The chameleon's mesmerizing, independently moving eyes have fascinated and baffled scientists since ancient Greek times. Following thousands of years of speculation and research, modern imaging technology has finally revealed the secret behind their nearly 360-degree vision and ability to look in two directions at once: Behind each bulging eye is a unique long, spiraling optic nerve that has never been found in other lizards.
"Chameleons' eyes are like surveillance cameras that can move in all directions," said Juan Daza, associate professor at Sam Houston State University and author of the latest study. "When they search for prey, their eyes can operate independently. Once the target is found, the two eyes will coordinate to point in the same direction to calculate the location of the tongue."
Although the chameleon's quick and agile gaze is easy to detect, its inner workings have long been a mystery. In 2017, Edward Stanley, director of the Digital Imaging Laboratory at the Florida Museum of Natural History, observed an unusual spiral structure in the optic nerve of the Brookesia minima through CT scanning in the Dasa laboratory, which he had never encountered before.
Both researchers were skeptical at first. Because chameleons have been dissected and studied so many times, it was hard for them to believe that they would be the first to discover this hidden trait. "I was surprised by the structure itself, and even more surprised that no one had noticed it before," Daza said. "Chameleons have been widely studied and have a long history of related anatomy research."
Chameleons, which span Africa, Europe and Asia, have evolved unique adaptations for living in trees. Their tails can grasp branches like hands, and their limbs are as dexterous as gloves, allowing them to move slowly in the treetops. Chameleons are not known for their speed, but they have a powerful spring tongue that can accelerate from standstill to 60 miles per hour in just 0.01 seconds. The tongue can be twice as long as their body, catching insects almost instantly.
The chameleon's unique shape and spiral tail are also clearly visible in ancient Egyptian petroglyphs. The research team believed that relevant descriptions should be found in various documents, so they extensively searched old documents and asked language experts to translate them into French, Italian and Latin.
Two thousand years ago, the Greek philosopher Aristotle mistakenly believed that chameleons had no optic nerve and that their eyes were directly connected to the brain, allowing them to move independently. Domenico Panaroli, a 17th-century Roman physician, believed that chameleons had optic nerves, but they were not crossed like other animals. Therefore, he theorized that chameleon eyes could move freely. Newton also mentioned chameleons many times in his book "Optiks" and adopted Panaroli's theory. In 1669, the French anatomist Claude Perrault drew an accurate image of the optic nerve crossing and then extending in a straight line. Unfortunately, it was ignored by everyone at the time.
In 1842, John Fisher also showed part of the spiral structure in his study of lizard brains and nerves, but it was not fully reflected in the diagram and was not described in detail. In 2015, Lev-Ari Thidar, a master's student at the University of Haifa, described the C-shaped structure of the chameleon's optic nerve in his paper, showing that the scientific community had previously been unable to fully reveal this secret.
Why have scientists failed to discover the true shape of the chameleon's optic nerve for so long? The key lies in CT scans and open data. Traditional dissection methods often destroy or displace the optic nerve, making it difficult to visualize the structure. Advanced X-ray CT technology is now widely available, allowing scientists to directly observe the internal structure of specimens without damaging them.
Using the oVert (openVertebrate) digital 3D vertebrate data platform, the research team analyzed more than 30 species of lizards and snakes, including 3 species of chameleons, and compared their optic nerve morphology. As a result all chameleons have significantly longer, more convoluted optic nerves.
Further research also found that this unique structure gradually forms during the embryonic development of chameleons. The optic nerve is straight at the earliest stage, and gradually elongates and spirals before hatching. When hatching, the eyes can move freely.
At the long-term evolutionary level, it is difficult to determine the specific time when this feature first appeared in chameleons. The earliest chameleon fossils date from 16 million to 23 million years ago, and many arboreal characteristics have evolved. The new findings help scientists speculate on why this structure evolved.
Animals with big eyes usually have two ways to expand their field of vision: turning their necks or turning their eyes. Owls and lemurs fall into the former category, while animals such as humans have developed stretchable optic nerves. Rodents also have wavy nerve fibers that promote flexibility.
Given that chameleons have limited neck movement, the convolution of the optic nerve is likely an attempt to relieve the physical stress of large eye movements. This is a rare evolution, similar to the neural form of the stalk-eye fly. Daza vividly said: "Just like the wires of an old telephone, they were originally connected in a straight line. Later, someone invented a coiled structure to facilitate movement. The same is true for the visual mechanism of the chameleon, which uses nerve coils to increase the space for the eyeballs to move."
Scientists say that despite thousands of years of observation and research, the animal world still has many mysteries to answer. In the future, they will also explore whether other arboreal lizards have similar evolutionary structures.
"Scientific giants such as Newton and Aristotle have inspired natural history scholars for centuries," Stanley said. "We are excited that we can take a new step in understanding the strange structure of chameleons."
Compiled from /ScitechDaily