Abstract
Cranial kinesis refers to movements of skeletal sub-units relative to one another at mobile sutures within the skull. The presence and functional significance of cranial kinesis has been investigated in various vertebrates, with much of our understanding coming from comparative studies and manipulation of ligamentous specimens. Drawing on these studies, cranial kinesis in lizards has been modeled as a four-bar linkage system involving streptostyly (rotation of the quadrate), hypokinesis (dorsoventral flexion and extension of the palato-maxillary sub-unit), mesokinesis (dorsoventral flexion and extension of the snout at the fronto-parietal suture) and metakinesis (sliding movements between parietal and supraocciptal bones). In vivo studies, although limited, suggest that cranial kinesis serves an important role during routine behaviors such as feeding. Here, we use X-ray Reconstruction Of Moving Morphology to further quantify mesokinesis in vivo in Gekko gecko during three routine behaviors: gape display, biting and post-ingestion feeding. During gape display, the snout rotates dorsally above rest position, with mesokinesis accounting for a 10% increase in maximum gape over that achieved solely by the depression of the lower jaw. During defensive biting, the snout rotates ventrally below rest position to participate in gape closure. Finally, ventroflexion of the snout also occurs during post-ingestion feeding, accounting for 42% of gape closure during intra-oral transport, 86% during puncture-crushing, and 61% during pharyngeal packing. Mesokinesis thus appears to facilitate prey puncturing by allowing the snout to rotate ventrally so that the upper teeth pierce the prey item, thus limiting the need for large movements of the lower jaw. This is suggested to maintain a firm grip on the prey and reduce the possibility of prey escape. More generally, this study demonstrates that mesokinesis is a key component of defensive biting and gape display behaviors, as well as post-ingestion feeding, all of which are linked to organismal fitness.
Highlights
The skull is a complex anatomical system involved in a variety of behaviors linked to organismal fitness and the adaptive profile of individual species, such as communication with conspecifics, defense, and feeding
We focus on mesokinesis because hypotheses based on the classical model for cranial kinesis suggest that it may be important for gape and bite force production, as well as for feeding [6,7,8,9, 33]
Cranial kinesis in squamate lizards has primarily been studied by manipulating ligamentous specimens [6, 25], analyzing the morphology of intracranial sutures [7,8, 10, 20] and simulation of strain regime [18, 26,27,28,29] in a variety of squamate taxa
Summary
The skull is a complex anatomical system involved in a variety of behaviors linked to organismal fitness and the adaptive profile of individual species, such as communication with conspecifics, defense, and feeding. Cranial kinesis has been of particular interest in large part due to the highly derived and kinetic skulls of ophidian squamates (i.e., snakes) [11,12,13], but the form and function of cranial kinesis has been studied in non-ophidians (i.e., Sphenodon and squamate lizards) (reviewed in [7,8]). Histological analyses reveal that cranial sutures in the squamate skull are more diverse than previously thought, suggesting different patterns of intra-cranial mobility [10, 18,19,20]. The skull of snakes has been shown to be very kinetic in vivo [11,12,13], but evidence is more scarce in lizards (but see [21,22,23])
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