Endocasts and brain volume of dogs

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.