One of the things chameleons are known for, aside from their colour-changing skin, is their rather marvellous eyes. With top and bottom eyelids fused together, the chameleon peers through a small hole left between them; but the entire structure can pivot, giving the chameleon 360-degree vision.
Moreover, each eye can move independently, which means that the chameleon can be looking at two different things at once. But, as it turns out, a chameleon's eyes aren't completely independent from one another after all: the left eye does know what the right eye is doing.
And the reason we know this is because of a computer game designed just for chameleons.
Ehud Rivlin, professor of computer science at the Technion-Israel Institute of Technology, and Gadi Katzir, professor of biology at the University of Haifa, Israel, developed a program designed to frustrate the animals, but also test whether their swivelling eyes can work together.
Their research was published in the July issue of the Journal of Experimental Biology.
The chameleons were shown a virtual insect moving across a screen. First, the chameleon focused the insect with one eye, while the other eye continued looking around elsewhere. But once the chameleon decided wanted to catch the virtual insect, it focused both eyes on it a split-second before shooting out its sticky tongue.
Then, the chameleons were shown two insects, moving in different directions across the screen. Initially, the chameleons were confused, but once they had decided to focus on one of the insects, the same thing happened: both eyes focused on the chosen insect a split second before the chameleon let its tongue fly.
"There were a few seconds of indecision when the chameleons were deciding which target to shoot at," Rivlin said in a statement.
"If the eyes were truly independent, one would not expect one eye to stay put and then have the other eye converge. But we found that once the chameleon made its decision about which target to fire on, it swivelled the second eye around to focus on the same simulated fly the first eye was locked on."
This suggests, the researchers said, that the two eyes engage in some kind of "cross-talk". When this happens, the two eyes focus together on the target. This gives the chameleon stereoscopic depth perception to gauge the distance to its prey, rather than monocular depth perception as had been previously supposed.
There are other animals that have truly independent eyes; some fish, for instance, and birds (although no mammals have monocular vision). In these animals, which aren't uncommon, each eye is linked to the opposite hemisphere of the brain, and no "cross-talk" occurs between the eyes. Because this model isn't rare, it was fair to assume chameleons worked the same way.
But, just like many other things about the chameleon, their vision works a little differently.
"When chameleons are presented with two small targets moving in opposite directions, they can perform simultaneous, smooth, monocular visual tracking," Rivlin said.
"To our knowledge, this is the first demonstration of this capacity. We suggest that in chameleons, eye movements are not simply 'independent,' but are disconjugate during scanning, conjugate during binocular (two-eye) tracking, and disconjuate but coordinated during monocular (one-eyed) tracking. Each eye is aware of its own location and the orientation of the other eye. It is just a different kind of cooperation that takes place in human binocular vision."