The Kyoto University Museum, Kyoto University, Kyoto 606-8501, Japan;
Centro Nacional de Investigación Sobre la Evolución Humana 09002 Burgos, Spain.
Proc Natl Acad Sci U S A. 2021 Nov 2;118(44). doi: 10.1073/pnas.2113943118.
Accurate characterization of sexual dimorphism is crucial in evolutionary biology because of its significance in understanding present and past adaptations involving reproductive and resource use strategies of species. However, inferring dimorphism in fossil assemblages is difficult, particularly with relatively low dimorphism. Commonly used methods of estimating dimorphism levels in fossils include the mean method, the binomial dimorphism index, and the coefficient of variation method. These methods have been reported to overestimate low levels of dimorphism, which is problematic when investigating issues such as canine size dimorphism in primates and its relation to reproductive strategies. Here, we introduce the posterior density peak (pdPeak) method that utilizes the Bayesian inference to provide posterior probability densities of dimorphism levels and within-sex variance. The highest posterior density point is termed the pdPeak. We investigated performance of the pdPeak method and made comparisons with the above-mentioned conventional methods via 1) computer-generated samples simulating a range of conditions and 2) application to canine crown-diameter datasets of extant known-sex anthropoids. Results showed that the pdPeak method is capable of unbiased estimates in a broader range of dimorphism levels than the other methods and uniquely provides reliable interval estimates. Although attention is required to its underestimation tendency when some of the distributional assumptions are violated, we demonstrate that the pdPeak method enables a more accurate dimorphism estimate at lower dimorphism levels than previously possible, which is important to illuminating human evolution.
准确描述性二型是进化生物学中的关键，因为它对于理解涉及物种生殖和资源利用策略的当前和过去的适应具有重要意义。然而，推断化石组合中的二型性是困难的，特别是在相对低的二型性的情况下。通常用于估计化石中二型性水平的方法包括均值法、二项式二型性指数和变异系数法。这些方法被报道会高估低水平的二型性，这在研究灵长类动物犬齿大小二型性及其与生殖策略的关系等问题时是有问题的。在这里，我们介绍了后密度峰 (pdPeak) 方法，该方法利用贝叶斯推断提供二型性水平和性别内方差的后验概率密度。后验密度最高点称为 pdPeak。我们通过 1）模拟一系列条件的计算机生成样本和 2）应用于现生物种已知性别的犬齿冠直径数据集，研究了 pdPeak 方法的性能并与上述传统方法进行了比较。结果表明，pdPeak 方法能够在比其他方法更广泛的二型性水平范围内进行无偏估计，并且独特地提供了可靠的区间估计。尽管在某些分布假设被违反时需要注意其低估趋势，但我们证明 pdPeak 方法能够在较低的二型性水平下比以前更准确地估计二型性，这对于阐明人类进化很重要。
