Histoenzymology and morphometry of the masticatory muscles of tufted capuchin monkey (Cebus apella Linnaeus, 1758)

The influence of jaw-muscle fibre-type phenotypes on estimating maximum muscle and bite forces in primates

Numerous anthropological studies have been aimed at estimating jaw-adductor muscle forces, which, in turn, are used to estimate bite force. While primate jaw adductors show considerable intra- and intermuscular heterogeneity in fibre types, studies generally model jaw-muscle forces by treating the jaw adductors as either homogeneously slow or homogeneously fast muscles. Here, we provide a novel extension of such studies by integrating fibre architecture, fibre types and fibre-specific tensions to estimate maximum muscle forces in the masseter and temporalis of five anthropoid primates: Sapajus apella (N = 3), Cercocebus atys (N = 4), Macaca fascicularis (N = 3), Gorilla gorilla (N = 1) and Pan troglodytes (N = 2). We calculated maximum muscle forces by proportionally adjusting muscle physiological cross-sectional areas by their fibre types and associated specific tensions. Our results show that the jaw adductors of our sample ubiquitously express MHC α-cardiac, which has low specific tension, and hybrid fibres. We find that treating the jaw adductors as either homogeneously slow or fast muscles potentially overestimates average maximum muscle forces by as much as approximately 44%. Including fibre types and their specific tensions is thus likely to improve jaw-muscle and bite force estimates in primates.

Fiber-type phenotype of the jaw-closing muscles in Gorilla gorilla, Pan troglodytes, and Pan paniscus: A test of the Frequent Recruitment Hypothesis

Skeletal muscle fiber types are important determinants of the contractile properties of muscle fibers, such as fatigue resistance and shortening velocity. Yet little is known about how jaw-adductor fiber types correlate with feeding behavior in primates. Compared with chimpanzees and bonobos, gorillas spend a greater percentage of their daily time feeding and shift to herbaceous vegetation when fruits are scarce. We thus used the African apes to test the hypothesis that chewing with unusually high frequency is correlated with the expression in the jaw adductors of a high proportion of type 1 (slow, fatigue-resistant) fibers at the expense of other fiber types (the Frequent Recruitment Hypothesis). We used immunohistochemistry to determine the presence and distribution of the four major myosin heavy chain (MHC) isoforms in the anterior superficial masseter (ASM), superficial anterior temporalis, and deep anterior temporalis of four Gorilla gorilla, two Pan paniscus, and four Pan troglodytes. Serial sections were stained against slow (MHC-1/-α-cardiac) and fast (MHC-2/-M) fibers. Fibers were counted and scored for staining intensity, and fiber cross-sectional areas (CSAs) were measured and used to estimate percentage of CSA of each MHC isoform. Hybrid fibers accounted for nearly 100% of fiber types in the masseter and temporalis of all three species, resulting in three main hybrid phenotypes. As predicted, the gorilla ASM and deep anterior temporalis comprised a greater percentage of CSA of the slower, fatigue-resistant hybrid fiber type, significantly so for the ASM (p = 0.015). Finally, the results suggest that fiber phenotype of the chewing muscles contributes to behavioral flexibility in ways that would go undetected in paleontological studies relying solely on morphology of the bony masticatory apparatus.

Jaw-muscle fiber architecture in tufted capuchins favors generating relatively large muscle forces without compromising jaw gape

Tufted capuchins (sensu lato) are renowned for their dietary flexibility and capacity to exploit hard and tough objects. Cebus apella differs from other capuchins in displaying a suite of craniodental features that have been functionally and adaptively linked to their feeding behavior, particularly the generation and dissipation of relatively large jaw forces. We compared fiber architecture of the masseter and temporalis muscles between C. apella (n=12) and two "untufted" capuchins (C. capucinus, n=3; C. albifrons, n=5). These three species share broadly similar diets, but tufted capuchins occasionally exploit mechanically challenging tissues. We tested the hypothesis that tufted capuchins exhibit architectural properties of their jaw muscles that facilitate relatively large forces including relatively greater physiologic cross-sectional areas (PCSA), more pinnate fibers, and lower ratios of mass to tetanic tension (Mass/P(0)). Results show some evidence supporting these predictions, as C. apella has relatively greater superficial masseter and temporalis PCSAs, significantly so only for the temporalis following Bonferroni adjustment. Capuchins did not differ in pinnation angle or Mass/P(0). As an architectural trade-off between maximizing muscle force and muscle excursion/contraction velocity, we also tested the hypothesis that C. apella exhibits relatively shorter muscle fibers. Contrary to our prediction, there are no significant differences in relative fiber lengths between tufted and untufted capuchins. Therefore, we attribute the relatively greater PCSAs in tufted capuchins primarily to their larger muscle masses. These findings suggest that relatively large jaw-muscle PCSAs can be added to the suite of masticatory features that have been functionally linked to the exploitation of a more resistant diet by C. apella. By enlarging jaw-muscle mass to increase PCSA, rather than reducing fiber lengths and increasing pinnation, tufted capuchins appear to have increased jaw-muscle and bite forces without markedly compromising muscle excursion and contraction velocity. One performance advantage of this morphology is that it promotes relatively large bite forces at wide jaw gapes, which may be useful for processing large food items along the posterior dentition. We further hypothesize that this morphological pattern may have the ecological benefit of facilitating the dietary diversity seen in tufted capuchins. Lastly, the observed feeding on large objects, coupled with a jaw-muscle architecture that facilitates this behavior, raises concerns about utilizing C. apella as an extant behavioral model for hominins that might have specialized on small objects in their diets.