Bone structure and the patterns of force transmission in the cat skull (Felis catus)

Projection microradiography was used to determine the density and orientation of the force transmitting structures, i.e., trabeculae and bone lying between approximately parallel vascular canals, within the bones of cat skulls. The organisation in the skulls was confirmed statistically for a total of ten cats. The results of the observations showed that within specific areas of the skull a high degree of structural orientation and an increased density of osseous structures was present. The distribution of these characters corresponded in contiguous bones such that a continuum of structural organisation was established between the alveolar region and the site of attachment of the temporalis and masseter muscles and the glenoid region. The patterns of force transmission during jaw closure were determined when a resistance was placed initially between the canines and then the carnassials. An analysis was first carried out on dry skulls using colophonium resin to determine the direction of the force distribution. The nature and the approximate magnitude of the forces were ascertained by replacing the resin with strain gauges. The basic similarities in the strain patterns recorded from the dry skulls and those from the ten anaesthetised cats in which strain gauges had been intra-vitally implanted, substantiated the recordings made on the dry skulls. Combination of the results from the three sets of experiments defined the patterns of force distribution in the cat skull during the closure of the mandible against a resistance. The results showed that: (1) the combined action of the temporalis and masseter muscles tended to reduce the overall strain in the skull bones, and that the deformations produced by the action of the masseter were greater than that exerted by the temporalis muscles; (2) during biting, whether the resistance was placed between the canines or carnassials, compressive forces predominated in the facial bones; (3) small movements observed between facial bones indicated the presence of a flexible component within the skull, thus allowing large forces to be exerted during biting without overstressing the facial bones; (4) the glenoid fossa is part of a force bearing joint; (5) forces generated during biting were resisted within the skull by forces of an opposite nature generated within the system, the incompressible nature of bone and by the effect of the soft tissues; (6) the nature and the magnitude of the strain altered when a resistance was placed at the canines and then at the carnassials; however, the pattern of force distribution within the skull remained the same; (7) there was a direct correspondence between the detailed structural organisation of the bones and the patterns of force distribution. This conclusion would appear to apply in general to mammalian skulls. The study also emphasises the importance, neglected hitherto, of carrying out a variety of experiments to determine the patterns of force distribution in bones. The Trajectorial Theory of bone organisation is discussed and, on the basis of the results obtained, a modified theory is proposed. This states that: the structural continuum is common to the compact and cancellous bone and comprises bony bars which are aligned in the optimum direction for the transmission of force to a region in the bone or bones where it is effectively resisted.     

Sexual Dimorphism and Facial Growth Beyond Dental Maturity in Great Apes and Gibbons

The great apes and gibbons are characterized by extensive variation in degree of body size and cranial dimorphism, but although some studies have investigated how sexual dimorphism in body mass is attained in these species, for the majority of taxa concerned, no corresponding work has explored the full extent of how sexual dimorphism is attained in the facial skeleton. In addition, most studies of sexual dimorphism combine dentally mature individuals into a single “adult” category, thereby assuming that no substantial changes in size or dimorphism take place after dental maturity. We investigated degree and pattern of male and female facial growth in Pan troglodytes troglodytes, Pan paniscus, Gorilla gorilla gorilla, Pongo pygmaeus, and Hylobates lar after dental maturity through cross-sectional analyses of linear measurements and geometric mean values of the facial skeleton and age-ranking of individuals based on molar occlusal wear. Results show that overall facial size continues to increase after dental maturity is reached in males and females of Gorilla gorilla gorilla and Pongo pygmaeus, as well as in the females of Hylobates lar. In male Pongo pygmaeus, adult growth patterns imply the presence of a secondary growth spurt in craniofacial dimensions. There is suggestive evidence of growth beyond dental maturity in the females of Pan troglodytes troglodytes and Pan paniscus, but not in the males of those species. The results show the presence of statistically significant facial size dimorphism in young adults of Pan paniscus and Hylobates lar, and of near statistical significance in Pan troglodytes troglodytes, but not in older adults of those species; adults of Gorilla gorilla gorilla and Pongo pygmaeus are sexually dimorphic at all ages after dental maturity. The presence of sex-specific growth patterns in these hominoid taxa indicates a complex relationship between socioecological selective pressures and growth of the facial skeleton.     

