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)
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Zoology
The location of the axis of rotation (AoR) of the mandible was quantified using the helical axis (HA) in eight individuals from three species of non-human primates: Papio anubis, Cebus apella, and Macaca mulatta. These data were used to test three hypotheses regarding the functional significance of anteroposterior condylar translation − an AoR located inferior to the temporomandibular joint (TMJ) − during chewing: minimizing impingement of the gonial region on cervical soft tissue structures during jaw opening; avoiding stretching of the inferior alveolar neurovascular bundle (IANB); and increasing jaw-elevator muscle torques. The results reveal that the HA is located near the occlusal plane in Papio and Cebus, but closer to the condyle in Macaca; is located anteroinferior to the TMJ during both opening and closing in Papio, as well as during opening in Macaca and Cebus; and varies in its location during closing in Macaca and Cebus. The impingement hypothesis is not supported by interspecific variation in HA location: species with larger gonial angles like Cebus do not have more inferiorly located HAs than species with more obtuse mandibular angles like Papio. However, intraspecific variation provides some support for the impingement hypothesis. The HA seldom passes near or through the lingula, falsifying the hypothesis that its location is determined by the sphenomandibular ligament, and the magnitudes of strain associated with a HA at the TMJ would not be large enough to cause problematic stretching of the IANB. HA location does affect muscle moment arms about the TMJ, with implications for the torque generation capability of the jaw-elevator muscles. In Cebus, a HA farther away from the TMJ is associated with larger jaw-elevator muscle moment arms about the joint than if it were at the TMJ. The effects of HA location on muscle strain and muscle moment arms are largest at large gapes and smallest at low gapes, suggesting that if HA location is of functional significance for primate feeding system performance, it is more likely to be in relation to large gape feeding behaviors than chewing. Its presence in humans is most parsimoniously interpreted as a primitive retention from non-human primate ancestors and explanations for the presence of anteroposterior condylar translation in humans need not invoke either the uniqueness of human speech or upright posture.
The instantaneous center of rotation of the mandible in nonhuman primates
Kinematic analyses of mandibular movement in humans demonstrate that the mandibular instantaneous center of rotation (ICoR) is commonly located near the level of the occlusal plane and varies in its position during a chewing sequence. Few data are available regarding the location of the ICoR in non-human primates and it remains unclear how the position of the ICoR varies in association with mastication and/or gape behaviors. ICoR was quantified throughout the gape cycle in five species of non-human primates (Macaca mulatta, Cebus apella, Chlorocebus aethiops, Eulemur fulvus, Varecia variegata). The ICoR is commonly located below the mandibular condyle close to the occlusal plane and varies considerably both superoinferiorly and anteroposteriorly through the gape cycle. The path of the ICoR, and by inference condylar movement, in Macaca and Chlorocebus differs from humans whereas movement in Cebus resembles that of humans. Similarities between humans and Cebus in articular eminence and occlusal morphology may explain these resemblances. Food material properties had little influence on ICoR movement parameters.
The relationship between primate mandibular form and diet has been previously analysed by applying a wide array of techniques and approaches. Nonetheless, most of these studies compared few species and/or infrequently aimed to elucidate function based on an explicit biomechanical framework. In this study, we generated and analysed 31 Finite Element planar models of different primate jaws under different loading scenarios (incisive, canine, premolar and molar bites) to test the hypothesis that there are significant differences in mandibular biomechanical performance due to food categories and/or food hardness. The obtained stress values show that in primates, hard food eaters have stiffer mandibles when compared to those that rely on softer diets. In addition, we find that folivores species have the weakest jaws, whilst omnivores have the strongest mandibles within the order Primates. These results are highly relevant because they show that there is a strong association between mandibular biomechanical performance, mandibular form, food hardness and diet categories and that these associations can be studied using biomechanical techniques rather than focusing solely on morphology. Diet is regarded as one of the main factor underlying the behavioural and ecological differences among living primates, and consequently primate diets have been more exhaustively documented than any other aspect of their behaviour 1. A substantial proportion of physiological and anatomical adaptations have as their fundamental objective the transformation of the ingesta that animals consume. Most primates have been habitually interpreted as mainly adapted to fruit consumption 2 , however it has been also acknowledged that some species occupy specific dietary niches ranging from omnivory to the pure folivory 1. Consequently, primates have been classified into three main diet categories: frugivores, folivores, and omnivores. These broad categories are coherent with much of the structural and nutritional characteristics of the food items observed in primates, and thus frugivores, foli-vores, and omnivores have characteristic features that enable them to process their different diets. Furthermore, some primates are adapted to the consumption of hard items (durophagy; hard-food eaters) whereas others are classified as soft-food consumers 3. The relationship between primate mandibular form and loading during biting has been analysed by numerous studies 4. This interest regarding shape and function in the mandible has not been restricted to primates; in fact, other mammalian clades such as Artiodactyla 5, 6 , Chiroptera 7 , and Carnivora 8, 9 have been studied as well. The close interaction between the mammalian feeding mechanism and the ingesta it processes represents a unique opportunity to study ecomorphological adaptations in extant species and potentially acquire valuable tools for the reconstruction of feeding behaviours in extinct taxa as well. The main function of the mammalian mandible is to transfer the forces generated by the masticatory muscles to the ingesta via the teeth. It has been proposed that mandibular shape is mostly involved in ensuring that the forces are transmitted without being dissipated or causing the mandible to fail structurally 10. Mandibular shape is related to diet through the frequency and magnitude of adductor muscle forces engaged during various oral activities. The greater the forces required to fracture food items (or their protective structures), and the more repeatedly such forces need to be produced (e.g. through repetitive biting), the stronger the mandible has to be to maintain its structural integrity 11. This has been experimentally tested by feeding animal with diets of different Published: xx xx xxxx OPEN
Scaling of rotational inertia of primate mandibles
The relative importance of pendulum mechanics and muscle mechanics in chewing dynamics has implications for understanding the optimality criteria driving the evolution of primate feeding systems. The Spring Model (Ross et al., 2009b), which modeled the primate chewing system as a forced mass-spring system, predicted that chew cycle time would increase faster than was actually observed. We hypothesized that if mandibular momentum plays an important role in chewing dynamics, more accurate estimates of the rotational inertia of the mandible would improve the accuracy with which the Spring Model predicts the scaling of primate chew cycle period. However, if mass-related momentum effects are of negligible importance in the scaling of primate chew cycle period, this hypothesis would be falsified. We also predicted that greater " robusticity " of anthropoid mandibles compared with prosimians would be associated with higher moments of inertia. From computed tomography scans, we estimated the scaling of the moment of inertia (I j) of the mandibles of thirty-one species of primates, including 22 anthropoid and nine prosimian species, separating I j into the moment about a transverse axis through the center of mass (I xx) and the moment of the center of mass about plausible axes of rotation. We found that across primates I j increases with positive allometry relative to jaw length, primarily due to positive allometry of jaw mass and I xx , and that anthropoid mandibles have greater rotational inertia compared with prosimian mandibles of similar length. Positive allometry of I j of primate mandibles actually lowers the predictive ability of the Spring Model, suggesting that scaling of primate chew cycle period, and chewing dynamics in general, are more strongly influenced by factors other than scaling of inertial properties of the mandible, such as the dynamic properties of the jaw muscles and neural control. Differences in cycle period scaling between chewing and locomotion systems reinforce the suggestion that displacement and force control are more important in the design of feeding systems than energetics and speed.
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).