Functional morphology of a highly specialised pivot joint in the cranio-cervical complex of the minute lizard Ablepharus kitaibelii in relation to feeding ecology and behaviour

Abstract

The snake-eyed skink Ablepharus kitaibelii is one of the smallest European lizards, but despite its minute size it is able to feed on comparatively large prey. Here we investigate the diet of A. kitaibelii and the mechanisms that allow the skink to overpower relatively large and even noxious prey. High-speed cinematography showed that A. kitaibelii uses a series of shaking and battering movements to immobilise and kill prey prior to swallowing. During this process, the skinks rises up on the hind limbs and then whacks the prey sidewise on the substrate by twisting the trunk, neck and head laterally. Our analysis showed that the shaking kinematics is very uniform among the investigated specimens. The morphological investigation of the cranio-cervical system revealed that A. kitaibelii possesses a well-developed synovial joint between the odontoid process of the axis, the atlas, and the basioccipital. The odontoid process is cylindrical and slim and together with the atlas and the basioccipital it forms a highly specialised pivot joint for lateral head rotation. We propose that the occipito-atlanto-axial complex of A. kitaibelii represents a functional adaptation for additional stabilisation of the cranio-cervical complex during prey shaking. Digital data from morphological databases showed that specialised joints of this type are very rare, but do also occur in other squamate groups. Thus we hypothesise that specialised cranio-cervical joints have evolved parallel as functional adaptations to different feeding and locomotion patterns. Future studies that link feeding kinematics and locomotion to cranio-cervical morphology might elucidate the function of various specialised occipito-atlanto-axial systems of squamates.