Terhune CE, Iriarte-Diaz J, Taylor AB, Ross CF. 2011. The instantaneous center of rotation of the mandible in nonhuman primates. Integ Comp Biol. 51, 320-332. (original) (raw)
The temporomandibular joint (TMJ) is a morphologically and functionally complex component of the skull. Temporomandibular joint shape varies considerably across mammals and within primates, and some aspects of the TMJ have been linked to differences in feeding behavior. However, a broad comparative context describing TMJ variation across primates is lacking. This dissertation therefore evaluated TMJ shape variation in the context of biomechanical hypotheses regarding TMJ function, and in light of phylogenetic and body size variation across anthropoid primates. Three-dimensional geometric morphometrics were used to quantify TMJ shape across a broad sample of 48 anthropoid primates, and more narrowly among small groups of closely related taxa with documented dietary differences. Linear measurements of the TMJ (e.g., glenoid length) were subsequently calculated and compared among taxa. Results of the dietary analyses indicate that taxa with more resistant diets tend to have larger joint surface areas, as well as mediolaterally wider and anteroposteriorly shorter TMJs. Strong correlations were found between glenoid length and measures of gape, suggesting that one way increased gape is achieved is through increased translation at the TMJ. Analyses of scaling in the TMJ found that many variables scaled with positive allometry against cranial and body size, although differences in scaling patterns among platyrrhines, cercopithecoids, and hominoids were identified. In the phylogenetic analysis, genetic and morphological phylogenies were compared and not found to be particularly congruent. This congruence varied across clades, however, and in many instances dietary and body size variation were correlated with morphology, suggesting that TMJ morphology is adaptive. These data highlight the myriad ways in which multiple factors may influence TMJ shape, which may or may not be congruent with known genetic relationships among taxa. Although the TMJ is only a small portion of the skeleton, the morphology of this joint can provide valuable information with which to infer or reconstruct the biology of primate taxa. Ultimately, these data will help to provide a framework for future analyses of primate, and particularly fossil hominin, TMJ variation, and more generally to contribute to the growing body of literature regarding form and function in the primate masticatory apparatus.
The Jaw Adductor Resultant and Estimated Bite Force in Primates
Anatomy Research International, 2011
We reconstructed the jaw adductor resultant in 34 primate species using new data on muscle physiological cross-sectional area (PCSA) and data on skull landmarks. Based on predictions by Greaves, the resultant should (1) cross the jaw at 30% of its length, (2) lie directly posterior to the last molar, and (3) incline more anteriorly in primates that need not resist large anteriorly-directed forces. We found that the resultant lies significantly posterior to its predicted location, is significantly posterior to the last molar, and is significantly more anteriorly inclined in folivores than in frugivores. Perhaps primates emphasize avoiding temporomandibular joint distraction and/or wide gapes at the expense of bite force. Our exploration of trends in the data revealed that estimated bite force varies with body mass (but not diet) and is significantly greater in strepsirrhines than in anthropoids. This might be related to greater contribution from the balancing-side jaw adductors in anthropoids.
Dietary correlates of temporomandibular joint morphology in the great apes
Behavioral observations of great apes have consistently identified differences in feeding behavior among species, and these differences have been linked to variation in masticatory form. As the point at which the mandible and cranium articulate, the temporomandibular joint (TMJ) is an important component of the masticatory apparatus. Forces are transmitted between the mandible and cranium via the TMJ, and this joint helps govern mandibular range of motion. This study examined the extent to which TMJ form covaries with feeding behavior in the great apes by testing a series of biomechanical hypotheses relating to specific components of joint shape using linear measurements extracted from three-dimensional coordinate data. Results of these analyses found that taxa differ significantly in TMJ shape, I use the term ''resistant'' here to collectively refer to foods that are fracture resistant (tough) and/or stress-limited (stiff) . in Wiley Online Library (wileyonlinelibrary.com).
Innovative approaches to the relationship between diet and mandibular morphology in primates.
