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Journal ArticleDOI

The Functional Significance of Primate Mandibular Form

01 May 1979-Journal of Morphology (J Morphol)-Vol. 160, Iss: 2, pp 223-239
TL;DR: A stress analysis of the primate mandible suggests that vertically deep jaws in the molar region are usually an adaptation to counter increased sagittal bending stress about the balancing‐side mandibular corpus during unilateral mastication.
Abstract: A stress analysis of the primate mandible suggests that vertically deep jaws in the molar region are usually an adaptation to counter increased sagittal bending stress about the balancing-side mandibular corpus during unilateral mastication. This increased bending stress about the balancing side is caused by an increase in the amount of balancing-side muscle force. Furthermore, this increased muscle force will also cause an increase in dorso-ventral shear stress along the mandibular symphysis. Since increased symphyseal stress can be countered by symphyseal fusion and as increased bending stress can be countered by a deeper jaw, deep jaws and symphyseal fusion are often part of the same functional pattern. In some primates (e.g., Cercocebus albigena), deep jaws are an adaptation to counter bending in the sagittal plane during powerful incisor biting, rather than during unilateral mastication. The stress analysis of the primate mandible also suggests that jaws which are transversely thick in the molar region are an adaptation to counter increased torsion about the long axis of the working-side mandibular corpus during unilateral mastication. Increased torsion of the mandibular corpus can be caused by an increase in masticatory muscle force, an increase in the transverse component of the postcanine bite force and/or an increase in premolar use during mastication. Patterns of masticatory muscle force were estimated for galagos and macaques, demonstrating that the ratio of working-side muscle force to balancing-side muscle force is approximately 1.5:1 in macaques and 3.5:1 in galagos during unilateral isometric molar biting. These data support the hypothesis that mandibular symphyseal fusion is an adaptative response to maximize unilateral molar bite force by utilizing a greater percentage of balancing-side muscle force.
Citations
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Journal ArticleDOI
TL;DR: Data from two experiments support the hypothesis that fatigue microdamage is a significant factor in the initiation of intracortical bone remodeling.

759 citations

Journal ArticleDOI
TL;DR: The details of occlusal microwear indicate that the diets of Australopithecus and Paranthropus were qualitatively different, and the inferred dietary differences and the differences in trophic adaptations do not appear to be related solely to purported differences in estimated body sizes.

432 citations

Journal ArticleDOI
Richard F. Kay1
TL;DR: Members of the Ramapithecinae, the stock which probably gave rise to Pliocene-Recent hominids, had very thick molar enamel, which suggests that they ate hard seeds, nuts, and fruits previously available only to arboreal rodents and forest-floor pigs.
Abstract: Molar enamel is thicker among frugivorous extant Old World monkeys and apes than among their folivorous close relatives. Furthermore, species that have the thickest molar enamel reportedly eat fruits, seeds, and nuts that are so hard that they cannot be broken by their sympatric thinner-enameled relatives. Species with relatively thick enamel show no tendency toward a terrestrial feeding pattern. Members of the Ramapithecinae, the stock which probably gave rise to Pliocene-Recent hominids, had very thick molar enamel. This suggests that they ate hard seeds, nuts, and fruits previously available only to arboreal rodents and forest-floor pigs. There is no reason to believe that these anatomical features had to evolve in non-rain-forest environments, as others have argued.

