Bird Brains Evolved in Unique Path, Study Finds

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CT scan of a bird's skull and brain with color-coded sections This CT scan shows a modern woodpecker (Melanerpes aurifrons) with its brain cast rendered opaque and the skull transparent. The endocast is partitioned into the following neuroanatomical regions: brain stem (yellow), cerebellum (blue), optic lobes (red), cerebrum (green), and olfactory bulbs (orange).
A. Balanoff/© AMNH

New research reveals some surprising details about how they have evolved relative to the brains of their close cousins, dinosaurs and alligators.

“The brains of birds are as large as many mammals and allow for intelligent behaviors such as learning songs and solving complex puzzles,” said Aki Watanabe, a Museum research associate and an assistant professor at the New York Institute of Technology who is the lead author of the new study, published this week in the journal eLife. “These features make bird brains an excellent system to compare with how humans and other mammals obtained large and unique brains, yet we know relatively very little about how bird brains got to be that way.”

To examine these dynamics in birds, the researchers focused on a sample of modern and extinct birds, their dinosaur precursors, and alligators, which are close living relatives to birds. The team also acquired developmental sampling of domestic chickens and American alligators.

High-resolution X-ray computed tomography (CT) imaging and high-density mathematical shape analysis were used to visualize, model, and statistically analyze how the shape of the brain cavity—an accurate proxy for the brain—changes through both evolutionary and developmental processes.

The researchers found that the way avian brains change with body size is different from that process in either dinosaurs or alligators. That "scaling relationship," as it is called, dictates how avian brains develop and evolve, on a unique path from dinosaurs or alligators.

Birds were also found to have a more integrated brain structure—a surprising result, because the researchers initially thought that bird brains, with their inflated forebrain, would be more modular than their dinosaurian counterparts, similar to the way modern humans’ brains are more modular than those of other primates.

“Bird brains develop and evolve in a more coordinated way, in which a change in one brain region is associated with predictable changes in another region, like squeezing on a balloon or an inflated toy,” Watanabe said.

It could be that the bird brain evolves and develops in a highly coordinated way because of shared functional, developmental, or physical forces, which could be investigated in future studies.

The researchers also found that brain evolution along the dinosaur-bird transition occurred in a stepwise fashion. Non-bird dinosaurs already had an “avian”-grade optic lobe and cerebellum with respect to their shape, which are important for visual input and motor coordination.

Historically, these regions were thought to have co-evolved with the evolution of powered flight, but the new study further supports the idea that dinosaurs already possessed an advanced cerebellum and optic lobes prior to the origin of birds.

“This suggests that vision was an important aspect of dinosaurian lifestyle, perhaps for recognizing an array of color patterns on their feathers, active predatory behaviors, or even an indication of limited wing-assisted locomotion capabilities in some of the dinosaurs we sampled that are very closely related to birds,” Watanabe said.

Other Museum authors on this study include Division of Paleontology and Macaulay Curator Mark Norell, and Research Associates Amy Balanoff, Paul Gignac, and Eugenia Gold.