Chameleons’ bulging eyes do strange things; they swivel in different directions like periscopes as they peer around their surroundings. One eye may be fixed on you, while the other eye is trained on an insect scuttling away.
Though scientists have been fascinated by this ability for centuries, the physiological mechanism that makes it possible has remained mysterious. Until now.
A team of researchers has discovered that chameleons possess two coiled optic nerves that operate independently to bring two separate scenes into the reptile’s focus. They report their findings in a Scientific Reports paper published today.
“Chameleon eyes are like security cameras, moving in all directions,” said biologist Juan Daza at Sam Houston State University, the lead author of the paper, in a statement. “They move their eyes independently while scanning their environment to find prey. And the moment they find their prey, their eyes coordinate and go in one direction so they can calculate where to shoot their tongues.”
Read more: “Seeing Through Animal Eyes”
In the new study, Daza and his colleagues examined the optic nerves of three species of chameleon that differ in body size, evolutionary history, and behavior. The brown leaf chameleon, Brookesia superciliaris, is about the length of a golf tee and lives on the ground or in shrubby vegetation. The bearded pygmy chameleon, Rieppeleon brevicaudatus, is a bit larger, about crayon-sized, and similar in its habitat use. The veiled chameleon, Chamaeleo calyptratus, is the big one of the bunch, nearly a foot long and thought to be entirely tree-dwelling. By comparing disparate species, the researchers hoped to ascertain which features of the optic nerve structure were common across chameleons.
High-resolution CT scanning corroborated an earlier observation that a chameleon controls each of its eyes independently but can coordinate eye movements when locking in on a target. Daza and his team found that this was possible though the use of optic nerves coiled up like old-fashioned telephone cords. The coiling showed up in all three species. This coiling gave each species far more optic nerve length than would typically be present, allowing wild swings of their eyes with plenty of slack in their nervous tissue.
When a chameleon’s eye swivels to look at something, the coiled optic nerve unfurls to give it the length it needs. Without coiling, the study authors hypothesize that the nerve would likely experience strain from these radical eye wanderings. Examining scans of chameleon embryos, the study authors noted that the optic nerves are initially straight during development—like in other reptiles—in the egg, but they lengthen and coil up by hatching.
The discovery sets chameleons apart from other animals, who employ different adaptations to swivel their gaze without straining the optic nerve. For example, owls dramatically rotate their necks while keeping their eyeballs steady. In rats, the optic nerves are stretchy, providing them with a little extra give for gazing around.
Chameleons, it appears, might have evolved an approach that is unique in the animal kingdom, making this startling creature even more of an evolutionary marvel. 
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Lead image: LuckyStep / Shutterstock
