Effects of loading on the biochemical behavior of molars of Homo,Pan, and Pongo

In a previous study, we found systematic differences in the biomechanical behavior of modern human molars using finite element stress analyses (FESA), which led us to propose that molars are adapted to differently-directed loads depending on their position within the mouth (Spears and Macho [1998] Am. J. Phys. Anthropol. 106:467–482 ). While the FESA results thus derived have not been verified experimentally, such an interpretation seemed reasonable. To refine the model previously presented, this study assessed the effects of 1) food particle size on the biomechanical behavior of molars, and those of 2) differences in morphology, particularly enamel thickness, on stress distribution. In order to appraise the evolutionary significance of the findings, the FESA results for modern humans were subsequently compared with those obtained for molars of one individual of Pan and Pongo, respectively. Bearing in mind limitations imposed by the FESA models created and analyzed in this study, constant cleavage-type loads and cuspal tip loads at different directions were employed on all teeth: this facilitated comparisons of patterns of stress distribution across molars and species. In Pan and Homo, cleavage-type loads exerted by big food particles tended to be better dissipated anteriorly than posteriorly, although trends in Pongo were less clear-cut. Furthermore, similar to modern humans, the buccal cusps of mandibular molars appeared to be able to dissipate the loads associated with a pestle-type action, while maxillary molars were better designed to dissipate the loads which would result if they acted as mortars against which the food is crushed/ground. While increases in enamel thickness lowered the overall stress values in teeth only slightly, changes in outer morphology could have a more profound effect on these stress levels. Overall, Pan appeared to be most generalized, while Homo and Pongo showed a number of unique specializations, which are in accordance with what is currently understood about their respective masticatory apparatus and dietary niche. Am J Phys Anthropol 109:211–227, 1999.© 1999 Wiley-Liss, Inc.     

Diet and locomotion of the armadillo Peltephilus: a new view

The Santacrucian armadillos Peltephilus (Mammalia, Dasypodidae, Peltephilinae) are reanalysed in order to test the traditional hypothesis that they were the most cursorial and active hunters of the dasypodids. The masticatory musculature is reconstructed, the form and distribution of teeth are studied, and a model of the jaw movements is proposed. Furthermore, the limb anatomy as well as their index of fossorial ability are compared with those of other armadillos. It is concluded that Peltephilus , and very likely other peltephilines, are poorly designed as carnivorous, actively cursorial mammals. An alternative habit is proposed, in which Peltephilus and the other peltephilines are viewed as having the conservative armadillo structure of diggers, feeding on soft but tough items, probably plant material of underground origin.     

Functional morphology of the human chin

The protruding chin is an attribute that defines modern Homo sapiens to the exclusion of all other primates, including fossil hominids. The functional significance of the chin has been contemplated for most of the 20th century, but as yet no compelling functional argument for its evolution has withstood careful scrutiny. Consequently, the human chin is often cited as an example of a nonadaptive trait. Past attempts to explain the chin in a functional or mechanical context have failed, largely as a result of an incomplete understanding of in vivo masticatory biomechanics. When the morphology of the chin is considered in light of experimental data on mastication, its evolution can be interpreted as a consequence of recent changes in mandibular proportions that have altered the relative importance of different masticatory stresses. Hypotheses proposing that chin morphology is the result of sexual selection or spatial constraints may be untestable. As with arguments that posit no functional role for the chin, the credibility of these hypotheses has depended, to a large degree, on the refutation of previous biomechanical explanations.     

Cerebral control of face,jaw, and tongue movements in awake cats: Changes in regional cerebral blood flow during lateral feeding

Mastication is achieved by cooperation among facial, masticatory, and lingual muscles. However, cortical control in cats for the masticatory performance is processed by two systems: facial movement processed by facial SI (the first somatosensory cortex), area C, and area M (motor areas), and jaw and tongue movements performed by intraoral SI, masticatory area, and area P (motor area). In particular, outputs from area P organized in the corticobulbar tract are projected bilaterally in the brainstem. In this present study, the aim is to explore changes in the regional cerebral blood flow (rCBF) in the facial SI, area M, and area P during trained lateral feeding (licking or chewing from the right or left side) of milk, fish paste, and small dry fish. The rCBF in area M showed contralateral dominance, and rCBF in area P during chewing or licking from the right or left side was almost the same value. Furthermore, activities of genioglossus and masseter muscles in the left side showed almost the same values during licking of milk and of fish paste, and chewing of small dry fish during lateral feeding. These findings suggest that the cortical process for facial, jaw, and tongue movements may be regulated by the contralateral dominance of area M and the bilateral one of area P.     

Scaling of chew cycle duration in primates

The biomechanical determinants of the scaling of chew cycle duration are important components of models of primate feeding systems at all levels, from the neuromechanical to the ecological. Chew cycle durations were estimated in 35 species of primates and analyzed in conjunction with data on morphological variables of the feeding system estimating moment of inertia of the mandible and force production capacity of the chewing muscles. Data on scaling of primate chew cycle duration were compared with the predictions of simple pendulum and forced mass-spring system models of the feeding system. The gravity-driven pendulum model best predicts the observed cycle duration scaling but is rejected as biomechanically unrealistic. The forced mass-spring model predicts larger increases in chew cycle duration with size than observed, but provides reasonable predictions of cycle duration scaling. We hypothesize that intrinsic properties of the muscles predict spring-like behavior of the jaw elevator muscles during opening and fast close phases of the jaw cycle and that modulation of stiffness by the central nervous system leads to spring-like properties during the slow close/power stroke phase. Strepsirrhines show no predictable relationship between chew cycle duration and jaw length. Anthropoids have longer chew cycle durations than nonprimate mammals with similar mandible lengths, possibly due to their enlarged symphyses, which increase the moment of inertia of the mandible. Deviations from general scaling trends suggest that both scaling of the jaw muscles and the inertial properties of the mandible are important in determining the scaling of chew cycle duration in primates.     

Maxillary complete denture movement during chewing in mandibular removable partial denture wearers

Objectives: To evaluate the pattern of maxillary complete denture movement during chewing for free‐end removable partial dentures (RPD) wearers, compared to maxillary and mandibular complete denture wearers. Materials and methods: Eighteen edentulous participants (group I) and 10 volunteers with bilateral posterior edentulous mandibles (group II) comprised the sample. Measures of mean denture movement and its variability were obtained by a kinesiographic instrument K6‐I Diagnostic System, during the mastication of bread and a polysulphide block. Data were analysed using two‐way anova (α = 0.05). Results: Upper movement during chewing was significantly lower for group II, regardless of the test food. The test food did not influence the vertical or lateral position of the denture bases, but more anterior dislocation was found when polysulphide blocks were chewed. Group II presented lower intra‐individual variability for the vertical axis. Vertical displacement was also more precise with bread as a test food. Conclusion: It can be concluded that mandibular free‐end RPD wearers show smaller and more precise movements than mandibular complete denture wearers.