How birds' oxygen-free retinas stay powered by sugar

Biologists at Aarhus University have resolved a long-standing mystery about how birds maintain sharp vision despite their retinas lacking direct blood vessels and thus oxygen supply. Unlike most neural tissues that require constant oxygen delivered via blood, bird retinas operate in an oxygen-free environment. This adaptation likely evolved to prevent light scattering and enhance visual clarity. Contrary to previous assumptions that the pecten oculi—a comb-like vascular structure in birds’ eyes—supplies oxygen to the retina, researchers found it does not deliver oxygen at all. Instead, bird retinas survive through anaerobic glycolysis, a less efficient metabolic process that breaks down sugar without oxygen. The team used spatial transcriptomics and metabolic imaging to reveal that bird retinas consume glucose at exceptionally high rates to compensate for the low energy yield of anaerobic metabolism. The pecten oculi functions as a metabolic gateway, supplying large amounts of sugar to fuel the retina’s anaerobic energy production and removing lactate waste to prevent toxicity. This unique