Mate choice is a taxonomically wide-spread phenomenon, mostly exerted by females although male mate choice occurs as well. While costs and benefits of choosiness have been well studied, the underlying mechanisms are largely unclear. Different models exist, namely best-of-n or threshold rules, based on sequential or simultaneous sampling, which differ in the required cognitive demands. We applied an experimental approach to shed light on the underlying mechanisms of male mate choice in the sexually cannibalistic spider Argiope bruennichi. Males are limited to two copulations and preferentially monopolise large females, while they may leave smaller females after a single copulation and resume mate search. Here, we utilised significant size-differences between females from Northern and Southern populations and presented males with three different-sized females that were matched for origin: all three females originated either from the same Northern European population as the males or from Southern populations where the smallest female was about the same size as the largest Northern female. This allowed testing the hypothesis that males base their mating tactic on a fixed local size threshold. We predicted Northern males to be choosy among Northern females, but to accept all Southern females since they would all be above that threshold. Males copulated with the first female they encountered, which was independent of her body size. Regardless of the females’ origins, males chose a monogynous tactic with the largest female in the trio, while they left the smallest female after one copulation. The same pattern applied to Southern females even though the smallest females in the trio were of a similar size as monopolised Northern females. Since males have poor eyesight and did not actively sample all females, they likely have gained information about relative size differences between females based on volatile chemical cues only. Our findings suggest that male A. bruennichi can assess relative differences in mate quality and adjust their mating tactic to the prevailing conditions (Northern vs. Southern). We reject the presence of a locally-adapted fixed threshold and argue that our results are best explained by an adjustable threshold that was raised under Southern conditions.

中文翻译：

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性食人蜘蛛的雄性化学评估雌性的相对质量。

配偶选择是一种分类学上普遍存在的现象，尽管男性也可以选择配偶，但大多数情况下是由女性施加的。虽然选择成本和收益已得到充分研究，但其潜在机制在很大程度上尚不清楚。存在基于连续或同时采样的不同模型，即n最佳或阈值规则，它们在所需的认知需求方面有所不同。我们应用了一种实验方法来揭示性食人蜘蛛Argiope bruennichi中男性伴侣选择的潜在机制。雄性只限于两次交配，并优先垄断大型雌性，而一次交配后，它们可能会留下较小的雌性，并继续寻找伴侣。这里，我们利用了北方和南方种群中雌性之间的显着大小差异，并为雄性提供了三只不同大小的雌性，这些雌性的来源相匹配：所有三名雌性都源自与雄性相同的北欧人口，或者起源于最小的雌性的南方种群与最大的北方雌性大小相同。这允许检验以下假设：雄性的交配策略基于固定的局部大小阈值。我们预测北方男性在北方女性中比较挑剔，但要接受所有南方女性，因为她们都将高于该阈值。雄性与他们遇到的第一个雌性交配，这与她的体型无关。不论女性的出身如何，男性都选择了单挑的策略，三人组合中最大的女性，而一次交配后，她们离开了最小的雌性。即使三人组中最小的雌性与垄断的北方雌性相近，同样的模式也适用于南方雌性。由于雄性的视力较差，并且没有对所有雌性进行积极采样，因此他们可能仅根据挥发性化学提示获得了雌性之间相对大小差异的信息。我们的研究结果表明，雄性布鲁氏杆菌可以评估配偶质量的相对差异，并根据流行条件（北方与南方）调整其交配策略。我们拒绝存在适应当地情况的固定阈值，并认为通过南方条件下提高的可调阈值可以最好地解释我们的结果。即使三人组中最小的雌性与垄断的北方雌性相近，同样的模式也适用于南方雌性。由于雄性的视力较差，并且没有对所有雌性进行积极采样，因此他们可能仅根据挥发性化学提示获得了雌性之间相对大小差异的信息。我们的研究结果表明，雄性布鲁氏杆菌可以评估配偶质量的相对差异，并根据流行条件（北方与南方）调整其交配策略。我们拒绝存在适应当地情况的固定阈值，并认为通过南方条件下提高的可调阈值可以最好地解释我们的结果。即使三人组中最小的雌性与垄断的北方雌性相近，同样的模式也适用于南方雌性。由于雄性的视力较差，并且没有对所有雌性进行积极采样，因此他们可能仅根据挥发性化学提示获得了雌性之间相对大小差异的信息。我们的研究结果表明，雄性布鲁氏杆菌可以评估配偶质量的相对差异，并根据流行条件（北方与南方）调整其交配策略。我们拒绝存在适应当地情况的固定阈值，并认为通过南方条件下提高的可调阈值可以最好地解释我们的结果。他们可能仅根据挥发性化学线索获得了关于雌性之间相对大小差异的信息。我们的研究结果表明，雄性布鲁氏杆菌可以评估配偶质量的相对差异，并根据流行条件（北方与南方）调整其交配策略。我们拒绝存在适应当地情况的固定阈值，并认为通过南方条件下提高的可调阈值可以最好地解释我们的结果。他们可能仅根据挥发性化学线索获得了关于雌性之间相对大小差异的信息。我们的研究结果表明，雄性布鲁氏杆菌可以评估配偶质量的相对差异，并根据流行条件（北方与南方）调整其交配策略。我们拒绝存在适应当地情况的固定阈值，并认为通过南方条件下提高的可调阈值可以最好地解释我们的结果。