Cranial kinesis in geckoes: Functional implications

Although it is generally assumed that cranial kinesis is a plesiomorphic ch aracteristic in squamates, experimental data tend to contradict this hypoth esis. In particular, coupled kinesis (i.e. streptostyly and mesokinesis) pr esumably arose independently in only a limited number of highly specialised groups. In this study, we investigated cranial kinesis in one of the most specialised of these groups: geckoes, On the basis of cineradiographic and electromyographic data, the fast opening and the slow closing/power stroke phases were modelled to elucidate possible functions of the observed kinesi s, The results of these analyses show that the retraction of the muzzle uni t during crushing is a self-reinforcing system that increases bite force an d reduces the joint forces; the active protraction of the kinetic system du ring jaw opening, in contrast, enhances opening speed through the coupling of the intracranial units. It can be argued that cranial kinesis in geckoes is probably not an adaptive trait as such but, instead, a consequence of t he 'Bauplan' of the cranial system in these animals. Presumably as a result of constructional constraints on the size of the jaw musculature and eyes, the supratemporal and postorbital bars were lost, which resulted in enormo us mobility in the skull. To counteract the potential negative factors asso ciated with this (decrease in bite force, skull damage), the kinetic system may have become coupled, and thus functional.