Mosaic evolution of avian brain compartments revealed by comparative MRI

Abstract

Birds evolved large, cognitively capable forebrains independently of mammals, yet comparative analyses of avian brain organization have been constrained by the lack of standardized resources capable of resolving internal parcellation and long-range connectivity across species. Here, we present a comparative MRI resource spanning 16 avian species representing major clades and diverse ecological niches. We analyzed high-resolution T2-weighted and diffusion-weighted datasets suitable for direct interspecific comparison. T2-weighted morphometry revealed pronounced region-specific variation in internal brain architecture, including lineage-dependent differences in the relative prominence of major brain divisions and commissural structures, supporting a pattern of mosaic diversification rather than uniform scaling. To validate MRI-derived anatomical boundaries, we compared MRI parcellations with complementary histological analyses in three representative taxa (the large-billed crow, gentoo penguin, and mandarin duck), demonstrating close correspondence between MRI-defined borders and cytoarchitectonic transitions identified by Nissl staining, as well as major myelinated compartments visualized by Luxol Fast Blue staining. Moreover, diffusion MRI tractography and fractional anisotropy (FA) mapping further revealed both conserved and species-specific features of large-scale brain organization. Seed-based tractography of the optic lobe, dorsal cortex, cerebellum, and anterior cortex in chick, gentoo penguin, and large-billed crow revealed conserved within-compartment trajectory patterns alongside marked region-specific interspecific differences, particularly in optic-lobe-associated long-range trajectories. Whole-brain FA maps revealed complementary variation in regional microstructural organization across taxa. Together, this comparative MRI framework provides a cross-validated foundation for linking internal brain anatomy and long-range connectivity to ecological and evolutionary diversification in birds, with broader applications to comparative neuroanatomy across amniotes. Significance StatementHow does the internal architecture of the brain reorganize as species adapt to diverse lifestyles? Traditional comparative neuroanatomy has largely relied on overall brain size and external morphology, providing limited insight into internal subdivisions and long-range connectivity. In this study, we present a standardized avian MRI resource and a reproducible analytical pipeline that enables direct cross-species comparisons of 3D morphometry and diffusion-derived connectivity across 16 bird species representing major clades and diverse ecological niches. MRI-derived anatomical boundaries are validated through complementary histological analyses, providing biological support for cross-species parcellation and white-matter measurements. Using this framework, we demonstrate that avian brains diversify in a strongly mosaic manner, with region-and pathway-specific changes that vary disproportionately across lineages rather than scaling uniformly with overall brain size. By moving beyond simple correlations between brain size and behavior, this resource provides a benchmark for testing hypotheses of mosaic brain evolution and for linking neural architecture to ecological and behavioral diversification across vertebrates.