Mysterious Structure in Bird Eyes May Explain a Curious Lack of Blood : ScienceAlert

Birds’ eyes have long fascinated scientists with their exceptional sharpness, but new research reveals an even more astonishing secret: their retinas operate without oxygen. In most vertebrates, the inner retina relies on oxygen delivered by blood vessels to convert glucose into energy. Birds, however, have evolved a radically different system, completely devoid of blood vessels in the retina. This allows light to pass unimpeded, maximizing visual clarity, but it also means oxygen must reach retinal cells through diffusion alone—a process that would normally be insufficient for sustaining life.

How, then, do birds manage to keep their retinas alive and functioning at peak performance? The answer lies in an obscure, vascular structure called the pecten oculi, long debated by anatomists but only now fully understood. Through meticulous experiments on live zebra finches, researchers discovered that bird retinal cells survive without oxygen by relying on anaerobic glycolysis—a metabolic pathway that extracts energy from glucose without using oxygen, though at lower efficiency and with the side effect of producing lactic acid. The pecten oculi acts as a specialized pump, delivering large amounts of glucose while simultaneously removing toxic lactic acid before it can damage delicate neural tissue.

This adaptation may have profound evolutionary advantages. By eliminating blood vessels from the retina, birds achieve unparalleled visual acuity, crucial for spotting prey or navigating vast distances. It may also enable high-altitude flight, as seen in short-toed snake eagles, whose retinas are over four times thicker than the maximum limit for oxygen diffusion in mammals. These birds soar hundreds of meters above the ground for hours, relying on their oxygen-free retinas to maintain sharp vision in thin air.

The discovery not only solves a centuries-old anatomical mystery but also opens new avenues for medical research. Understanding how bird retinas survive without oxygen could inform treatments for human conditions like stroke, where brain cells are similarly deprived of oxygen. The pecten oculi’s ability to sustain high metabolic demands without traditional blood supply is a testament to the ingenuity of evolution and may inspire innovative approaches to preserving neural tissue under stress.

This groundbreaking study, published in Nature, is the result of eight years of collaborative research across multiple scientific disciplines. It underscores the remarkable ways in which evolution has fine-tuned bird physiology, offering insights that could benefit both biology and medicine.

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