378 citations

Journal ArticleDOI
TL;DR: The data suggest that during the power stroke of mastication, the macaque symphysis is predominately sheared dorsoventrally and/or twisted about a transverse axis and bent by lateral transverse bending of the mandibular corpora.
Abstract: The primary purpose of this study was to test various hypotheses about symphyseal stress in primates. First, those patterns of symphyseal strain that would be associated with various hypothetical patterns of symphyseal stress were formulated. Then these hypothetical patterns of stress and strain were tested by comparing the formulated bone strain pattern with actual in vivo symphyseal bone strain patterns. Patterns of in vivo symphyseal bone strain were determined by bonding rosette and/or single-element strain gages to the midline of the middle and lower third of the labial aspect of the symphysis of six adult Macaca fascicularis. Following recovery from the anesthetic, bone strain was recorded during mastication, incision, and isometric biting. Symphyseal bone strain was also recorded during yawning, licking, and threat behaviors. The data suggest that during the power stroke of mastication, the macaque symphysis is predominately sheared dorsoventrally and/or twisted about a transverse axis and bent by lateral transverse bending of the mandibular corpora. During lateral transverse bending of the mandibular corpora, the labial aspect of the macaque symphysis experiences compressive bending stress, while the lingual aspect experiences tensile bending stress. During the opening stroke of mastication and during other jaw opening behaviors, the macaque symphysis is bent by medial transverse bending of the mandibular corpora. At this time the labial aspect of the symphysis experiences tensile bending stress, while its lingual aspect experiences compressive bending stress. During both the power and opening strokes of mastication, the macaque mandible is bent in the plane of its curvature, and therefore the mandible acts as a curved beam. This is important because it results in elevated levels of stress along the lingual aspect of the macaque symphysis, particularly during the power stroke of mastication. During the power stroke of incision, the local effects of the bite force are unknown; however, at this time the lower half of the macaque symphysis is both sheared dorsoventrally and bent due to twisting of the mandibular corpora about their long axes. The results of this stress analysis have implications for understanding the mechanical attributes of symphyseal structure. In order to counter dorsoventral shear, the most important symphyseal attribute is to have adequate cross-sectional area of bone in the plane of the applied stress. In contrast, both the cross-sectional area of bone and symphyseal shape is important in order to counter stress effectively during symphyseal torsion and the three symphyseal bending regimes.(ABSTRACT TRUNCATED AT 400 WORDS)

364 citations

Journal ArticleDOI
TL;DR: The stress analysis and an allometric analysis of mandibular dimensions in female cercopithecine (Old World) monkeys indicates that allometric changes in the symphysis are readily understood if the mandible is modelled as a curved beam.
Abstract: Patterns of stress were analyzed in the mandibular symphysis of Macaca fascicularis using rosette strain gages. During jaw opening, the mandibular symphysis is bent due to medial transverse bending of the mandibular corpora. Levels of stress and strain are relatively low at this time, and the source of this stress is the medially-directed component of force from the lateral pterygoid muscles. During the power stroke of mastication, the symphysis is maximally stressed. At this time the symphysis experiences dorsoventral shear and bending due to lateral transverse bending of the mandibular corpora, i.e., “wishboning.” The dorsoventral shear is due to the vertical component of the balancingside adductor muscle force; the “wishboning” is due to the laterally-directed components of the bite and jaw adductor muscle forces. Unlike dorsoventral shear, “wishboning” results in considerable levels of stress and strain, particularly along the most lingual aspect of the symphysis. The most effective way to counter this stress is to increase the thickness of the symphysis in the labio-lingual direction. The stress analysis and an allometric analysis of mandibular dimensions in female cercopithecine (Old World) monkeys indicates that allometric changes in the symphysis are readily understood if the mandible is modelled as a curved beam. With increasing body size, symphyseal thickness in cercopithecines must increase in a positively allometric fashion so as to prevent the occurrence of dangerously high levels of stress along the most lingual aspect of the symphysis. This is because increasing body size is associated with three factors thathave important consequences within the context of the biomechanics of curved beams: (1) jaw length is positively allometric to body size, (2) mandibular-arch width is negatively allometric to body size, and (3) there is a tendency to use relatively greater amounts of balancing-side jaw muscle force with increased body size because of dietary changes and allometricconstraints on total jaw muscle force.

