The Relationships Among Jaw-Muscle Fiber Architecture, Jaw Morphology, and Feeding Behavior in Extant Apes and Modern Humans
african apes; anthropoid primates; Anthropology; consequences; cross-sectional area; diet; evolution; Evolutionary Biology; finite-element-analysis; functional; hominoids; internal architecture; macaca-fascicularis; mandibular symphysis; masseter; mountain gorilla; pan-troglodytes; Temporalis
The jaw-closing muscles are responsible for generating many of the forces and movements associated with feeding. Muscle physiologic cross-sectional area (PCSA) and fiber length are two architectural parameters that heavily influence muscle function. While there have been numerous comparative studies of hominoid and hominin craniodental and mandibular morphology, little is known about hominoid jaw-muscle fiber architecture. We present novel data on masseter and temporalis internal muscle architecture for small-and large-bodied hominoids. Hominoid scaling patterns are evaluated and compared with representative New- (Cebus) and Old-World (Macaca) monkeys. Variation in hominoid jaw-muscle fiber architecture is related to both absolute size and allometry. PCSAs scale close to isometry relative to jaw length in anthropoids, but likely with positive allometry in hominoids. Thus, large-bodied apes may be capable of generating both absolutely and relatively greater muscle forces compared with smaller-bodied apes and monkeys. Compared with extant apes, modern humans exhibit a reduction in masseter PCSA relative to condyle-M-1 length but retain relatively long fibers, suggesting humans may have sacrificed relative masseter muscle force during chewing without appreciably altering muscle excursion/contraction velocity. Lastly, craniometric estimates of PCSAs underestimate hominoid masseter and temporalis PCSAs by more than 50% in gorillas, and overestimate masseter PCSA by as much as 30% in humans. These findings underscore the difficulty of accurately estimating jaw-muscle fiber architecture from craniometric measures and suggest models of fossil hominin and hominoid bite forces will be improved by incorporating architectural data in estimating jaw-muscle forces. Am J Phys Anthropol 151:120-134, 2013. (C) 2013 Wiley Periodicals, Inc.
Taylor A B; Vinyard C J
American Journal of Physical Anthropology
2013
2013-05
Journal Article
<a href="http://doi.org/10.1002/ajpa.22260" target="_blank" rel="noreferrer noopener">10.1002/ajpa.22260</a>
Masticatory motor patterns in ungulates: A quantitative assessment of jaw-muscle coordination in goats, alpacas and horses
electromyography; emg; force; fusion; mandibular symphysis; masseter muscle; movements; pigs; Zoology
We investigated patterns of jaw-muscle coordination during rhythmic mastication in three species of ungulates displaying the marked transverse jaw movements typical of many large mammalian herbivores. In order to quantify consistent motor patterns during chewing, electromyograms were recorded from the superficial masseter, deep masseter, posterior temporalis and medial pterygoid muscles of goats, alpacas and horses. Timing differences between muscle pairs were evaluated in the context of an evolutionary model of jaw-muscle function. In this model, the closing and food reduction phases of mastication are primarily controlled by two distinct muscle groups, triplet I (balancing-side superficial masseter and medial pterygoid and working-side posterior temporalis) and triplet II (working-side superficial masseter and medial pterygoid and balancing-side posterior temporalis), and the asynchronous activity of the working- and balancing-side deep masseters. The three species differ in the extent to which the jaw muscles are coordinated as triplet I and triplet II. Alpacas, And to a lesser extent, goats, exhibit the triplet pattern whereas horses do not. In contrast, all three species show marked asynchrony of the working-side and balancing-side deep masseters, with jaw closing initiated by the working-side muscle and the balancing-side muscle firing much later during closing. However, goats differ from alpacas and horses in the timing of the balancing-side deep masseter relative to the triplet II muscles. This study highlights interspecific differences in the coordination of jaw muscles to influence transverse jaw movements and the production of bite force in herbivorous ungulates.
