GoKawiilWhen an optometrist shines a bright light into your eyes, a vast, branching tree sprouts in your field of vision. This is the shadow of blood vessels. Though we normally can’t perceive them, these vessels always occlude a portion of what we see, and for an important reason. They power the retina, a thin layer of nerve tissue in the back of the eye that communicates light signals to the brain.
The retina is one of the body’s most energetically expensive tissues. Built from complex networks of sometimes more than 100 different types of neurons, retinal tissue consumes two to three times more energy than the same mass of typical brain tissue. That’s why most vertebrate retinas, including our own, are furrowed with dense, branching networks of blood vessels: to deliver oxygen and other ingredients for producing energy.
But there’s a significant exception to this rule. Birds have retinas that mostly lack blood vessels. This may seem especially strange given birds’ exceptional vision. The bird retina is “one of the most metabolically active tissues in the animal kingdom, yet it worked with no apparent blood perfusion,” said Christian Damsgaard, an evolutionary physiologist at Aarhus University. “It was a complete paradox.” For centuries this has puzzled scientists, who figured that the bird retina must obtain oxygen through a unique, undiscovered process.
Damsgaard is the lead author of a study, published in the journal Nature in January 2026, that showed for the first time that bird retinas don’t have some unusual adaptation for acquiring oxygen — they survive without it entirely. Instead, to bring energy to the tissue, they use a process called anaerobic glycolysis that is significantly less efficient than oxygen-powered metabolism but gets the job done.
The evolutionary physiologist Christian Damsgaard measured gas exchange in bird eyes with microsensors. Surprisingly, the inner retina, a highly active tissue, used no oxygen. Jesper Ekmann
By studying how tissues can survive without oxygen, researchers can potentially develop therapeutics to treat conditions of oxygen deprivation, such as strokes. More fundamentally, they want to understand the limits of evolution.
“What are the extremes of life?” Damsgaard said. “How far can we bend the conditions under which highly metabolically active tissues can actually survive?”
A bird, he learned, can bend them pretty far.
Oxygenated Life
Around 3.4 billion years ago, cyanobacteria invented photosynthesis. Slowly at first, then quickly, their newly evolved method of making energy from sunlight succeeded and spread. The cells pumped so much oxygen, a by-product of photosynthesis, into the atmosphere that it changed the course of life on Earth.
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