Macroevolutionary patterns of sexual size dimorphism among African tree frogs (Family: Hyperoliidae)

Sexual size dimorphism (SSD) is shaped by multiple selective forces that drive the evolution of sex-specific body size, resulting in male or female-biased SSD. Stronger selection on one sex can result in an allometric body-size scaling relationship consistent with Rensch's rule or its converse. Anurans (frogs and toads) generally display female-biased SSD, but there is variation across clades and the mechanisms driving the evolution of SSD remain poorly understood. We investigated these topics in a diverse family of African treefrogs (Hyperoliidae). Hyperoliids display traits considered rare among amphibians, including sexual dichromatism and protogynous sex change. Using phylogenetic comparative methods, we tested if adult ecology, sexual dichromatism, and sex change were predictors of body size or SSD. We also tested whether hyperoliids displayed allometric interspecific body-size-scaling relationships. We found a majority of hyperoliid taxa display female-biased SSD, but that adult ecology and sexual dichromatism are poor predictors of sex-specific body size and SSD. Regardless of the groupings analyzed (partitioned by clades or traits), we found support for isometric body-size scaling. However, we found that sex change is a significant predictor of SSD variation. Species in the Hyperolius viridiflavus complex, which putatively display this trait, show a significant reduction in SSD and tend to be sexually monomorphic in size. Although protogynous sex change needs to be validated for several of these species, we tentatively propose this trait is a novel mechanism influencing anuran body size evolution. Beyond this association, additional factors that shape the evolution of anuran body size and SSD remain elusive.

性大小二态性（Sexual size dimorphism, SSD）由多种驱动性特异性体型演化的选择压力所形塑，最终形成雄性偏倚或雌性偏倚的性大小二态性。对某一性的更强选择压力，可催生与伦施法则（Rensch's rule）或其反向版本相符的异速体型缩放关系。无尾目（Anurans，即蛙与蟾蜍）普遍呈现雌性偏倚的性大小二态性，但不同演化支间存在性状变异，而驱动性大小二态性演化的相关机制仍未得到充分阐释。我们针对一类多样的非洲树蛙类群——树蛙科（Hyperoliidae），对上述问题展开了研究。树蛙科类群具备两栖动物中较为罕见的特征，包括性二色性与雌性先熟性转变。本研究采用系统发育比较方法，检验了成体生态、性二色性以及性转变能否作为体型或性大小二态性的预测因子；同时验证了树蛙科类群是否存在种间异速体型缩放关系。研究结果显示，多数树蛙科类群呈现雌性偏倚的性大小二态性，但成体生态与性二色性并非性特异性体型及性大小二态性的有效预测因子。无论以演化支或性状进行分组分析，我们均获得了支持等速体型缩放的证据。但我们发现，性转变是性大小二态性变异的重要预测因子。绿纹树蛙复合群（Hyperolius viridiflavus complex）中被认为具备该性状的物种，其性大小二态性显著降低，体型多呈现单态性。尽管该类群中的多个物种尚需验证是否存在雌性先熟性转变，但我们初步提出，该性状是影响无尾目体型演化的一种全新机制。除该关联之外，其余塑造无尾目体型与性大小二态性演化的额外因素仍有待进一步探明。