Their findings provide compelling evidence for the role of the pecten oculi in supporting anaerobic glycolysis, which “has been a mystery for a long time,” said Thomas Baden, a neuroscientist at the University of Sussex who was not involved in the study. “The insight that the retina basically goes oxygen-free, at least in some layers, is surprising. … It really gets properly down to zero.”
This pathway is used by cancer cells and temporarily by our muscles when they’re strained and can’t get enough oxygen — such as when we’re running. But no known vertebrate tissue was known to survive in completely anoxic conditions for a lifetime.
Eyes Like a Hawk
The bird’s retina and its no-oxygen power system are so unusual that they naturally raise questions about how they could have evolved.
This is “a series of beautiful experiments,” said Karthik Shekhar of the University of California, Berkeley, who was not involved in the research. It’s an example of how an animal took the vertebrate eye — a highly conserved structure whose origins go back some 560 million years to a light-sensitive patch on a primitive creature — and tinkered with it to fit its own needs. “Evolution is not really like an inventor; it acts more like a tinkerer,” he said, citing a 1977 essay, “Evolution and Tinkering,” by the French biologist François Jacob. “It takes parts that have existed long before, and it recombines, reinvents, and reshapes.”
The researchers tried to pinpoint when the pecten oculi might have arisen by comparing oxygen levels in the bird retina to those in not-so-distant relatives: two reptile species, Chinese pond turtles and broad-snouted caimans. The reptile retinas had normal oxygen levels and no indication of anaerobic glycolysis. This led Damsgaard’s team to conclude that the oxygen-free tissue likely evolved sometime during the dinosaur era, after the avian lineage had split from crocodiles but hadn’t yet evolved into modern birds. This was around the same time that the retina thickened.
Still, that rough time estimate can’t explain what evolutionary pressure might have selected for the unusual retinal tissue. Researchers can only speculate. “I think the system evolved in theropod dinosaurs in response to selection for sharp vision for tracking prey and identifying mates,” Damsgaard suggested. Then, later, when birds took to the skies, it “served as the physiological basis for maintaining retinal function” during high-altitude flights, when oxygen levels are low, he speculated.
The lack of blood vessels could also offer birds the advantage of better vision. The bird retina is complex and densely packed with more than a hundred cell types that work to render the world in great resolution. Birds use their exceptional visual sense for hunting and foraging — consider an owl tracking a mouse from the sky, an albatross watching for signs of fish on the ocean’s surface, or a hummingbird locating hundreds of flowers every day — as well as for following landmarks across the landscape during migration. Without blood vessels obstructing their view, birds’ retinal cells might be able to take in more visual information.
Could this be an adaptation, or is it a coincidence of evolutionary history? There’s no way to know for sure how birds’ incredible vision evolved. There’s this mystery “that has lingered around us,” Baden said. “What is it about birds that makes their eyes so special?” Their retinal power system seems as if it could explain what makes them so unique. However, Lewin, the physiologist, is cautious about overextending the results and interpretations to every bird, given that the researchers haven’t looked at any migratory species.
The implications stretch well beyond bird adaptations to biomedicine. A common thread in many medical conditions is a drop in oxygen delivery to tissues, which, depending on where it occurs, can lead to scars or brain damage. Human brains can tolerate maybe a minute of total anoxia, Lewin said. That’s what makes strokes, which cut off blood and oxygen supply to parts of the brain, so devastating. By studying low-oxygen conditions in creatures such as naked mole rats and birds, scientists can gain insight into how tissues can tolerate low-oxygen conditions.
“Maybe we can get inspiration for how nature solved these problems by millions of years of natural selection,” Damsgaard said. “There’s so much to be learned from these animals that are able to do something that we cannot do.”