Endocast Morphological Variation and Its Driving Forces in Scutiger boulengeri

Cranial endocasts can be used as a reliable proxy for brain size, reflecting the evolutionary and environmental selection pressure of species. Although studies on endocasts in amphibians have increased in recent years, those performed on endocasts of Anura are comparatively rare, especially at the intraspecific level. Here, using a high-altitude endemic toadScutiger boulengeri-as a model, through the application of integrative methods (morphology, anatomy, phylogeny, and ecology), we studied intraspecific variations in endocast morphology and explored its driving forces. Three-dimensional reconstruction and the brain-to-endocranial cavity (BEC) index suggested that the endocast of S. boulengeri can reflect brain morphology to a large extent. Elliptic Fourier analysis and principal component analysis revealed great variability in the cranial endocast morphology among individuals, as well as the variation concentrated in the regions of telencephalon and optic tectum. In the species, individuals with large bodies are accompanied by a larger endocast size; the relative endocast sizes have significant clade differences but no sexual dimorphism. Additionally, the relative endocast sizes of S. boulengeri were not associated with phylogenetic history and aquatic preference but were positively correlated with altitude and negatively correlated with oxygen content, temperature, and precipitation factors (annual mean temperature, temperature seasonality, annual precipitation, and precipitation seasonality). These findings suggested that high-altitude and extreme environmental conditions acted as important selective forces in morphological variation of the cranial endocast of S. boulengeri.

Automated summary

In this study, the intraspecific variations in endocast morphology of the high-altitude endemic toad Scutiger boulengeri were investigated. Results showed that individuals with larger body sizes had larger endocast sizes, and there were significant differences in endocast sizes among different clades. Moreover, the relative endocast sizes of S. boulengeri were positively correlated with altitude and negatively correlated with oxygen content, temperature, and precipitation factors. These findings suggested that high-altitude and extreme environmental conditions played important roles in the morphological variation of the cranial endocast of S. boulengeri.