Chameleons have a distinctive visual system that enables them to see their environment in almost 360 degrees (180 degrees horizontally and +/-90 degrees vertically). They do this in two ways. The first is with anatomical specializations that enable the eyes to rotate with a high degree of freedom. The second is the chameleon’s ability to transition between monocular and binocular vision, meaning they can view objects with either eye independently, or with both eyes together.
Several anatomical features enable chameleons to rotate their eyes to such a high degree. The eyes are located on opposite sides of the head, providing a view to the sides and behind or toward the front. Internally, the eye balls are mounted in twin conical turrets (like two upside down ice cream cones). Without a deep orbital socket to keep the eye from falling out (as in humans), the chameleon has evolved a thick, muscular lid. This lid surrounds each eye turret, leaving only the pupil exposed. This provides a “safety net” that enables the eye to bulge out of the conical turret. Without the restriction of a deep orbital socket, each eye can rotate nearly 180 degrees, giving a much wider range of vision than animals whose eyes are secured in socket structures.
The ability to transition between monocular and binocular vision also enables the chameleon to view objects panoramically. While searching for prey, the chameleon uses monocular vision, with each eye functioning independently of the other. The eye movements–or saccades–are referred to as ‘uncoupled’ when functioning this way. Two separate bundles of nerves control the musculature of the eyes, and two separate images are sent to the brain. Once the chameleon spots its prey, the saccades synchronize, in a process called “coupling,” and both eyes lock on the object. For coupling to occur, visual signals are first sent to the brain through two non-coupled neural bundles. The brain reads these signals, and the eye that has spotted the prey sends stronger electrical impulses to the brain than the eye still searching for the target. The neuron from the eye that does not see the prey syncs with the one that does, forming a larger neural bundle. Once the eye movements are synchronized, the eyes fix on the object and only the head rotates.
The chameleon’s ability to switch freely between synchronous and uncoupled saccadic eye movement is like having two movies playing in your head, and if you wanted to only watch one, you could. This enables the chameleon to operate as both a binocular and monocular organism in a remarkably efficient way for protection, food gathering, and reflexes.
This summary was contributed by Allie Miller.
“Perhaps the strangest of animal eyes belong to the chameleon. They are mounted in twin conical turrets and can move independently of each other, giving the chameleon the ability to see all round itself when seeking prey, and binocular vision in front when it is preparing to strike with its long, sticky tongue.” (Foy and Oxford Scientific Films 1982:127)
“…[T]he chameleon has [sic] highly independent eye movements, with a pattern of alternation of saccadic eye movements between each eye. In the Chameleon, the number of saccades in one eye before the switch to the other, is usually one, but three or four could be observed (see Supplementary material). Photoretinoscopy measurements show that the alternation of each eye also extends to accommodation, which uses visual feedback to control retinal image focus only in the ‘active’ eye. These observations support the interpretation that attention is alternating from eye to eye along with the oculomotor switch…[I]t seems likely that the switching mechanism helps eliminate the ambiguity that would result if both eyes were to simultaneously acquire different prey targets.” (Pettigrew et al. 1999:421-422)
“When no prey item was fixated, disconjugate saccades were observed which was in accordance with earlier observations in chameleons. During prey tracking the chameleons switched to a different oculomotor behaviour and pursued the moving prey with synchronous saccades. At higher target velocities, the tracking movement of the head was also saccadic and was synchronised with the two eyes.” (Ott 2001:173)
“The scleral cartilage (ring) is present and in chamaeleon is formed by 11 scleral ossicles creating a conical form. It is confined to the orbital hemisphere in the scleral layer of eye with the cornea extending out of center. This scleral ossicle is coated with fine muscle fibers from the M. depressor palpebralis inferior of the eyelid just below the surface of the skin. This eyelid depressor muscle extends from the rim of the eyelid ventromedially around the eye in a thin sheet to the ventral and medial aspect of the orbit where it originated on the palatine and interorbital membrane.” (Tolley and Herrel 2013: 44)