Biology Department, Science Museum of Minnesota, Saint Paul, MN, USA.
Department of Biological Sciences, Ohio University, Athens, OH, USA.
J Anat. 2020 Dec;237(6):1162-1176. doi: 10.1111/joa.13285. Epub 2020 Sep 6.
Endocasts are increasingly relied upon to examine avian brain evolution because they can be used across extant and extinct species. The endocasts of birds appear to be relatively faithful representatives of the external morphology of their brains, but it is unclear how well the size of a surface feature visible on endocasts reflects the volume of the underlying brain region. The optic lobe and the Wulst are two endocast structures that are clearly visible on the external surface of avian endocasts. As they overlie two major visual regions of the brain, the optic tectum and hyperpallium, the surface areas of the optic lobe and Wulst, respectively, are often used to infer visual abilities. To determine whether the surface area of these features reflects the volume of the underlying brain regions, we compared the surface areas of the optic lobes and Wulsts from digital endocasts with the volumes of the optic tecta and hyperpallia from the literature or measured from histological series of brains of the same species. Regression analyses revealed strong, statistically significant correlations between the volumes of the brain regions and the surface areas of the overlying endocast structures. In other words, the size of the hyperpallium and optic tectum can be reliably inferred from the surface areas of the Wulst and optic lobe, respectively. This validation opens the possibility of estimating brain-region volumes for extinct species in order to gain better insights in their visual ecology. It also emphasizes the importance of adopting a quantitative approach to the analysis of endocasts in the study of brain evolution.
内骨铸型越来越多地被用于研究鸟类大脑进化，因为它们可以用于现生和已灭绝的物种。鸟类的内骨铸型似乎是其大脑外部形态的相对忠实代表，但尚不清楚内骨铸型上可见表面特征的大小与该特征下的大脑区域体积之间的关系。视叶和 Wulst 是两种在内骨铸型外部表面清晰可见的结构。由于它们位于大脑两个主要的视觉区域——视顶盖和上纹状体的上方，因此视叶和 Wulst 的表面积通常用于推断视觉能力。为了确定这些特征的表面积是否反映了大脑区域的体积，我们将数字内骨铸型上视叶和 Wulst 的表面积与文献中或从同一物种的脑组织的组织学系列中测量得到的视顶盖和上纹状体的体积进行了比较。回归分析显示，大脑区域的体积与覆盖的内骨铸型结构的表面积之间存在着很强的、统计学上显著的相关性。换句话说，Wulst 和视叶的表面积可以分别可靠地推断出上纹状体和视顶盖的大小。这种验证为评估已灭绝物种的脑区体积提供了可能性，以便更好地了解它们的视觉生态。它还强调了在研究大脑进化时对内骨铸型进行定量分析的重要性。
