Despite the growing number of studies describing digit ratio patterns in tetrapods, knowledge concerning certain basic issues is still scarce. In lower vertebrates such as tailless amphibians (Anura), the numbering of individual fingers on the forelimbs and their homology with the fingers of other vertebrates pose an unsolved problem. Based on reviewed data on anuran limb development, we argue that the correct finger numbering scheme should be based on the assumption that the first finger, not the fifth finger, was reduced on the forelimbs. We analyzed the digit ratio in the common toad (Bufo bufo, Bufonidae), a species characterized by well-developed sexual dimorphism whereby females are larger than males, using both numbering schemes present in the literature. We found that the digit ratio on hindlimbs differed significantly between the sexes only in the cases of left 2D:3D, with lower digit ratios in females, and of left 3D:4D, with lower digit ratios in males. We found that sex was the only significant variable for forelimbs, differentiating 2D:3D on the left forelimb, with lower digit ratios in females; 2D:4D on the right forelimb, with lower digit ratios in males; and 3D:4D on both forelimbs, with lower digit ratios in males. These results relate to variant II reflecting the hypothesis that the first digit was reduced during phylogenesis. There was no relationship between the body size (SVL) of individuals and any digit ratio, excluding 2D:4D on the right forelimbs in models with age variables. Additionally, for a subset of data where individual age was known, the models indicated that age was linked to significant differences in 2D:4D and 3D:4D on the left hindlimbs, while age, SVL, and sex influenced 2D:4D on the right forelimbs. We emphasize the importance of the problem of the correct numbering of forelimb digits in Anura and, under the assumption that it was the fifth digit that was reduced, argue that earlier results on digit ratio in this group should be interpreted with caution. The detected relationship between digit ratio and age in amphibians expands our knowledge, indicating that the age of individuals should be included in future digit ratio studies. This relationship may also apply to studies using digit ratio as a noninvasive indicator of endocrine disruption in amphibians.

中文翻译：

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普通蟾蜍的指趾比例：手指减少和年龄依赖性的影响


尽管越来越多的研究描述了四足动物的手指比例模式，但有关某些基本问题的知识仍然很少。在低等脊椎动物中，例如无尾两栖动物（无尾目），前肢上各个手指的编号及其与其他脊椎动物手指的同源性构成了一个未解决的问题。根据有关无尾动物肢体发育的回顾数据，我们认为正确的手指编号方案应该基于这样的假设：前肢上的第一根手指而不是第五根手指被减少。我们使用文献中存在的两种编号方案分析了普通蟾蜍（Bufo bufo，Bufonidae）的指趾比例，该物种具有发达的两性二态性，雌性大于雄性。我们发现，后肢的指比率在性别之间存在显着差异，仅在左2D：3D的情况下，女性的指比率较低，而在左3D：4D的情况下，男性的指比率较低。我们发现性别是前肢唯一显着的变量，左前肢区分 2D:3D，女性的指比率较低；右前肢 2D:4D，雄性的指比率较低；双前肢均为 3D:4D，雄性的指比率较低。这些结果与变体 II 相关，反映了第一个数字在系统发育过程中减少的假设。个体的体型 (SVL) 与任何手指比例之间没有关系，不包括年龄变量模型中右前肢的 2D:4D。此外，对于已知个体年龄的数据子集，模型表明年龄与左后肢 2D:4D 和 3D:4D 的显着差异相关，而年龄、SVL 和性别影响右后肢 2D:4D前肢。 我们强调无尾前肢数字正确编号问题的重要性，并假设减少的是第五个数字，认为应谨慎解释该组中早期数字比例的结果。在两栖动物中检测到的指比率和年龄之间的关系扩展了我们的知识，表明个体的年龄应该被纳入未来的指比率研究中。这种关系也可能适用于使用趾比作为两栖动物内分泌干扰的非侵入性指标的研究。