Normally, neural tissue dies quickly without the supply of oxygen. That is why most animals have a dense network of tiny blood vessels in their neural tissue to deliver the necessary oxygen and nutrients. However, the retina of birds has up until now remained a mystery. It is among the most energy intensive tissues in the animal kingdom, but has no blood vessels inside the retina itself.

In the study published in the scientific journal Nature this week, first-author Christian Damsgaard, associate professor at the Department of Biology, section for Zoophysiology at Aarhus University together with an interdisciplinary team of researchers shows that the inner retina of birds works in a permanent state of oxygen deprivation. Yet, their study reveals that it functions because the nerve cells switch to anaerobic energy production, also known as glycolysis, which is the breakdown of sugar without oxygen.

Crucial interdisciplinary platform 

Comparative physiologist Christian Damsgaard began working on the mystery of the missing oxygen in the bird eye during his AIAS-MSCA COFUND fellowship at the Aarhus Institute of Advanced Studies (AIAS) in his project ‘Sensory evolution from a bird's eye perspective – elucidating the physiological basis for superior eyesight.’ Since then, more than 20 other Danish and German researchers from a wide range of disciplines have contributed to the study, among these several  AIAS fellows.

"The AIAS fellowship provided a constructive platform to engage with researchers from other disciplines. Together with AIAS-COFUND fellow Joanna Kalucka (Biomedicine), we set up retinal spatial transcriptomics and together with AIAS Associate fellow Catherine Williams (Animal and Veterinary Sciences) we developed the proper anaesthetic protocols needed to study these non-traditional model organisms,” Christian Damsgaard explains about this interdisciplinary research effort.

The AIAS fellowship further provided a window into the labs of other research fields, opening up collaborations that led us to answer questions that would have been impossible in isolation.”

An evolutionary answer with broad implications

In the study, the team of researchers also find the explanation for how tolerance to oxygen deprivation is possible. The pecten oculi, a “comb” in the vitreous of the eye with a previously unknown function, transports sugar into the eye and removes the waste that the retina produces. Together, the results of this study point to a completely new way in which neural tissue can live without oxygen, a chronic anoxia tolerance, and provide an evolutionary answer to how birds have been able to develop sharp vision through a retina without blood vessels.

Extending known boundaries for cellular function

The finding of chronic anoxia tolerance challenges the textbook knowledge that neural tissues are inherently dependent on oxygen.

"We extend the known boundaries for cellular function under extreme conditions. While these findings are important for understanding basic cell biology and animal physiology, they may further stimulate future research within biomedical and clinical research, where oxygen deprivation is a major factor associated with a range of human diseases,” explains Christian Damsgaard. 

In conditions like stroke, human tissues suffer because oxygen delivery is reduced and metabolic waste accumulates. In the bird retina, oxygen deprivation appears to be tolerate in a completely different way as the study reveals.

Co-author of the study, Professor Jens Randel Nyengaard from the Department of Clinical Medicine, Aarhus University, elaborates:

“Nature has solved a physiological problem in birds that makes humans sick. We hope that understanding this evolutionary solution can inspire new ways of thinking about why tissues fail under oxygen deprivation in disease, and how such diseases can be treated.”

The scientific paper behind the results

‘Oxygen-free metabolism in the bird inner retina supported by the pecten’ by Damsgaard et al in: Nature, 21 January 2026.
https://doi.org/10.1038/s41586-025-09978-w

A detailed popular science journalism article on the results

‘Bird retinas function without oxygen – solving a centuries-old biological mystery’ by science journalist Peter F. Gammelby: 
https://bio.au.dk/en/about-biology/news-and-events/show/artikel/fugles-nethinder-lever-uden-ilt-aarhundredgammelt-biologisk-mysterium-er-opklaret

Funding sources

Christian Damsgaard’s study has received funding support through the AIAS-COFUND MSCA Fellowship II programme from the EU Horizon 2020 Research and Innovation programme under the Marie Skłodowska-Curie grant agreement no 754513 and Aarhus Unisversity Research Foundation as well as the Carlsberg Foundation (CF18-0658), the Lundbeck Foundation (R346-2020-1210), the Novo Nordisk Foundation and the Villum Foundation.

Contact

Christian Damsgaard, 2020-2023 AIAS-COFUND MSCA Fellow, Associate Professor
E-mail: cd@bio.au.dk
Department of Biology,
Section for Zoophysiology
Aarhus University
Denmark