Data from: The rarity of size-assortative mating in animals: assessing the evidence with anuran amphibians

Assortative mating in animals can have substantial evolutionary impact. Numerous reports also make it appear to be pervasive in occurrence. In assortative mating, defined here in behavioral terms, animals select their mates according to a particular shared trait such that mated individuals resemble each other phenotypically more than expected by chance. Body size is a widely studied assortment trait. This is especially relevant for anuran amphibians (frogs and toads), among whom reproductive advantages may accrue to large individuals of both sexes. Anurans also exhibit discrete forms of male mating behavior. Sedentary calling behavior of “sitters” allows for female choice whereas fighting for possession of mates by “scramblers” precludes female choice. Size-assortative mating in anurans, therefore, should be a property of sitters, not scramblers. I used meta-analysis to assess the occurrence of true size-assortative mating in relation to mating behaviour and other variables in 282 studies of 68 species of anurans. I found publication bias against reporting non-significant results and analytical bias resulting from pooling of samples collected at different times or places (Simpson’s Paradox). Pooled samples significantly inflated the apparent occurrence and strength of size-assortative mating. Controlling for such biases left little credible evidence for size-assortative mating behavior in any anurans. Instead, large-male advantage among scramblers was associated with a 2° pattern of concomitant non-random mating. In this “disproportionate” mating, neither sex behaves according to mate choice rules that could lead to consistently strong assortment. It should thus have relatively little evolutionary impact compared to true assortative mating.

动物中的选型交配（Assortative mating）可对演化过程产生显著影响，且大量研究报告显示其发生普遍。本研究从行为学角度对选型交配定义如下：动物依据某一共同性状选择配偶，使得交配个体间的表型相似性显著高于随机交配的预期水平。体型是当前被广泛研究的选型交配性状之一，这一点在无尾两栖动物（Anuran amphibians，即蛙类与蟾蜍类）中尤为关键——该类群中雌雄两性的大型个体均可获得繁殖收益。无尾两栖动物的雄性交配行为存在多种离散模式：‘坐等型’个体采取静伏鸣叫的策略，为雌性择偶提供了可能；而‘争抢型’个体通过争夺配偶占据权的行为，则直接排除了雌性选择的机会。由此而言，无尾两栖动物的体型选型交配应仅存在于坐等型个体中，而非争抢型个体。本研究采用元分析（Meta-analysis）方法，整合了68种无尾两栖动物的282项研究数据，评估了与交配行为及其他变量相关的真实体型选型交配发生情况。研究发现存在两类偏倚：一是针对非显著结果的发表偏倚，二是因合并不同时间或地点采集的样本所引发的分析偏倚（即辛普森悖论（Simpson’s Paradox））。合并样本的做法显著夸大了体型选型交配的表观发生率与效应强度。在控制此类偏倚后，几乎没有可信证据表明任何无尾两栖动物存在体型选型交配行为。与之相反，争抢型个体中的雄性体型优势，与一种伴随性的非随机交配二级模式相关联。在这种‘不成比例交配’模式中，两性均未遵循可促成持续强选型交配的择偶规则。因此，相较于真实的选型交配，该模式的演化影响相对有限。