How Bird Retinas Stay Powered by Sugar: A Scientific Breakthrough

Biologists have finally solved a centuries-old mystery regarding how birds maintain sharp eyesight despite their retinas lacking a direct blood supply. This means that bird retinas work without oxygen. Aarhus University in Denmark has found that birds solve the problem of oxygen deprivation by switching their eyes to anaerobic power. A bloodless retina that refuses to fail Neural tissues, especially the retina, are among the most energy-demanding tissues in the body. These tissues typically require a constant supply of oxygen via blood vessels to function. However, bird retinas lack blood vessels, likely an evolutionary adaptation to prevent light scattering and improve visual clarity. Logically, these cells should be dead. Without blood, there is no oxygen. Without oxygen, neural tissue usually withers in minutes. “According to everything we know about physiology, this tissue should not be able to function,” said Christian Damsgaard, biologist and first author. Yet, birds see better than many other creatures on Earth. Now, after eight years of investigation, the Aarhus team has finally decoded this vascular paradox. But first, a quick rewind. Since the 1600s, biologists have pointed to the pecten oculi — a strange, comb-like structure protruding into the bird’s eye — as the secret oxygen tank. They assumed it leaked oxygen into the surrounding fluid to keep the retina alive. It turns out, the experts’ assumptions were wrong. The researchers stated that the “pecten does not deliver oxygen to the retina at all.” Instead, the tissue exists in a state of chronic deprivation that would cause a human brain to undergo a massive stroke. Energy without oxygen This raised a serious question: how does the retina stay powered? The team turned to “molecular GPS” called spatial transcriptomics, which enables mapping thousands of genes in the eye. It was found that oxygen-deprived inner layers have switched from oxygen-based metabolism to anaerobic glycolysis — a process that breaks down sugar in the absence of oxygen. This is a desperate, inefficient way to make energy. It produces 15 times less power per sugar molecule than oxygen-based breathing. “This mismatch raised yet another question: How can one of the most energy-hungry tissues in the body survive on such an inefficient process?” questioned Jens Randel Nyengaard, senior author. To solve the mystery, researchers used metabolic imaging to track sugar movement, discovering that bird retinas consume glucose at far higher rates than the rest of the brain. Here, the role of the pecten oculi — a structure long misunderstood — comes into play. Pecten acts as a high-capacity “metabolic gateway.” The structure floods the retina with sugar to fuel its anaerobic engine and rapidly vacuums out lactate waste, preventing the build-up of toxic byproducts in the bloodless eye. “The pecten is not an oxygen supplier. It is a transport system for fuel in and waste out,” said Nyengaard. Understanding brain stroke Evolutionary evidence suggests that birds inherited these bloodless retinas from their dinosaur ancestors to provide superior visual clarity. The team says the findings could offer a potential medical blueprint for treating human strokes. In a brain stroke, human tissues die quickly when deprived of oxygen and choked by waste. Birds have evolved a natural strategy for surviving these exact conditions. “Nature has solved a physiological problem in birds that makes humans sick,” Nyengaard added. One day, it could help develop new strategies for protecting human brain tissue when blood flow is restricted or cut off. The study was published in the journal Nature on January 21.