Animals do the most amazing things. Read about them here in this series by Janaki Lenin.
It’s impossible to sneak up behind a chameleon. Without taking an eye off the juicy grasshopper, it can swivel the other eye around to watch you.
The chameleon is an ambush predator. It perches motionlessly in trees for interminable lengths of time for insects to come within reach of its long, sticky tongue. If it has to move its head to look in another direction, its cover may be blown. If the startled insect flies away, the chameleon goes hungry. If a raptor spots the slow-moving lizard, it’s dead meat. Independently moving eyes is a splendid trick to overcome this problem. Some birds, fish, and seahorses also have the same talent.
However, chameleon eyes are the coolest among vertebrates in more ways than one. Its eyelids are fused to the eyeballs, leaving a tiny hole for the pupils. The lens within the eye makes the chameleon near-sighted, but the lizard can spot prey 5 to 10 metres away without needing spectacles. A unique combination of a minus-powered lens with a far-sighted cornea turns the chameleon eye into a telephoto lens, projecting a magnified image on the retina. A small insect must appear as big as a bird to a chameleon.
Its domed eyes bulge outwards, so the flat, thin body doesn’t block the view. Scientists say its range of vision is more than 180 degree view horizontally and 90 degree view vertically on each side. In comparison, humans see 80 degrees across and 70 vertically. Translated in English, a chameleon has unrestricted view in any direction without moving its head.
When each eye is looking elsewhere, does one know at what the other is gazing?
Hadas Ketter Katz and Gadi Katzir along with colleagues in the University of Haifa, Israel, have discovered the answer. Their paper, ‘Eye movements in chameleons are not truly independent –evidence from simultaneous monocular tracking of two targets,’ was published recently in The Journal of Experimental Biology.
Ketter Katz explained to The Wire that the most accurate way to test sight abilities is by using computerised images so the experimenter has control over size, speed, contrast, and distance. She trained 13 common chameleons, found around the Mediterranean, to focus on an animated target on a computer screen. Many creatures like toads, praying mantises, and archerfish try to catch virtual insects. For instance, see this toad attempt to catch ants on a phone.
https://www.youtube.com/watch?v=LbNl3J8HXw4
When there’s only a single object to track, chameleons focus both their eyes on it. Ketter Katz’s black rectangle with two little dangling legs apparently resembled an insect closely enough that the chameleons leaned forward and smacked it with their long tongues.
Once Ketter Katz made sure the computer-generated “insect” had their undivided attention, she moved to the main part of the experiment. She split the black rectangle in two, and each travelled across the screen in opposite directions. She says, “They immediately split their gaze and tracked each target monocularly.”
The chameleons continued to watch the two parts of the insect drifting away from each other until they decided which black rectangle they liked better. Once they made that decision, the other eye swung around to track the favoured target along with the first eye. The authors say this demonstrates that each eye knew what the other was watching.
Ketter Katz says she didn’t know what to expect before the experiment. “The unnatural situation of splitting targets has not been tested. I had no idea whether the chameleons will binocularly track one of the targets, or if they will split their gaze and track each of them monocularly as they did.” The lizards weren’t fazed by this strange self-replicating, confounding insect, as if they had seen it all their lives. If anyone was excited, it was Ketter Katz. She confesses, “Just imagine me all alone in the lab screaming and jumping.”
While this is not the first study of chameleons’ astonishing eyeball acrobatics, Ketter Katz says this is the first time anyone has demonstrated how chameleon eyes work “under unnatural conditions, with such high accuracy, and showing that the two eyes are not truly independent as they were thought to be.”
It took Ketter Katz a whole year to build the experimental apparatus in a way that chameleons would be comfortable enough to voluntarily perform the experiment. She didn’t use restraints, but she lured them out of their enclosure with a live cockroach to the experiment area. She rewarded them randomly between tests, so their performance was not influenced by treats.
Professor Hans-Joachim Bischof of Universität Bielefeld, Germany, who studied visual tracking in birds says, “The technique used is very good. It helps to control the initial position of the eyes. What is new is the claim that the animals are tracking two items at the same time with smooth pursuit. This has not been shown before.”
While one puzzle is solved, it raises other questions. How does the brain deal with two different visual inputs at the same time?
Bischof says, if they can watch two objects at the same time, it indicates “they can split the attention of the left and the right [brain] hemispheres, which I doubt without any scientific backing. Katzir and colleagues, however, do not treat this question. But they show the two eyes are not acting fully independently, which has been shown already in birds where there were also claims about independence of eye movements. Overall, the paper nicely refines our knowledge on eye interaction.”
Why do chameleons focus both eyes on the target before sticking out their tongues? Unlike us, they don’t need binocular vision to gauge distances. In other studies, scientists have taped one eye shut, and chameleons shot out their tongues to snag insects accurately. Each remarkable eye is fully capable of estimating distance on its own. So why are their eyes not completely independent of each other?
Nobody knows.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.