Chameleon vision

A suggested theory for the evolution of squamate vision is that corneal accommodation and monocular depth perception are "primitive" mechanisms in comparison to binocular vision and stereopsis.[3] Chameleons use an alternative strategy to stereopsis in functional coupling of the eyes immediately before the tongue shot. This differs from stereopsis in that the images from both eyes are not reconciled into one. However, it is possible that this was first used for neural static reduction.[3] This suggests that chameleons could be seen as a transition between independent and coupled eye use.[8] However, it is also possible that the chameleon vision system is an alternative, equally successful mode of prey capture and predator avoidance, and perhaps more appropriate for the chameleon's niche as a camouflaged, arboreal hunter than other vision systems.

The chameleon, a camouflaged, slow-moving lizard, is an arboreal hunter that hides and ambushes prey.[2] Prey and predators alike can be sighted and monitored using monocular depth perception. Also, nodal point separation allows distance to be judged with one eye, so minimal head movement is needed by the chameleon in watching its surroundings, reinforcing the chameleon strategy of inconspicuousness.[5]

The specialized strategy by which chameleons capture prey is reflected in their sensory anatomy and use, in particular that of vision.[2] First, prey is sighted and distance assessed using one eye.[6] To avoid detection by prey, a chameleon uses minimal head movement, made possible by nodal point separation.[5] The chameleon then slowly turns its head toward the prey. Both eyes focus independently on the prey before the tongue shot.[3]

The chameleon predator avoidance response is vision-mediated.[1] In predator avoidance, chameleons use minimal head movement and a unique method to monitor potential threats. Due to nodal point separation, a chameleon can judge distance to a potential threat with minimal head movement needed. When confronted with a potential threat, chameleons rotate their slender bodies to the opposite side of their perch to avoid detection.[1] They will keep moving around the branch to keep the branch between themselves and the threat and to keep the threat in their line of sight.[1] If the branch is narrow, a chameleon can observe a threat binocularly around the branch. While a wide branch might present a difficulty in depth perception to another lizard as it is forced to view the threat monocularly, a chameleon due to corneal accommodation and nodal point separation can judge distance between itself and a potential threat with only one eye viewing the threat.[1]

While the chameleon eye is unique in lizards, parallels exist in other animals. In particular, the barred sandburrower fish shares key vision features with the chameleon. This is because the environmental circumstances such as the need for camouflaged quick prey capture that led to the development of the chameleon eye seem to have acted on the sandburrower fish as well.[7] Rapid predatory attacks are made possible through the chameleon and the sandlances' striated corneal muscles allowing for corneal accommodation, a reduced power lens, and increased corneal power.[3] A nearly complete eyelid covering and lack of head movement due to nodal separation reduce conspicuousness to prey and predators.[7]