DOI: 10.5061/dryad.nzs7h44wn
title: Endocasts and brain volume of dogs

description: Domestication is a well-known example of the relaxation of
 environmentally-based cognitive selection that leads to reductions in
 brain size. However, little is known about how brain size evolves after
 domestication and whether subsequent directional/artificial selection can
 compensate for domestication effects. The first animal to be domesticated
 was the dog, and recent directional breeding generated the extensive
 phenotypic variation among breeds we observe today. Here we use a novel
 endocranial dataset based on high-resolution CT scans to estimate brain
 size in 159 dog breeds and analyze how relative brain size varies across
 breeds in relation to functional selection, longevity, and litter size. In
 our analyses, we controlled for potential confounding factors such as
 common descent, gene flow, body size, and skull shape. We found that dogs
 have consistently smaller relative brain size than wolves supporting the
 domestication effect, but breeds that are more distantly related to wolves
 have relatively larger brains than breeds that are more closely related to
 wolves. Neither functional category, skull shape, longevity, nor litter
 size was associated with relative brain size, which implies that selection
 for performing specific tasks, morphology, and life history do not
 necessarily influence brain size evolution in domesticated species.
description: We
 processed the collection of dog skulls that is maintained at the
 Department of Anatomy, Cell and Developmental Biology, Eötvös Loránd
 University (Budapest, Hungary). This private collection (owned by TC) is
 composed of specimens that have been obtained mostly in the last 10 years
 by the appropriate preparation of the heads of deceased dogs (which were
 donated post-mortem), from which the soft materials have been removed a
 priori. TC systematically collected the prepared skulls with the aim of
 having both male and female samples from as many breeds as possible. Breed
 identity was usually verified upon the collection of cadavers/skulls,
 given that these materials originate from known dog breeders.
 Alternatively, we checked the appropriate breed certificates/chips for
 pedigree. Currently, the collection consists of 383 individual skulls
 (including males, females and unknown sexes) from 146 breeds. We selected
 172 skulls (38 females, 83 males and 50 unknown sexes) across all breeds
 represented in the collection for subsequent CT scan analysis (see
 Supplementary Material, Table S1). Skulls were selected from adult
 individuals, which we verified using morphological characteristics (i.e.,
 the presence of permanent teeth, as dogs should replace all baby teeth
 before 6-7 months of age). The
 selected skulls were transferred to the Diagnostic and Oncoradiology
 Centre in Kaposvár
 (Hungary) for CT scanning. We used a Siemens Somatom Definition AS+ CT
 machine (Siemens, Erlangen, Germany) to digitalize the skulls with high
 resolution (170 mAs, 140 kV, pixel size 0.323 × 0.322 mm, slice thickness
 0.6 mm, with a v80u bone kernel). The resulting DICOM image series were
 imported into the 3D Slicer software (freeware, www.slicer.org), and using its segmentation and modelling tools, the endocranial volumes (=endocast) were reconstructed (see details in Czeibert et al. 2020). These endocasts reflect the surface morphology of the brain in such detail that external blood vessels and differences in gyrification can be observed (Figure 1). In parallel, we calculated the volume of the endocasts for the analysis (Czeibert et al. 2020) in this study. We also extracted additional data on brain volumes from the literature for some dog breeds.
description: Microsoft Excel, R