By creating a computer game for chameleons, scientists have found that the creatures’ eyes are not really independent, as long believed, but rather engage in some sort of “cross talk.”
Called “voluntary strabismus,” the ability to swivel their eyes each in a different direction enables the chameleon to minimise its body movements to hide from those it hunts as well as to those that might hunt them.
To find whether the reptile’s eyes really move independent of each other, researchers got chameleons to play a computer game to possibly solve the riddle of whether chameleons really enjoy ‘eye independence.’
When the researchers showed chameleons a double image of a tiny insect moving opposite directions across a computer screen, the reptiles focused first on one image with one eye while the other eye “wandered.”
Suddenly, both eyes locked on one image a nanosecond before the reptile cocked its dart-like, sticky tongue and fired at-will.
“There were a few seconds of indecision when the chameleons were deciding which target to shoot at,” said Ehud Rivlin of the Technion-Israel Institute of Technology.
“If the eyes were truly independent, one would not expect one eye to stay put and then have the other eye converge,” Rivlin said.
This behaviour pattern, said the researchers, suggests that the second eye has knowledge of where the first eye is directed.
That the chameleons are able to track objects moving in opposite directions before deciding which one to target suggests that their eyes are not really independent, as many have believed.
“Their eyes possibly engage in some kind of ‘cross talk,'” said Rivlin.
When chameleon eye cross talk occurs, when the second eye locks onto the same target as the first, the expert predator of tiny insects gets the necessary stereoscopic depth perception at just the right moment for accurate striking.
According to the researchers, many animals – especially fish and birds, but not mammals – have wandering, independent eyes, with each eye connected to the opposite side of the brain so that the left part of the brain knows what the right eye is doing, and vice-versa. But chameleons are different.
“We demonstrated that when chameleons are presented with two small targets moving in opposite directions, they can perform simultaneous, smooth, monocular visual tracking,” said Rivlin.
“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,” Rivlin said.
The study was published in the Journal of Experimental Biology.