Chameleon eye position obeys Listing's law

Listing's law (LL) states that 3D-eye positions lie in a plane, when they are described as single-axis rotations from the primary position. This implies that the degrees of freedom of eye movements are reduced from three to two. Various hypotheses exist, regarding the implementation of LL. These include facilitation of binocular vision, optimization of oculomotor control, and mechanical constraints in the orbit. We recorded the 3D-eye position during saccadic scanning in the chameleon, to investigate whether LL is valid in an animal with different anatomical and behavioral characteristics compared to primates. We show that in chameleons, the eye position obeys LL with a high precision. Since the anatomical arrangement of the orbit in chameleons is very different from that in primates, and binocular fused vision is virtually absent, we suggest that in the chameleon, LL mainly optimizes oculomotor control.