Bird Retinas Run Without Oxygen, Solving a 300-Year-Old Anatomy Puzzle

A study published in Nature in January 2026 by Christian Damsgaard and colleagues at Aarhus University resolves a long-standing paradox: bird retinas, among the most metabolically demanding tissues in any animal, lack the dense blood vessel networks that fuel vision in other vertebrates. Using microsensors on zebra finches, pigeons, and chickens, the team confirmed that the inner retina contains no oxygen at all. Instead of relying on aerobic respiration, which yields roughly 15 times more ATP per glucose molecule, bird retinas survive on anaerobic glycolysis — the same inefficient pathway used by tumor cells and overexerted muscles.

Spatial transcriptomics revealed that genes for aerobic metabolism were active only in the outer retina, while the oxygen-starved inner layer expressed exclusively anaerobic-respiration genes and consumed 2.5 times more glucose than other bird brain tissue. The researchers also cracked the function of the pecten oculi, a comb-shaped vascular structure inside the bird eye that has puzzled anatomists since the 17th century and inspired roughly 30 competing theories. It turns out not to deliver oxygen but to pump glucose into the retina and flush out toxic lactic acid byproducts.

The finding is significant because no vertebrate tissue was previously known to function permanently without oxygen — most mammals die within minutes of anoxia. Understanding how bird retinas tolerate this state could inform therapies for stroke and other oxygen-deprivation conditions, and it expands the known limits of how metabolically active tissue can operate under extreme evolutionary pressure.