Williams S H; Vinyard C J; Wall C E; Hylander W L
Journal of Experimental Zoology Part a-Ecological Genetics and Physiology
2007
2007-04
Journal Article
<a href="http://doi.org/10.1002/jez.a.362" target="_blank" rel="noreferrer noopener">10.1002/jez.a.362</a>
Mechanical properties of foods used in experimental studies of primate masticatory function
biomechanical analysis; elastic modulus; etectromyography; fracture-toughness; fracture-toughness; genus cebus; jaw; lateral pterygoid muscle; macaca-fascicularis; mandibular symphysis; masseter force; mechanical properties; movement; old-world monkeys; periodontal mechanoreceptive afferents; Zoology
In vivo studies, of jaw-muscle behavior have been integral factors in the development of our current understanding of the primate masticatory apparatus. However, even though it has been shown that food textures and mechanical properties influence jaw-muscle activity during mastication, very little effort has been made to quantify the relationship between the elicited masticatory responses of the subject and the mechanical proper-ties of the foods that are eaten. Recent work on human mastication highlights the importance of two mechanical properties-toughness and elastic modulus (i.e., stiffness)-for food breakdown during mastication. Here we provide data on the toughness and elastic modulus of the majority of foods used in experimental studies of the nonhuman primate masticatory apparatus. Food toughness ranges from approximately 56.97Jm(-2) (apple pulp) to 4355.45 Jm(-2) (prune pit). The elastic modulus of the experimental foods ranges from 0.07 MPa for gummy bears to 346 MPa for popcorn kernels. These data can help researchers studying primate mastication select among several potential foods with broadly similar mechanical properties. Moreover, they provide a framework for understanding how jaw-muscle activity varies with food mechanical properties in these studies.
Williams S H; Wright B W; Den Truong V; Daubert C R; Vinyard C J
American Journal of Primatology
2005
2005-11
Journal Article
<a href="http://doi.org/10.1002/ajp.20189" target="_blank" rel="noreferrer noopener">10.1002/ajp.20189</a>
Telemetry System for Assessing Jaw-Muscle Function in Free-ranging Primates
Alouatta; analysis; biomechanical; bone strain; electromyography; food properties; form; galago crassicaudatus; macaca-fascicularis; mandibular symphysis; mastication; morphology; old-world monkeys; telemetry; Zoology
In vivo laboratory-based studies describing jaw-muscle activity and mandibular bone strain during mastication provide the empirical basis for most evolutionary hypotheses linking primate masticatory apparatus form to diet. However, the laboratory data pose a potential problem for testing predictions of these hypotheses because estimates of masticatory function and performance recorded in the laboratory may lack the appropriate ecological context for understanding adaptation and evolution. For example, in laboratory studies researchers elicit rhythmic chewing using foods that may differ significantly from the diets of wild primates. Because the textural and mechanical properties of foods influence jaw-muscle activity and the resulting strains, chewing behaviors studied in the laboratory may not adequately reflect chewing behaviors of primates feeding in their natural habitats. To circumvent this limitation of laboratory-based studies of primate mastication, we developed a system for recording jaw-muscle electromyograms (EMGs) from free-ranging primates so that researchers can conduct studies of primate jaw-muscle function in vivo in the field. We used the system to record jaw-muscle EMGs from mantled howlers (Alouatta palliata) at Hacienda La Pacifica, Costa Rica. These are the first EMGs recorded from a noncaptive primate feeding in its natural habitat. Further refinements of the system will allow long-term EMG data collection so that researchers can correlate jaw-muscle function with food mechanical properties and behavioral observations. In addition to furthering understanding of primate feeding biology, our work will foster improved adaptive hypotheses explaining the evolution of primate jaw form.
Williams S; Vinyard C; Glander K; Deffenbaugh M; Teaford M; Thompson C
International Journal of Primatology
2008
2008-12
Journal Article
<a href="http://doi.org/10.1007/s10764-008-9292-3" target="_blank" rel="noreferrer noopener">10.1007/s10764-008-9292-3</a>
Functional And Evolutionary Significance Of The Recruitment And Firing Patterns Of The Jaw Adductors During Chewing In Verreaux's Sifaka (propithecus Verreauxi)
Anthropology; biomechanics; bone-strain; electromyography; Evolutionary Biology; Force; fusion; lemurs lemur-catta; macaca-fascicularis; mandibular symphysis; masseter force; mastication; muscle; primates; strepsirrhines; wishboning
Jaw-muscle electromyographic (EMG) patterns indicate that compared with thick-tailed galagos and ring-tailed lemurs, anthropoids recruit more relative EMG from their balancing-side deep masseter, and that this muscle peaks late in the power stroke. These recruitment and firing patterns in anthropoids are thought to cause the mandibular symphysis to wishbone (lateral transverse bending), resulting in relatively high symphyseal stresses. We test the hypothesis that living strepsirrhines with robust, partially fused symphyses have muscle recruitment and firing patterns more similar to anthropoids, unlike those strepsirrhines with highly mobile unfused symphyses. Electromyographic (EMG) activity of the superficial and deep masseter, anterior and posterior temporalis, and medial pterygoid muscles were recorded in four dentally adult Verreaux's sifakas (Propithecus verreauxi). As predicted, we find that sifaka motor patterns are more similar to anthropoids. For example, among sifakas, recruitment levels of the balancing-side (b-s) deep masseter are high, and the b-s deep masseter fires late during the power stroke. As adult sifakas often exhibit nearly complete symphyseal fusion, these data support the hypothesis that the evolution of symphyseal fusion in primates is functionally linked to wishboning. Furthermore, these data provide compelling evidence for the convergent evolution of the wishboning motor patterns in anthropoids and sifakas. Am J Phys Anthropol 145:531-547, 2011. (C) 2011 Wiley-Liss, Inc.