The ontogeny of sexual dimorphism in the facial skeleton of the African apes

This paper aims to test the contribution of ontogenetic scaling to sexual dimorphism of the facial skeleton in the African apes. Specifically, it addresses whether males and females of each species share a common postnatal ontogenetic shape trajectory for the facial skeleton. Where trajectories are found to differ, it is tested whether male and female trajectories: 1) diverge early, or 2) diverge later after sharing a common trajectory earlier in the postnatal period. Where ontogenetic shape trajectories are found to be shared, it is also tested whether males and females are ontogenetically scaled. This study uses geometric morphometric analyses of 28 landmarks from the facial skeletons of 137 G. g. gorilla (62 adults; 75 juveniles), 95 P. paniscus (34 adults; 61 juveniles), and 115 P. t. troglodytes (58 adults; 57 juveniles). On average, males and females share a common ontogenetic shape trajectory until around the eruption of the second permanent molars. In addition, for the same period, males and females in each species share a common ontogenetic scaling trajectory. After this period, males and females diverge both from each other and from the common juvenile ontogenetic shape and scaling trajectories within each species. Thus, the male and female facial skeleton shows ontogenetic scaling until around the point of the eruption of the second molar (i.e., around puberty and the development of secondary sexual characteristics), but subsequent sexual dimorphism occurs via divergent trajectories and not via ontogenetic scaling.     

Genetic control of feeding preferences in the mosquitoes Aedes (Stegomyia) simpsoni and aegypti

ABSTRACT. The biting rate of a non-anthropophilic (Bwayise) population of Aedes simpsoni was found to be approximately 0.3 mosquitoes per catcher per hour, whereas that of an anthropophilic (Bwamba) population was approximately 101 per catcher per hour. Population density indices, as determined by the number of pupae per wet plant axil, were 0.70 in Bwayise and 1.00 in Bwamba. The big difference in anthropophilic behaviour between these populations was therefore unlikely to be derived from this small population difference. Larval density was higher at Bwamba than at Bwayise, but isolation or crowding of the larvae in the laboratory did not affect the biting behaviour of adult Ae. simpsoni. Laboratory studies also failed to confirm field observations that temperature might play a part in determining anthropophily and non-anthropophily in this species. In choice-chamber landing tests, using a rat and a human hand, Ae. simpsoni females derived from wild larvae and reared in the laboratory showed that 83% of the Bwamba strain landed on man, whereas only 38% of the Bwayise strain did so. In Aedes aegypti , 71% of a long-established laboratory strain (Ilobi) landed on man, whereas 47% of a relatively non-anthropophilic wild (Kampala) strain did so. These preferences persisted in culture. Selective breeding increased the preference for the rodent significantly in the Kampala strain of Ae. aegypti , but had no significant effect on the Ilobi strain. Crossbreeding showed that the F₁ and F₂ hybrids between the anthropophilic and non-anthropophilic strains were intermediate in their preference between the parental pure bred strains; the reciprocal crosses were not significantly different from each other. The behaviour of the backcross progenies, at least in Ae. aegypti , appeared to indicate that the genotype of the male parent might be the main determining factor.     

Concealing facial evidence of mood: Perspective-taking in a captive gorilla?

A captive female lowland gorilla was observed repeatedly to hide or inhibit her playface by placing one or both hands over the face. When this behaviour was seen play usually did not follow immediately, even if other signals associated with play were simultaneously being made by the gorilla. By contrast, a playface predicted that play would follow within a few seconds; this difference was statistically reliable. Several levels of interpretation of the behaviour are possible: hiding the playface may have functioned as a form of deception, a meta-communication, or merely an attempt to suppress the playface. However, by any of these interpretations, the behaviour implies that the gorilla is aware of her spontaneous facial expressions and the consequences they entail. Among the great apes, manual suppression of a facial expression has previously been reported once for chimpanzees but never for gorillas.     

