Avian telencephalon and cerebellum volumes can be accurately estimated from digital brain endocasts

Abstract

For studies of the evolution of vertebrate brain anatomy and potentially associated behaviours, reconstructions of digital brain endocasts from computed tomography scans have revolutionized our capacity to collect neuroanatomical data. However, measurements from digital endocasts must be validated as reflecting actual brain anatomy, which is difficult because the collection of soft tissue information through histology is laborious and time-consuming. In birds, the reliability of digital endocast measurements as volume proxies for the two largest brain regions-the telencephalon and cerebellum-remains to be validated despite their use as proxies, e.g. of cognitive performance or flight ability. We here use the largest dataset of histology and digital endocasts to date, including 136 species from 25 avian orders, to compare digital endocast surface area measurements with actual brain volumes of the telencephalon, cerebellum and whole-brain endocast. Using linear and phylogenetically informed regression analyses, we demonstrate that endocast surfaces are strongly correlated with their brain volume counterparts for both absolute and relative size. This provides empirical support for using endocast-derived cerebellar and telencephalic surface areas in existing and future studies of living and extinct birds, with potential to expand to the dinosaur-bird transition in the future.

Keywords: birds; cerebellum; comparative neuroanatomy; endocast; telencephalon; vertebrate brain.

Figure 1.

The endocast of a Eurasian treecreeper (Certhia familiaris) in rostral (a) and caudal views (b). Both images show osteological landmarks used to delineate the endocast telencephalon and cerebellum for measurements. The telencephalon is blue, the cerebellum is red and the rest of the endocast is grey. (c) Endocasts of a representative species of each of the orders examined in this study using the same colour coding as in (a) and (b). Note that endocast images shown here are of reduced resolution and are not to scale.

Figure 2.

Log-transformed measurements of the (a) whole endocast, (b) telencephalon and (c) cerebellum surface areas measured from endocasts regressed against their brain volume counterparts for 136 species of birds. The orders from these data are represented by unique combinations of colours with point shapes. The black line represents the ordinary least-squares linear model regression; dark grey area represents 95% confidence intervals; light grey area represents 95% prediction intervals.

Figure 3.

Log-transformed measurements comparing the scaling relationships of the telencephalon for (a) the surface area against endocast remainder and (b) the volume against brain remainder, as well as the cerebellum for (c) the surface area against endocast remainder and (d) the volume against brain remainder for 136 species of birds. The orders from these data are represented by unique combinations of colours with point shapes. Ordinary least-squares regression (LM slope) is shown by the black line; dark grey for the 95% confidence intervals; light grey for the 95% prediction intervals. Phylogenetic generalized least-squares (PGLS) regression is shown by the dashed red line; pink area shows the 95% confidence intervals of the PGLS.