Accelerated brain shape evolution is associated with rapid diversification in an avian radiation

Niche expansion is a critical step in the speciation process. Large brains
linked to improved cognitive ability may enable species to expand their
niches and forage in new ways, thereby promoting speciation. Despite
considerable work on ecological divergence in brain size and its
importance in speciation, relatively little is known about how brain shape
relates to behavioral, ecological, and taxonomic diversity at
macroevolutionary scales. This is due, in part, to inherent challenges
with quantifying brain shape across many species. Here, we present a
novel, semiautomated approach for rapidly phenotyping brain shape using
semilandmarks derived from X-ray computed micro-tomography (microCT)
scans. We then test its utility by parsing evolutionary trends within a
diverse radiation of birds, kingfishers (Aves: Alcedinidae). Multivariate
comparative analyses reveal that rates of brain shape evolution, but not
beak shape, are positively correlated with lineage diversification rates.
Distinct brain shapes are further associated with changes in body size and
foraging behavior, suggesting both allometric and ecological constraints
on brain shape evolution. These results are in line with the idea of
brains acting as a "master regulator" of critical processes
governing speciation, such as dispersal, foraging behavior, and dietary
niche.