Strain in the galago facial skull

Little experimental work has been directed at understanding the distribution of stresses along the facial skull during routine masticatory behaviors. Such information is important for understanding the functional significance of the mammalian circumorbital region. In this study, bone strain was recorded along the dorsal interorbit, postorbital bar, and mandibular corpus in Otolemur garnettii and O. crassicaudatus (greater galagos) during molar chewing and biting. We determined principal-strain magnitudes and directions, compared peak shear-strain magnitudes between various regions of the face, and compared galago strain patterns with similar experimental data for anthropoids. This suite of analyses were used to test the facial torsion model (Greaves [1985] J Zool (Lond) 207:125-136; [1991] Zool J Linn Soc 101:121-129; [1995] Functional morphology in vertebrate paleontology. Cambridge: Cambridge University Press, p 99-115). A comparison of galago circumorbital and mandibular peak strains during powerful mastication indicates that circumorbital strains are very low in magnitude. This demonstrates that, as in anthropoids, the strepsirhine circumorbital region is highly overbuilt for countering routine masticatory loads. The fact that circumorbital peak-strain magnitudes are uniformly low in both primate suborders undermines any model that emphasizes the importance of masticatory stresses as a determinant of circumorbital form, function, and evolution. Preliminary data also suggest that the difference between mandibular and circumorbital strains is greater in larger-bodied primates. This pattern is interpreted to mean that sufficient cortical bone must exist in the circumorbital region to prevent structural failure due to nonmasticatory traumatic forces. During unilateral mastication, the direction of epsilon(1) at the galago dorsal interorbit indicates the presence of facial torsion combined with bending in the frontal plane. Postorbital bar principal-strain directions during mastication are oriented, on average, very close to 45 degrees relative to the skull's long axis, much as predicted by the facial torsion model. When chewing shifts from one side of the face to the other, there is a characteristic reversal or flip-flop in principal-strain directions for both the interorbit and postorbital bar. Although anthropoids also exhibit an interorbital reversal pattern, peak-strain directions for this clade are opposite those for galagos. The presence of such variation may be due to suborder differences in relative balancing-side jaw-muscle force recruitment. Most importantly, although the strain-direction data for the galago circumorbital region offer support for the occurrence of facial torsion, the low magnitude of these strains suggests that this loading pattern may not be an important determinant of circumorbital morphology.     

Mandibular function in Galago crassicaudatus and Macaca fascicularis: an in vivo approach to stress analysis of the mandible.

Single-element and/or rosette strain gages were bonded to mandibular cortical bone in Galago crassicaudatus and Macaca fascicularis. Five galago and eleven macaque bone strain experiments were performed and analyzed. In vivo bone strain was recorded from the lateral surface of the mandibular corpus below the postcanine tooth row during transducer biting and during mastication and ingestion of food objects. In macaques and galagos, the mandibular corpus on the balancing side is primarily bent in the sagittal plane during mastication and is both twisted about its long axis and bent in the sagittal plane during transducer biting. On the working side, it is primarily twisted about its long axis and directly sheared perpendicular to its long axis, and portions of it are bent in the sagittal plane during mastication and molar transducer biting. In macaques, the mandibular corpus on each side is primarily bent in the sagittal plane and twisted during incisal transducer biting and ingestion of food objects, and it is transversely bent and slightly twisted during jaw opening. Since galagos usually refused to bite the transducer or food objects with their incisors, an adequate characterization of mandibular stress patterns during these behaviors was not possible. In galagos the mandibular corpus experiences very little transverse bending stress during jaw opening, perhaps in part due to its unfused mandibular symphysis. Marked differences in the patterns of mandibular bone strain were present between galagos and macaques during the masticatory power stroke and during transducer biting. Galagos consistently had much more strain on the working side of the mandibular corpus than on the balancing side. These experiments support the hypothesis that galagos, in contrast to macaques, employ a larger amount of working-side muscle force relative to the balancing-side muscle force during unilateral biting and mastication, and that the fused mandibular symphysis is an adaption to use a maximal amount of balancing-side muscle force during unilateral biting and mastication. These experiments also demonstrate the effects that rosette position, bite force magnitudes, and types of food eaten have on recorded mandibular strain patterns.     

Incisal bite force direction in humans and the functional significance of mammalian mandibular translation

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).     

In vivo bone strain in the mandible of Galago crassicaudatus.