Hylander W L; Vinyard C J; Wall C E; Williams S H; Johnsonl K R
American Journal of Physical Anthropology
2011
2011-08
Journal Article or Conference Abstract Publication
<a href="http://doi.org/10.1002/ajpa.21529" target="_blank" rel="noreferrer noopener">10.1002/ajpa.21529</a>
Ternporalis Function In Anthropoids And Strepsirrhines: An Emg Study
adductor muscle force; Anthropology; biomechanics; electromyography; Evolutionary Biology; fusion; galago crassicaudatus; invivo bone strain; jaw-adductor muscle force; macaca-fascicularis; mandibular symphysis; masseter force; mastication; patterns; postorbital septum; primates; temporalis
The major purpose of this study is to analyze anterior and posterior temporalis muscle force recruitment and firing patterns in various anthropoid and strepsirrhine primates. There are two specific goals for this project. First, we test the hypothesis that in addition to transversely directed muscle force, the evolution of symphyseal fusion in primates may also be linked to vertically directed balancing-side muscle force during chewing (Hylander et al. [2000] Am. J. Phys. Anthropol. 112:469-492). Second, we test the hypothesis of whether strepsirrhines retain the hypothesized primitive mammalian condition for the firing of the anterior temporalis, whereas anthropoids have the derived condition (Weijs [1994] Biomechanics of Feeding in Vertebrates; Berlin: Springer-Verlag, p. 282-320). Electromyographic (EMG) activities of the left and right anterior and posterior temporalis muscles were recorded and analyzed in baboons, macaques, owl monkeys, thick-tailed galagos, and ring-tailed lemurs. In addition, as we used the working-side superficial masseter as a reference muscle, we also recorded and analyzed EMG activity of the left and right superficial masseter in these primates. The data for the anterior temporalis provided no support for the hypothesis that symphyseal fusion in primates is linked to vertically directed jaw muscle forces during mastication. Thus, symphyseal fusion in primates is most likely mainly linked to the timing and recruitment of transversely directed forces from the balancing-side deep masseter (Hylander et al. [2000] Am. J. Phys. Anthropol. 112:469-492). In addition, our data demonstrate that the firing patterns for the working- and balancing-side anterior temporalis muscles are near identical in both strepsirrhines and anthropoids. Their working- and balancing-side anterior temporalis muscles fire asynchronously and reach peak activity during the power stroke. Similarly, their working- and balancing-side posterior temporalis muscles also fire asynchronously and reach peak activity during the power stroke. Compared to these strepsirrhines, however, the balancing-side posterior temporalis of anthropoids appears to have a relatively delayed firing pattern. Moreover, based on their smaller W/B ratios, anthropoids demonstrate a relative increase in muscle-force recruitment of the balancing-side posterior temporalis. This in turn suggests that anthropoids may emphasize the duration and magnitude of the power stroke during mastication. This hypothesis, however, requires additional testing. Furthermore, during the latter portion of the power stroke, the late activity of the balancing-side posterior temporalis of anthropoids apparently assists the balancing-side deep masseter in driving the working-side molars through the terminal portion of occlusion.
Hylander W L; Wall C E; Vinyard C J; Ross C; Ravosa M R; Williams S H; Johnson K R
American Journal of Physical Anthropology
2005
2005-09
Journal Article or Conference Abstract Publication
<a href="http://doi.org/10.1002/ajpa.20058" target="_blank" rel="noreferrer noopener">10.1002/ajpa.20058</a>