Beaty Lynne E, Emmering Quinn C, Bernal Ximena E
Department of Integrative Biology, Oklahoma State University, Stillwater, Oklahoma, USA.
Federal Energy Regulatory Commission, Washington, DC.
Anat Rec (Hoboken). 2016 Apr;299(4):421-7. doi: 10.1002/ar.23322. Epub 2016 Feb 17.
Sexual dimorphism in the ratio of digit lengths has been correlated to behavioral, physiological, and morphological traits in a variety of taxa. While sexual dimorphism in the second-to-fourth digit length ratio (2D:4D) is a well-established indicator of prenatal androgen exposure in mammals, investigations into the patterns of 2D:4D and the drivers of such variation in other taxa are lacking. We used linear mixed effects models to gain a mechanistic understanding of the factors that drive variation in the scaling relationship between the lengths of the second and fourth digits in two species of anurans: túngara frogs (Engystomops pustulosus) and cane toads (Rhinella marina). We found evidence for sexual dimorphism of the 2D:4D scaling relationship on the front feet of túngara frogs, with female frogs having a larger ratio than males resulting from a relatively longer second digit on females. To our knowledge, this mammal-like pattern of sex differences in digit ratio has not yet been reported for anurans. However, given the reduced number of digits on the front feet of anurans, and uncertainty about which digit was lost during evolutionary history, this apparent sexual dimorphism in the front feet of túngara frogs should be treated with caution. In contrast, we found no evidence of sexual dimorphism in 2D:4D on either the front or rear feet of cane toads. This study highlights ambiguities in 2D:4D across taxa and suggests that further research is needed to evaluate the effect of androgens on 2D:4D in animals other than placental mammals.
在多种生物分类群中，指长比例的两性异形与行为、生理和形态特征相关。虽然第二至第四指长比例（2D:4D）的两性异形是哺乳动物产前雄激素暴露的一个公认指标，但对于其他生物分类群中2D:4D的模式以及这种变异的驱动因素的研究却很缺乏。我们使用线性混合效应模型，以深入了解驱动两种无尾目动物（饰纹姬蛙和海蟾蜍）第二和第四指长度比例缩放关系变异的因素。我们发现饰纹姬蛙前足的2D:4D缩放关系存在两性异形的证据，雌性蛙的比例大于雄性，这是因为雌性的第二指相对较长。据我们所知，无尾目动物中这种类似哺乳动物的指比例性别差异模式尚未见报道。然而，鉴于无尾目前足的指数量减少，以及在进化历史中丢失了哪根手指存在不确定性，饰纹姬蛙前足这种明显的两性异形应谨慎对待。相比之下，我们在海蟾蜍的前足或后足上均未发现2D:4D两性异形的证据。这项研究凸显了不同生物分类群中2D:4D的模糊性，并表明需要进一步研究以评估雄激素对胎盘哺乳动物以外的动物2D:4D的影响。