Single element foil strain gages were bonded to mandibular cortical bone in eight specimens of Galago crassicaudatus. The gage was bonded below the Pm4 or M2 adjacent to the lower border of the mandible. The bonded strain gage was connected to form one arm of a Wheatstone bridge. Following recovery from the general anesthetic, the restrained Galago bit either a piece of wood, a food object, or a bite force transducer. During these biting episodes, mandibular bone strain deformed the strain gage and the resulting change in electrical resistance of the gage caused voltage changes across the Wheatstone bridge. These changes, directly proportional to the amount of bone strain along the gage site, were recovered by a strip chart recorder. Bone strain was measured on both the working and balancing sides of the jaws. Maximum values of bone strain and bite force were 435 microstrain (compression) and 8.2 kilograms respectively. During bending of the mandible, the correlation between bone strain (tension or compression) and bite force ranged from -0.893 (tension) to 0.997 (compression). The experiments reported here demonstrate that only a small percentage of the Galago bite force is due to balancing side muscle force during isometric unilateral molar biting. In addition, these experiments demonstrate that the Galago mandible is bent in a predictable manner during biting. The amount of apparent mandibular bone strain is dependent on (1) the magnitude of the bite force and (2) the position of the bite point.     

Heterogeneity of the ribosomal genes in mice and men

The structures of mouse and human ribosomal DNA were studied using the restriction enzymes Eco R1 and Hind III. Individual mice or humans showed a heterogeneous pattern of restriction fragments resulting from differences in the non-transcribed spacer DNA. Six individual mice from the inbred strain CBA/H-T6 had identical patterns. The same pattern was shown by another CBA strain and by C3H. These strains were originally derived from a BALB X DBA cross made in 1920. Different patterns were found for BALB/c, C57BL and Mus poschiavinus. Cultured cells derived from C3H mice (L cells) showed a pattern quantitatively different from that of the parent strain, but two myeloma cell lines derived from BALB/c showed the same pattern as BALB/c mice. Ribosomal DNA in man is also heterogeneous. Differences were observed between human DNAs in the amounts of the different spacer classes. Studies on mouse-human cell hybrids suggest that some spacer classes are present on more than one of the five human nucleolus organizers.     

Characterization of CR1- and membrane cofactor protein-like proteins of two primates

We have identified and characterized C3b binding proteins of two primates, orangutan (Pongo pygmaeus) and gorilla (Gorilla gorilla). Detergent solubilized 125I surface-labeled E and PBMC were subjected to affinity chromatography with homologous or human iC3/C3b. These ligands bound a 225,000 single chain protein from orangutan E and PBMC and a 220,000 protein from gorilla E. Proteins of the same Mr were immunoprecipitated by a rabbit polyclonal and two murine mAb to the human CR1 (CD35). The C3b binding protein of gorilla E aligned with that of the common human CR1 polymorphic size variant. Human or orangutan iC3 was also a ligand for a surface-labeled protein doublet of 59,000 and 65,000 from orangutan E. The doublet pattern and mol wts are similar to membrane cofactor protein (or CD46). Further, this doublet was immunoprecipitated by a mAb to human MCP. The MCP-like protein doublet was not isolated from gorilla or human E. Decay accelerating factor (DAF) of orangutan E was also identified and was structurally and antigenically distinct from the MCP-like protein. Orangutan or gorilla E preparations were a cofactor for the cleavage of human iC3 by human factor I and produced the same cleavage fragments as human CR1. Cofactor activity of orangutan E was partially inhibited by preclearance of CR1 and more completely inhibited by preclearance of MCP. Cofactor activity of gorilla E was inhibited by coincubation with a monoclonal antibody to human CR1. These data indicate that the orangutan and gorilla high m.w. proteins are equivalent to human CR1. The orangutan E membrane protein doublet with m.w. of 59,000 and 65,000 possesses biochemical, antigenic, and functional properties of human membrane cofactor protein.     

