (ratio of ready-to-mate male to ready-to-mate female) that are often not representative of natural conditions. Here, we first measured the range of adult sex ratio (proportion of adult males to adult females; ASR) in two populations of Trinidadian guppies (Poecilia reticulata) differing in ambient predation risk (high vs. low). We then explored, under semi-wild conditions, the effect of ASR (i.e., 0.17, 0.50, 0.83) on mating competition patterns in these populations. ASR in the wild was female-biased and did not significantly differ between the two populations. The range of ASR in our experiment was representative of natural ASRs. As expected, we observed an increase in intrasexual aggression rates in both sexes as the relative abundance of competitors increased. In support of the risky competition hypothesis, all measured behaviors had lower rates in a high versus low predation-risk population, likely due to the costs of predation. In terms of mating tactics, a male-biased ASR did not lead males to favor forced mating over courtship, indicating that males did not compensate for the cost of competition by switching to a less costly alternative mating tactic. Overall, this study highlights the need for field experiments using natural ranges of ASRs to test the validity of mating systems theory in a more complex, ecologically relevant context.

中文翻译：

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野生特立尼达孔雀鱼的交配竞争和成年性别比

大多数交配系统理论的实验测试都是在实验室中进行的，使用通常不代表自然条件的可操作性别比（准备交配的雄性与准备交配的雌性的比率）。在这里，我们首先测量了环境捕食风险（高与低）不同的两个特立尼达孔雀鱼（Poecilia reticulata）种群的成年性别比范围（成年男性与成年女性的比例；ASR）。然后，我们在半野生条件下探索了 ASR（即 0.17、0.50、0.83）对这些种群交配竞争模式的影响。野外的 ASR 偏向于女性，两个种群之间没有显着差异。我们实验中的 ASR 范围是自然 ASR 的代表。正如预期的那样，我们观察到随着竞争者的相对丰富程度的增加，两性的性内攻击率都有所增加。为了支持风险竞争假设，所有测量的行为在高捕食风险人群和低捕食风险人群中的比率都较低，这可能是由于捕食的成本。在交配策略方面，偏向雄性的 ASR 并没有导致雄性更喜欢强迫交配而不是求爱，这表明雄性并没有通过转向成本较低的替代交配策略来补偿竞争成本。总体而言，这项研究强调了使用自然范围的 ASR 进行现场实验的必要性，以在更复杂、生态相关的环境中测试交配系统理论的有效性。所有测量的行为在高捕食风险人群和低捕食风险人群中的比率都较低，这可能是由于捕食的成本。在交配策略方面，偏向雄性的 ASR 并没有导致雄性更喜欢强迫交配而不是求爱，这表明雄性并没有通过转向成本较低的替代交配策略来补偿竞争成本。总体而言，这项研究强调了使用自然范围的 ASR 进行现场实验的必要性，以在更复杂、生态相关的环境中测试交配系统理论的有效性。所有测量的行为在高捕食风险人群和低捕食风险人群中的比率都较低，这可能是由于捕食的成本。在交配策略方面，偏向雄性的 ASR 并没有导致雄性更喜欢强迫交配而不是求爱，这表明雄性并没有通过转向成本较低的替代交配策略来补偿竞争成本。总体而言，这项研究强调了使用自然范围的 ASR 进行现场实验的必要性，以在更复杂、生态相关的环境中测试交配系统理论的有效性。