New study reveals spiraling optic nerves that finally explain the extraordinary, independent vision of colorful reptiles. Only with computerized tomography could the great “brains” have unraveled this mystery.
Chameleons have always intrigued scientists, with their highly mobile and independent eyes, capable of observing almost 360 degrees of the environment.
Now, high-precision imaging research has finally revealed the secret behind this ability: long, spirally coiled optic nerves — a “telephone cord” structure never before seen in other reptiles.
The study, in Scientific Reports and led by Juan Dazaprofessor at Sam Houston State University, and Edward Stanley, director of the digital imaging laboratory at the Florida Museum of Natural History, show that these “spiral nerves” are unique among lizards and fundamental to their unusual vision. According to Daza, chameleons’ eyes function as surveillance camerasmoving in all directions independently. When they detect prey, the eyes coordinate to precisely calculate the point where the tongue — which can reach 100 km/h in just one hundredth of a second — should be projected.
The discovery came about almost by chance, when Stanley was analyzing a CT scan of a tiny leaf chameleon (Brookesia minima). The spiral shape of the nerves caught his attention. Researchers even wondered how it was possible that something so obvious had gone unnoticed during centuries of anatomical studies.
In fact, since Ancient Greece, figures such as Aristotle, Newton and other thinkers had already looked into the ocular anatomy of chameleonsalbeit with wrong interpretations.
Aristotle believed that these animals did not even have optic nerves, directly connecting the eyes to the brain. In the 17th century, the Roman doctor Domenico Panaroli corrected part of this idea, suggesting that the nerves existed, but did not cross each other — which would explain the independence of the eyes. Isaac Newton took up and disseminated this theory in his famous Opticks (1704).
Other scientists, such as French anatomist Claude Perrault, came closer to the truth by drawing nerves crossed and then aligned in a straight line. But none of them identified the winding that is now confirmed. Even in 1852, Johann Fischer partially designed the structure but omitted the spiral. Only in 2015, an Israeli academic thesis described the nerve as “C-shaped”, but without understanding its real complexity, recalls the .
The reason this enigma has persisted for more than two millennia is the limitation of the dissection methods used until recently: when cutting the tissue, the optic nerves ended up deformed or destroyed. Current computerized tomography scans have made it possible to visualize the interior of the skull for the first time without damaging the samples.
The team analyzed more than thirty species of lizards and snakes, including three chameleons. In all, researchers confirmed the presence of longer, more coiled optic nerves than in any other lizard.
When studying embryos of the veiled chameleon (Chamaeleo calyptratus), scientists observed that the nerves are initially straight, forming spirals shortly before hatching. This suggests that curling is a trait acquired during development and essential for full eye mobility in adults.
From an evolutionary point of view, This adaptation appears to have emerged between 16 and 23 million years agowhen chameleons already dominated the arboreal environment. As their necks are not very flexible, they needed another way to expand their field of vision without excessive muscular effort. The spiral optic nerve works like a “coiled telephone cord”, explained Daza: it gives more freedom to the movement of the eyes, allowing them to rotate in all directions without tension.
Since chameleons have limited neck mobility, they probably needed another way to reduce the physical effort of moving their eyes. And the solution appears to be the spiral optic nerve, which provides greater space for the eyes.