Immune and pathophysiological responses to different strains of Giardia duodenalis in neonatal mice

Numerous studies have demonstrated various strain differences between Giardia isolates, but little is known about the immunology and pathogenesis of infections. This study aimed to compare host responses to strains of Giardi duodenalis differing in levels of virulence and pathogenicity and, by doing so, elucidate the mechanisms via which pathogenic strains establish infections. Marked differences were found in the infection dynamics, histopathological responses and serum antibody responses of neonatal mice infected with either G. duodenalis strain BRIS/83/HEPU/106 (isolated from a human) or BRIS/95/HEPU/2041 (isolated from a sulphur-crested cockatoo, Cacatua galerita). Infections with the bird strain were more intense (6.7-times greater) and persisted longer (by 14days) than infections with the human strain. The bird strain was more pathogenic and caused greater pathophysiological alteration to the gut mucosa, including increased villous atrophy, hyperplasia of goblet cells and vacuolated epithelial cells. Mice infected with the bird strain produced less serum anti-Giardia IgA and IgM, but more total (non-specific) serum IgA than those infected with the human strain of Giardia. This suggests that avian G. duodenalis strains are infective for mammalian hosts and may contribute to zoonotic infections. Furthermore, infection of mice with BRIS/95/HEPU/2041 serves as a good experimental model to provide further insight into the mechanisms via which G. duodenalis causes disease.     

Facilitating play through communication: significance of teeth exposure in the gorilla play face

Primate facial expressions (FEs) likely play an important role in primate society: through facial signals, individuals can potentially send and receive information and may benefit from coordinating their behavior accordingly. Many primates use a relaxed open mouth (ROM) facial display or “play face” (PF) during play behavior, where the mouth is open but teeth are covered. In addition to this conventional PF, however, Western Lowland gorillas (Gorilla gorilla gorilla) also use a full PF where the upper teeth are exposed. As the teeth are similarly exposed in the bared-teeth expression (which is a signal of appeasement, submission and/or affiliation), the full PF may be a blend of the PF and bared-teeth face, and have a different signal function to the PF alone. Focal animal sampling of captive Western Lowland gorillas (N=10) showed that the full PF was more often observed in intense rather than gentle play, and intense play bouts that featured the full PF were longer than those that featured only the PF. Both expressions were associated with an increase in affinitive behavior between sender and receiver postplay, but only the full PF was associated with an increase higher than that of play alone. Overall, the findings suggest that the full PF has an additional role in coordinating and maintaining play, possibly though reducing uncertainty in the receiver and confirming that play is only play.     

Experimental analysis of temporomandibular joint reaction force in macaques

Mandibular bone strain in the region immediately below the temporomandibular ligament was analyzed in adult and sub-adult Macaca fascicularis and Macaca mulatta. Following recovery from the general anesthetic, the monkeys were presented food objects, a wooden rod, or a specially designed bite-force transducer. Bone strain was recorded during incisal biting and mastication of food, and also during isometric biting of the rod and/or the transducer. The bone strain data suggest the following: The macaque TMJ is loaded by a compressive reaction force during the power stroke of mastication and incision of food, and during isometric molar and incisor biting. TMJ reaction forces are larger on the contralateral side during both mastication and isometric molar biting. Patterns of ipsilateral TMJ reaction force in macaques during isometric biting vary markedly in response to the position of the bite point. During biting along the premolars or first two molars a compressive reaction force acts about the ipsilateral TMJ; however, when the bite point is positioned along the M3, the ipsilateral TMJ has either very little compressive stress, no stress, or it is loaded in tension.     

In vivo function of the craniofacial haft: the interorbital "pillar"