International Journal of Primatology , 2012
Attempts to establish relationships between mandibular morphology and either traditional dietary categories or geometric and material properties of primate diets have not been particularly successful. Using our conceptual framework of the feeding factors impacting mandibular morphology, we argue that this is because dietary categories and food geometric and material properties affect mandibular morphology only through intervening variables that are currently poorly understood, i.e., feeding behavior, mandibular loading, and stress and strain regimes. Our studies of 3-dimensional jaw kinematics in macaques and capuchins show that, although jaw movement profiles during chewing are affected by food material properties and species-level effects, patterns of jaw movements in these two species are broadly similar. However, because mandibular loading, stress, and strain regimes are determined by interactions between feeding behavior (such as jaw kinematics) and mandibular morphology, it is difficult to say whether these similarities in chewing kinematics also mean similarities in loading, stress, and strain. Comparative analyses of the scaling of daily feeding time and chew cycle duration reveal only weak support for the hypothesis that larger primates chew more than smaller primates. Consideration of these results suggests that better data are needed on the relationship between dietary categories, food material and geometric properties, the amount of time/cycles associated with different feeding behaviors (ingestion, premolar biting, mastication), and mandible stress and strain patterns if we are to understand fully relationships between mandibular morphology and diet in primates.
Jaw muscle function and wishboning of the mandible during mastication in macaques and baboons
American Journal of Physical Anthropology, 1994
An analysis of in vivo bone strain indicates that the mandibular symphysis of macaques experiences lateral transverse bending or "wishboning" during the power stroke of mastication, and this loading regime results in relatively intense concentrations of stress along the lingual aspect of the symphysis . It has been hypothesized that peak wishboning of the macaque mandible, which probably occurs at the very end of the power stroke, that is, after the initial occurrence of maximum intercuspation, is associated with the late peak activity of the balancing-side deep masseter muscle couded with the raDid decline in the activity of the balancing-side medial pterygoid and superficial masseter muscles .
A number of researchers have suggested a functional relationship between dietary variation and temporomandibular joint (TMJ) morphology, yet few studies have evaluated TMJ form in the African apes. In this study, I compare TMJ morphology in adults and during ontogeny in Gorilla (G.g. beringei, G.g. graueri, and G.g. gorilla) and Pan (P. paniscus, P. troglodytes troglodytes, P.t. schweinfurthii, and P.t. verus). I test two hypotheses: first, compared to all other African apes, G.g. beringei exhibits TMJ morphologies that would be predicted for a primate that consumes a diet comprised primarily of moderately to very tough, leafy vegetation; and second, all gorillas exhibit the same predicted morphologies compared to Pan. Compared to all adult African apes, G.g. beringei has higher rami and condyles positioned further above the occlusal plane of the mandible, relative to jaw length. Thus, mountain gorillas have the potential to generate relatively more muscle force, more evenly distribute occlusal forces along the postcanine teeth, and generate relatively greater jaw adductor moment. G.g. beringei also exhibits relatively wider mandibular condyles, suggesting these folivorous apes are able to resist relatively greater compressive loads along the lateral and/or medial aspect of the condyle. All gorillas likewise exhibit these same shape differences compared to Pan. These morphological responses are the predicted consequences of intensification of folivory and, as such, provide support for functional hypotheses linking these TMJ morphologies to degree of folivory. The African apes to not, however, demonstrate a systematic pattern of divergence in relative condylar area as a function of intensification of folivory. The ontogenetic trajectories for gorillas are significantly elevated above those of Pan, and to a lesser but still significant degree, mountain gorillas similarly deviate from lowland gorillas (G.g. gorilla and G.g. graueri). Thus, adult shape differences in ramal and condylar heights do not result from the simple extrapolation of common growth allometries relative to jaw length. As such, they are suggestive of an adaptive shift towards a tougher, more folivorous diet. However, the allometric patterning for condylar area and condylar width does not systematically conform to predictions based on dietary specialization. Thus, while differences in condylar shapes may confer functional advantages both during growth and as * Tel.: C44 919 6683016/44 919 6683021; fax: C44 919 6683024. Journal of Human Evolution 48 (2005) 555e574
American Journal of Physical Anthropology, 1978
Incisal bite force direction was recorded and analyzed in ten human subjects using a specially designed force transducer. In all ten subjects the maxillary incisal bite force was vertically and anteriorly directed both during static biting and during biting associated with simultaneous mandibular translation and rotation. Since the resultant muscle force could not have been equal and opposite to the mandibular bite force, the mandibular condyles must have been loaded. These data demonstrate that the mandible acts as a lever during incisal biting and that there is no consistent relationship between incisal bite force direction and object size. In some individuals the bite force direction was more vertical during biting on a large transducer (30 mm high), while in other subjects it was more vertical during biting on a small transducer (10 mm high).