326 citations

References
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Book
01 Jan 1976
TL;DR: This book reviews biological structural materials and systems and their mechanically important features and demonstrates that function at any particular level of biological integration is permitted and controlled by structure at lower levels of integration.
Abstract: This book deals with an interface between mechanical engineering and biology. Available for the first time in paperback, it reviews biological structural materials and systems and their mechanically important features and demonstrates that function at any particular level of biological integration is permitted and controlled by structure at lower levels of integration. Five chapters discuss the properties of materials in general and those of biomaterials in particular. The authors examine the design of skeletal elements and discuss animal and plant systems in terms of mechanical design. In a concluding chapter they investigate organisms in their environments and the insights gained from study of the mechanical aspects of their lives.

1,407 citations

Book
01 Jan 1953

750 citations

Book ChapterDOI
01 Mar 1970
TL;DR: In this article, an attempt to reopen the problem of origins by examining critically some of the existing models of hominid differentiation, and to suggest a new one based on a fresh approach.
Abstract: Despite years of theorising, and a rapidly accumulating body of fossil evidence, physical anthropology still lacks a convincing causal model of hominid origins. Diverse lines of evidence point to a later common ancestry with the African pongids than with any other living primate, and studies of hominid fossils of the Basal and Early Lower Pleistocene (Howell I967) have elucidated the complex of characters which at that time distinguished the family from African and other Pongidae (Le Gros Clark I964). It is also possible to argue that the elements of the complex form a mutually reinforcing positive feedback system. Bipedalism frees the forelimb to make and use artefacts; regular use of tools and weapons permits (or causes) reduction of the anterior teeth by taking over their functions; the elaboration of material culture and associated learning is correlated with a cerebral reorganisation of which increase in relative cranial capacity is one aspect. Bipedalism is needed to permit handling of the relatively helpless young through the long period of cultural conditioning, and so on. Preoccupied with the apparent elegance of the feedback model, we tend to forget that to demonstrate the mutual relationship between the elements is not to account for their origin, and hence does not explain whty the hominids became differentiated from the pongids, or why this was achieved in the hominid way. From their very circularity, feedback models cannot explain their own beginnings, except by tautology, which is no explanation at all. In fact, the more closely the elements of the hominid complex are shown to interlock, the more difficult it becomes to say what was responsible for setting the feedback spiral in motion, and for accumulating the elements of the cycle in the first place. Most authors seem either to avoid the problem of origins and causes altogether (beyond vague references to 'open country' life), or to fall back upon reasoning that tends to be teleological and often also illogical. This article is an attempt to reopen the problem of origins by examining critically some of the existing models of hominid differentiation, and to suggest a new one based on a fresh approach.

541 citations

Journal ArticleDOI
TL;DR: The results suggest that the observed changes in the morphology of the jaw apparatus have probably occurred within the limits set by a pre-existing behavioral pattern.
Abstract: Masticatory movements and molar wear facets in species of Tupaia, Galago, Saimiri, and Ateles have been examined using cinefluorography and occlusal analysis. The molars have been compared with those of a fossil series: Palenochtha, Pelycodus and Aegyptopithecus. The extant primates are almost identical in their feeding behaviour, the movements and timing of the masticatory cycle. Food is first puncture-crushed where the cycle is elongated, the power stroke attenuated and abrasion facets are produced on the molars. Chewing follows, the movements are more complex, the power stroke has two distinct parts and attrition facets are produced. In the primitive forms (Tupaia, Palenochtha), shearing blades, arranged in series (en echelon) were used to cut the food during the first part (Phase I) of the power stroke as the lower teeth move into centric occlusion. This mechanism has been progressively replaced by a system of blade-ringed compression chambers which cut and compartmentalise the food in Phase I. This is followed by an anteromedially and inferiorly directed movement away from centric occlusion (Phase II) in which the food is ground. In both extant and fossil series there has been a clear trend towards the elongation of Phase II with a corresponding reduction in Phase I. These results suggest that the observed changes in the morphology of the jaw apparatus have probably occurred within the limits set by a pre-existing behavioral pattern.

407 citations