The craniofacial haft resists forces generated in the face during feeding, but the importance of these forces for the form of the craniofacial haft remains to be determined. In vivo bone strain data were recorded from the medial orbital wall in an owl monkey (Aotus), rhesus macaques (Macaca mulatta), and a galago (Otolemur) during feeding. These data were used to determine whether: the interorbital region can be modeled as a simple beam under bending or shear; the face is twisting on the brain case during unilateral biting or mastication; the interorbital "pillar" is being axially compressed during incisor loading and both axially compressed and laterally bent during mastication; and the interorbital "pillar" transmits axial compressive forces from the toothrow to the braincase. The strain data reveal that the interorbital region cannot be modeled as a anteroposteriorly oriented beam bent superiorly in the sagittal plane during incision or mastication. The strain orientations recorded in the majority of experiments are concordant with those predicted for a short beam under shear, although the anthropoids displayed evidence of multiple loading regimes in the medial orbital wall. Strain orientation data corroborate the hypothesis that the strepsirrhine face is twisted during mastication. The hypothesis that the interorbital region is a member in a rigid frame subjected to axial compression during mastication receives some support. The hypothesis that the interorbital region is a member in a rigid frame subjected to lateral bending during mastication is supported by the epsilon1/absolute value epsilon2 ratio data but not by the strain orientation data. The timing of peak shear strains in the medial orbital wall of anthropoids does not bear a consistent relationship to the timing of peak shear strain in the mandibular corpus, suggesting that bite force is not the only external force influencing the medial orbital wall. Strain orientation data suggest the existence of two distinct loading regimes, possibly associated with masseter or medial pterygoid contraction. Regardless of the loading regime, all taxa showed low strain magnitudes in the medial orbital wall relative to the anterior root of the zygoma and the mandibular corpus. The strain gradients documented here and elsewhere suggest that, in anthropoids at least, local effects of external forces are more important than a single global loading regime. The low strain magnitudes in the medial orbital wall and in other thin bony plates around the orbit suggest that these structures are not optimally designed for resisting feeding forces. It is hypothesized that their function is to provide rigid support and protection for soft-tissue structures such as the nasal epithelium, the brain, meninges, and the eye and its adnexa. In contrast with the face of Otolemur, which appears to be subjected to a single predominant loading regime, anthropoids may experience different loading regimes in different parts of the face. This implies that the anthropoid and strepsirrhine facial skulls might be optimized for different functions.     

Bite force production capability and efficiency in Neandertals and modern humans

Although there is consensus that Neandertal craniofacial morphology is unique in the genus Homo, debate continues regarding the precise anatomical basis for this uniqueness and the evolutionary mechanism that produced it. In recent years, biomechanical explanations have received the most attention. Some proponents of the "anterior dental loading hypothesis" (ADLH) maintain that Neandertal facial anatomy was an adaptive response to high-magnitude forces resulting from both masticatory and paramasticatory activity. However, while many have argued that Neandertal facial structure was well-adapted to dissipate heavy occlusal loads, few have considered, much less demonstrated, the ability of the Neandertal masticatory system to generate these presumably heavy loads. In fact, the Neandertal masticatory configuration has often been simultaneously interpreted as being disadvantageous for producing large bite forces. With rare exception, analyses that attempted to resolve this conflict were qualitative rather than quantitative. Using a three-dimensional digitizer, we recorded a sequence of points on the cranium and associated mandible of the Amud 1, La Chapelle-aux-Saints, and La Ferrassie 1 Neandertals, and a sample of early and recent modern humans (n = 29), including a subsample with heavy dental wear and documented paramasticatory behavior. From these points, we calculated measures of force-production capability (i.e., magnitudes of muscle force, bite force, and condylar reaction force), measures of force production efficiency (i.e., ratios of force magnitudes and muscle mechanical advantages), and a measure of overall size (i.e., the geometric mean of all linear craniofacial measurements taken). In contrast to the expectations set forth by the ADLH, the primary dichotomy in force-production capability was not between Neandertal and modern specimens, but rather between large (robust) and small (gracile) specimens overall. Our results further suggest that the masticatory system in the genus Homo scales such that a certain level of force-production efficiency is maintained across a considerable range of size and robusticity. Natural selection was probably not acting on Neandertal facial architecture in terms of peak bite force dissipation, but rather on large tooth size to better resist wear and abrasion from submaximal (but more frequent) biting and grinding forces. We conclude that masticatory biomechanical adaptation does not underlie variation in the facial skeleton of later Pleistocene Homo in general, and that continued exploration of alternative explanations for Neandertal facial architecture (e.g., climatic, respiratory, developmental, and/or stochastic mechanisms) seems warranted.     

A model of stress and strain in the interosseous ligament of the forearm based on fiber network theory

