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ABSTRACT ObjectivesBite force has received significant attention in biological anthropology, but maximum bite force estimates for a single primate species often span hundreds of newtons. In this synthesis, we discuss the definitions of maximum bite force, review and highlight the variability in methods used to assess bite force in primates, and compare bite force ranges in macaques to bracket maximum force estimates between physiological and mechanical maxima. Materials and MethodsMethods of estimating bite force in primates were gathered from the literature along with published estimates of maximum bite force for macaques (Macacasp.). ResultsMaximum bite force can be defined physiologically or mechanically, and methods of estimating bite force can be grouped as in vivo, muscle‐based, and craniodental within these two definitions. Physiological estimates occur under natural conditions modulated by sensorimotor feedback, whereas mechanical maximum bite forces ignore muscular and neural limitations. Published maximum bite forces for macaques at the molars vary from 127 N to 898 N, a 771 N range. Using a bracketing approach suggested here, we narrow the estimated bite force range at the incisors to 487–503 N and 503–898 N for the molars. DiscussionThis synthesis emphasizes the need for comparisons between in vivo, muscle‐based, and craniodental bite force methods in living primates. We propose bracketing bite force estimates between physiological and mechanical maxima in order to provide more reliable bite force estimates and improve understanding of how bite force relates to primate functional morphology and feeding ecology. 

ABSTRACT The jaw‐adductor muscles drive the movements and forces associated with primate feeding behaviors such as biting and chewing as well as social signaling behaviors such as wide‐mouth canine display. The past several decades have seen a rise in research aimed at the anatomy and physiology of primate chewing muscles to better understand the functional and evolutionary significance of the primate masticatory apparatus. This review summarizes variation in jaw‐adductor fiber types and muscle architecture in primates, focusing on physiological, architectural, and behavioral performance variables such as specific tension, fatigue resistance, muscle and bite force, and muscle stretch and gape.Paranthropus andAustralopithecusare used as one paleontological example to showcase the importance of these data for addressing paleobiological questions. The high degree of morphological variation related to sex, age, muscle, and species suggests future research should bracket ranges of performance variables rather than focus on single estimates of performance. 

ABSTRACT Bite force and gape are two important performance metrics of the feeding system, and these metrics are inversely related for a given muscle size because of fundamental constraints in sarcomere length–tension relationships. How these competing performance metrics change in developing primates is largely unknown. Here, we quantified in vivo bite forces and gapes across ontogeny and examined these data in relation to body mass and cranial measurements in captive tufted capuchins, Sapajus spp. Bite force and gape were also compared across geometric and mechanical properties of mechanically challenging foods to investigate relationships between bite force, gape and food accessibility (defined here as the ability to breach shelled nuts). Bite forces at a range of gapes and feeding behavioral data were collected from a cross-sectional ontogenetic series of 20 captive and semi-wild tufted capuchins at the Núcleo de Procriação de Macacos-Prego Research Center in Araçatuba, Brazil. These data were paired with body mass, photogrammetric measures of jaw length and facial width, and food geometric and material properties. Tufted capuchins with larger body masses had absolutely higher in vivo bite forces and gapes, and animals with wider faces had absolutely higher bite forces. Bite forces and gapes were significantly smaller in juveniles compared with subadults and adults. These are the first primate data to empirically demonstrate the gapes at which maximum active bite force is generated and to demonstrate relationships to food accessibility. These data advance our understanding of how primates meet the changing performance demands of the feeding system during development. 

Abstract The ontogeny of feeding is characterized by shifting functional demands concurrent with changes in craniofacial anatomy; relationships between these factors will look different in primates with disparate feeding behaviors during development. This study examines the ontogeny of skull morphology and jaw leverage in tufted (Sapajus) and untufted (Cebus) capuchin monkeys. UnlikeCebus,Sapajushave a mechanically challenging diet and behavioral observations of juvenileSapajussuggest these foods are exploited early in development. Landmarks were placed on three‐dimensional surface models of an ontogenetic series ofSapajusandCebusskulls (n = 53) and used to generate shape data and jaw‐leverage estimates across the tooth row for three jaw‐closing muscles (temporalis, masseter, medial pterygoid) as well as a weighted combined estimate. Using geometric morphometric methods, we found that skull shape diverges early and shape is significantly different betweenSapajusandCebusthroughout ontogeny. Additionally, jaw leverage varies with age and position on the tooth row and is greater inSapajuscompared toCebuswhen calculated at the permanent dentition. We used two‐block partial least squares analyses to identify covariance between skull shape and each of our jaw muscle leverage estimates.Sapajus, but notCebus, has significant covariance between all leverage estimates at the anterior dentition. Our findings show thatSapajusandCebusexhibit distinct craniofacial morphologies early in ontogeny and strong covariance between leverage estimates and craniofacial shape inSapajus. These results are consistent with prior behavioral and comparative work suggesting these differences are a function of selection for exploiting mechanically challenging foods inSapajus, and further emphasize that these differences appear quite early in ontogeny. This research builds on prior work that has highlighted the importance of understanding ontogeny for interpreting adult morphology.