In a Rare Biological Twist, Chameleons’ Optic Nerves Coil Like a Telephone Cord

Chameleons see the world in a way that no other animal can, with eyes that move independently but synchronize perfectly when focused. A new study in Scientific reports discovered the structure behind this ability: optic nerves coiled like telephone cords, giving each eye time to rotate nearly 360 degrees without damage.

“Chameleon eyes are like security cameras, moving in all directions,” study author Juan Daza said in a press release. “They move their eyes independently while scanning their surroundings for prey. And as soon as they find their prey, their eyes coordinate and go in one direction so they can calculate where to stick their tongue out.”

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CT scans reveal hidden anatomy of a chameleon

The breakthrough came in 2017, when Edward Stanley, director of the imaging laboratory at the Florida Museum of Natural History, noticed something unusual in a CT scan of a tiny leaf chameleon (Brookesia minima): his optic nerves were coiled and not straight.

“I was surprised by the structure itself, but I was even more surprised that no one else noticed it,” Daza said in the press release. “Chameleons are well studied and people have been doing anatomical studies of them for a long time.”

Traditional dissections had damaged or displaced the delicate nerves, hiding their true shape. Only with modern CT scans have scientists been able to visualize them intact.

“Throughout history, people have looked at the eyes of chameleons because they are interesting,” said Edward Stanley, co-author of the study. “But if you physically dissect the animal, you lose information that can tell the whole story.”

Chameleons’ vision is essential to their slow, deliberate hunting style. They have a prehensile tail, grasping legs, and a tongue that strikes prey at more than twice its body length. Their short, rigid neck limits head movement, so their eyes do most of the work.

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Powerful eye movement of coiled optic nerves

Using contrast-enhanced microCT imaging, the team scanned more than 30 species of lizards and snakes, including three chameleons from different evolutionary lineages. The analyses, part of the open access project oVert (openVertebrate), revealed that each chameleon had optic nerves that looped several times before reaching the eyes, a design not seen in other reptiles.

The nerves were up to three times longer than the straight distance between the brain and the eye, forming spirals that act like springs to prevent strain when the eyes rotate. When the team examined the veiled chameleon’s embryos (Chamaeleo calyptratus), they saw the nerves start straight and twist into loops just before hatching.

“You can compare optic nerves to old phones,” Daza said. “Early phones just had a simple straight cord attached to the headset, but then someone had the idea of ​​coiling the cord and giving it more slack so people could walk further while holding it. That’s what these animals do: they maximize the range of motion of the eye by creating this coiled structure.”

Scientists study chameleon vision in more depth

Even after centuries of study, chameleons still hold surprises. The researchers now want to know whether other arboreal lizards have developed similar optic nerve coils.

“These giants we cited – Newton, Aristotle and others – have inspired natural historians for centuries,” Stanley said. “It’s exciting to be the ones who take the next step on the long road to understanding what’s going on in chameleons.”

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