To many, pigeons seem like little more than trash-eating denizens of the sidewalk without a synapse between them. But scientists from the Johns Hopkins University say that’s wrong. There is new evidence they are, in fact, big brained.
And this may be just the information needed to answer a long-running biological question: How did birds and other vertebrates, those with backbones, evolve to fly?
“This was a rare event,” said Amy Balanoff, assistant professor of functional anatomy and evolution in Hopkins’ School of Medicine and first author on the published research. “It’s happened only three times.”
Flying is the domain of just birds, bats and pterosaurs, the extinct reptilian predator and cousin to dinosaurs that lived during the Mesozoic period, which ended over 65 million years ago.
Scientists long suspected the evolutionary ability started with the brain, specifically the cerebellum, responsible for movements and motor control. Then other changes followed in limb proportions, growth of a wishbone and development of proper feathers.
Balanoff said none of these things evolved specifically for flight, but together allowed for it.
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For example, feathers likely came about first to regulate body temperature and then aided birds in thrust and lift. Wishbones provided structural stability and then gave birds their spring motion for wing flapping.
Brains likely expanded to manage more complex terrain that was uneven with obstacles. (Human brains evolved, too, but not the body parts.)
The attributes didn’t develop specifically to enable flying, she said. “But once it happened, it was there so they could use it.”
To get the proof about the big cerebellums, Balanoff and colleagues used brain scans from modern pigeons and dinosaur fossils.
First, Hopkins scientists worked with biomedical engineers at Stony Brook University in New York to coax eight good-natured pigeons to either sit on a perch or fly between perches. Then they anesthetized the birds and injected them with a tracing compound similar to glucose that can be absorbed by brain cells and tracked. The tracer is excreted from the body in a day or two.
They put the pigeons in a mini version of a positron emission tomography, or PET, scanner, a tube-type imaging machine commonly used on humans, and compared activity in 26 regions of the brain during rest and just after flying (a window where the brain activity continues).
They saw the cerebellum was not only larger but had significantly increased activity when birds were flying. They also saw big increases in the network of brain cells connecting the retina in the eye to the cerebellum.
This indicated information from the birds’ visual field was combining with other sensory information in the cerebellum, which informed their bird brains about how to fly rather than just glide or walk.
Next they examined some ancient dinosaur brains to confirm a similar evolution. They used a database of endocasts, or molds of the space inside a dino skull, that resemble brains. They had similar results, which were all published in the Jan. 31 issue of the journal “Proceedings of the Royal Society B: Biological Sciences.”
“We were able to design a study that used PET to effectively capture the brain activity during flight, and then discovered the primary role of the cerebellum,” said Paul Vaska, a professor in the Departments of Biomedical Engineering and Radiology in the Renaissance School of Medicine at Stony Brook.
The findings, he said, lay the groundwork for future studies to better understand brain evolution across species.
And perhaps for pigeons, it adds a feather in their cap.
