Morphology and variability of masticatory structures in the orangutan

Abstract

The present study examines and describes variability and ontogenetic change in the masticatory structures of the orangutan. Mm. temporalis, masseter, pterygoideus lateralis, and pterygoideus medialis as well as m. digastricus are dissected, described, and removed unilaterally for drying from a sample of 10 orangutans. The temporomandibular joint and related structures are also described. Results indicate substantial interindividual variability in muscle morphology as well as possible age-related changes. Age-related changes in m. temporalis include an expansion of attachment area, adjustments in fiber direction, and changes in related bony structures. An increased attachment area is likewise seen in m. masseter in older individuals. Age-related variation is also seen in the position and extent of the origin of m. digastricus as well as in its fiber direction. A comparison of muscle weights indicates that m. pterygoideus lateralis is relatively larger in the neonate, whereas in the older individuals, it is relatively smaller compared to other masticatory muscles. The general morphology of the masticatory structures in the orangutan resembles that of other extant great apes and humans with the exception of m. digastricus. The orangutan differs in possessing only a posterior belly of this muscle.