Fiber network theory was developed to describe cloth, a thin material with strength in the fiber directions. The interosseous ligament (IOL) of the forearm is a broad, thin ligament with highly aligned fibers. The objectives of this study were to develop a model of the stress and strain distributions in the IOL, based on fiber network theory, to compare the strains from the model with the experimentally measured strains, and to evaluate the force distribution across the ligament fibers from the model. The geometries of the radius, ulna, and IOL were reconstructed from CT scans. Position and orientation of IOL insertion sites and force in the IOL were measured during a forearm compression experiment in pronation, neutral rotation, and supination. An optical image-based technique was used to directly measure strain in two regions of the IOL in neutral rotation. For the network model, the IOL was represented as a parametric ruled three-dimensional surface, with rulings along local fiber directions. Fiber strains were calculated from the deformation field, and fiber stresses were calculated from the strains using average IOL tensile properties from a previous study. The in situ strain in the IOL was assumed uniform and was calculated so that the net force predicted by the network model in neutral rotation matched the experimental result. The net force in the IOL was comparable to experimental results in supination and pronation. The model predicted higher stress and strain in fibers near the elbow in neutral rotation, and higher stresses in fibers near the wrist in supination. Strains in neutral forearm rotation followed the same trends as those measured experimentally. In this study, a model of stress and strain in the IOL utilizing fiber network theory was successfully implemented. The model illustrates variations in the stress and strain distribution in the IOL. This model can be used to show surgeons how different fibers are taut in different forearm rotation positions-this information is important for understanding the biomechanical role of the IOL and for planning an IOL reconstruction.     

Masticatory biomechanics and its relevance to early hominid phylogeny: an examination of palatal thickness using finite-element analysis

It has been proposed that morphological characters functionally related to mastication may be unreliable indicators of early hominid phylogeny. One hypothesis states that masticatory characters are highly prone to homoplasy. A second hypothesis states that such characters are likely to be morphologically integrated and thus violate the assumption of character independence implicit in all phylogenetic analyses. Evaluation of these hypotheses requires that masticatory features be accurately identified, but, to date, there have been relatively few attempts to test precisely which early hominid features are functionally related to chewing. This paper uses finite-element analysis to evaluate the functional relationships of a character--palatal thickness--that is one of several Paranthropus synapomorphies putatively related to mastication. A finite-element model of 145,680 elements was created from sixty-one 2-mm-thick CT scans of a Macaca fascicularis skull. The model was assigned the elastic properties of facial bone and loaded with muscle forces corresponding to the moment of centric occlusion during mastication. The model was constrained so as to produce a reaction force (corresponding to the bite force) at M(1). With a few exceptions, the strain patterns in the finite-element model compare well with those gathered from published and unpublished bone-strain experiments. The model was then modified to have a thick palate. The model was reloaded using an identical loading regime, and the strain patterns of the original and thick-palate models were compared. Although a thickened palate acts to reduce palatal strain, strains are elevated in other facial regions. This suggests that a thick palate would not have evolved in isolation as an adaptation to withstand masticatory stress. Rather, a thick palate may have evolved in concert with a suite of other facial features that share a stress-resistance function. This appears to be consistent with hypotheses positing that at least some facial features related to chewing evolved in an integrated fashion. More functional studies of other facial features are needed, as are formal studies of morphological integration.     

Endo's stress analysis of the primate skull and the functional significance of the supraorbital region

A review of Endo's experimental and theoretical procedures and data indicates that the magnitude of the principal strains in the glabella region of both humans and gorillas are low as compared to other parts of the face. Therefore, his data do not provide support for the hypothesis that the glabella region is a highly stressed region during biting. In addition, increased levels of strain in the supraorbital region are directly related to increased levels of masticatory muscle and reaction forces, and not necessarily to anterior tooth loading as opposed to posterior tooth loading. His data also indicate that the supraorbital region in extant humans cannot be accurately modeled as a beam. These conclusions either differ from those of Endo or are not clearly presented or emphasized throughout any of Endo's papers. Therefore, we suggest that a number of investigators have made unsupported or erroneous conclusions based on Endo's work. This is particularly true for those studies that have emphasized the existence of powerful bending stress in the glabella region during incisor biting in both humans and non-human primates.     

Gorilla society: What we know and don't know

Science is fairly certain that the gorilla lineage separated from the remainder of the hominoid clade about eight million years ago, 2 , 4 and that the chimpanzee lineage and hominin clade did so about a million years after that. 1 , 2 However, just this year, 2007, it was discovered that although the human head louse separated from the congeneric chimpanzee body louse (Pediculus) around the same time as the chimpanzee and hominin lineages split, 3 the human pubic louse apparently split from its sister species, the congeneric gorilla louse, Pthirus, 4.5 million years after their host lineages split. 3 No tested explanations exist for the discrepancy. Much is known about hominin evolution, but much remains to be discovered. The same is true of primate socioecology in general and gorilla socioecology in particular.