Scientists have discovered how bird retinas operate without oxygen, relying instead on a surge of glucose for energy. This finding, based on studies of zebra finches, resolves a 400-year-old puzzle about avian eye physiology. The adaptation highlights evolution's unexpected solutions for high-energy visual demands.
Bird retinas represent a unique departure from typical vertebrate eye tissues. Unlike most animals, where blood vessels deliver oxygen to the light-sensitive layer at the back of the eye, birds' thick retinas lack such vessels. Instead, they sustain themselves through glycolysis, a process that breaks down sugars without oxygen, though it demands far more glucose—15 times as much—to produce equivalent energy.
Researchers led by Christian Damsgaard at Aarhus University in Denmark examined zebra finches, or Taeniopygia guttata, using tiny oxygen sensors inserted into their eyes. The sensors revealed that inner retinal layers receive no oxygen, as it diffuses only from the eye's rear and cannot penetrate the full thickness. "They get oxygen from the back of the eye, but it cannot diffuse all the way through the retina," Damsgaard explained.
Metabolic gene analysis confirmed heightened glycolysis in oxygen-deprived zones. The key enabler is the pecten oculi, a rake-like structure of blood vessels in birds' eyes, long suspected of oxygen delivery but now shown to flood the retina with glucose—four times the rate of brain cells.
This mechanism addresses how birds maintain vital nerve cell function despite the retina's immense energy needs. "The retina—especially a bird retina—is one of the most energy-needy tissues in all of the animal kingdom," noted Luke Tyrrell at the State University of New York at Plattsburgh, expressing surprise at the inefficiency yet acknowledging potential benefits for visual sharpness and high-altitude flights unaffected by low oxygen.
Pavel Němec at Charles University in Prague described the discovery as a "clear case that reminds us that evolution brings very counterintuitive solutions." Damsgaard's team suggests implications for human medicine, such as engineering cells to endure oxygen scarcity post-stroke. The findings appear in Nature (DOI: 10.1038/s41586-025-09978-w), marking a neurobiological shift: "We have the first evidence that some neurons can work without any oxygen, and they’re found in the birds that fly around in our gardens."
The avascular design likely evolved to boost acuity, trading efficiency for clarity in flight-critical vision.
