Craniofacial integration, plasticity and biomechanics in the mouse masticatory system

The craniomandibular skeleton is a complex, dynamic structure, housing many vital tissues and required to perform critical functions. This region is however subject to substantial morphological change during development, and required to adapt to its environment and individual variance. The capacity of this region to maintain correlated form and appropriate functional performance despite these challenges is not fully understood. The sample consists of three strains of mice; a wild-type strain and two mutant strains from the same genetic background strain. Both mutations selectively affect chondrocranial growth, and thus influence of both are limited to the crania. The brachymorph mutant phenotype is characterised by a shortened cranium, while the pten is elongated. This sample therefore allows exploration of a potential plastic response in terms of the mandible, the masticatory lever system, and in turn mechanical advantage, when cranial length and the out-lever are varied. Three dimensional landmarks were applied to micro-CT scans and partial-least-squares analysis carried out to determine covariance between crania and mandibles. Mechanical advantage was calculated as a ratio of muscle in-lever and jaw out-lever for three key masticatory muscles. A common pattern of both variance and covariance was found among all three strains, with mandibular morphology in each strain covarying with cranial phenotypes. Jaw out-lever lengths were found to be significantly different in all three strains, and yet little significant difference between strains was found in mechanical advantage for any muscles. This maintenance of mechanical advantage is attributed to plastic adaptation in regions influencing muscle in-lever length, the latter which were found to be significantly different in the three strains. These results show the potential of the craniomandibular complex to plastically adapt to maintain both correlated form and functionality when variation occurs in one region, and thus these results have significant implications for the evolvability of the complex

Masticatory muscle anatomy and feeding efficiency of the American beaver, Castor canadensis (Rodentia, Castoridae)

Beavers are well-known for their ability to fell large trees through gnawing. Yet, despite this impressive behavior, little information exists on their masticatory musculature or the biomechanics of their jaw movements. It was hypothesized that beavers would have a highly efficient arrangement of the masticatory apparatus, and that gnawing efficiency would be maintained at large gape. The head of an American beaver, Castor canadensis, was dissected to reveal the masticatory musculature. Muscle origins and insertions were noted, the muscles were weighed and fiber lengths measured. Physiological cross-sectional areas were determined, and along with the muscle vectors, were used to calculate the length of the muscle moment arms, the maximum incisor bite force, and the proportion of the bite force projected along the long axis of the lower incisor, at occlusion and 30° gape. Compared to other sciuromorph rodents, the American beaver was found to have large superficial masseter and temporalis muscles, but a relatively smaller anterior deep masseter. The incisor bite force calculated for the beaver (550–740 N) was much higher than would be predicted from body mass or incisor dimensions. This is not a result of the mechanical advantage of the muscles, which is lower than most other sciuromorphs, but is likely related to the very high percentage (>96 %) of bite force directed along the lower incisor long axis. The morphology of the skull, mandible and jaw-closing muscles enable the beaver to produce a very effective and efficient bite, which has permitted beavers to become highly successful ecosystem engineers