A new twist in the evolution of chameleons uncovers an extremely specialized optic nerve morphology

Chameleons are unique among lizards for their exceptional suite of anatomical modifications which has allowed them to adapt to and diversify in arboreal environments. Specializations shared by members of this lineage include a trunk with a reduced number of presacral vertebrae, a body that can be mediolaterally compressed or expanded, reduced flexibility in the trunk and neck, grasping hands and feet, a prehensile and non-autotomizing tail, highly developed and independently movable eyes, and a ballistic tongue. Here we use contrast enhanced computed tomography to describe the arrangement of optic nerves of chameleons and compare them with those of other squamates. Results from dissection and 3D modeling of the brain and associated structures reveal that, in comparison with other squamates (and across tetrapods in general), chameleons have evolved a longer, coiled optic nerve that is likely to provide "slack" to reduce tension on the optic nerve during the extensive rotations characteristic of the chameleon eye. Despite significant study on the visual system of chameleons, our use of non-destructive CT in examining neural cranial anatomy of whole specimens has revealed a previously unreported adaptation that is part of an interconnected suite of features that chameleons have evolved in conjunction with life in a complex three-dimensional arboreal environment.