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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 ObjectivesWild juvenile capuchins exhibit lower feeding success than adults, particularly for mechanically challenging foods, but ontogenetic changes in oral food processing behaviors related to this reduced success are unknown. We test how oral food processing efficiency varies across development in an experimental setting in tufted capuchins (Sapajusspp.). Further, we simulate discontinuous feeding observations to test the comparability of behaviors measured in wild and captive settings. Materials and MethodsTwenty‐nine captive and semi‐wild infants (n = 2), juveniles (n = 12), older juveniles (n = 4), and subadults‐adults (n = 11) were video recorded while feeding at the Núcleo de Procriação de Macacos‐Prego Research Center (Araçatuba, Brazil). Each animal was offered a series of five foods ranging in volume, toughness, and elastic modulus. ResultsMeasures of oral food processing inconsistently varied with sex; however, younger animals were less efficient in food processing than older individuals. Larger and more mechanically challenging foods were associated with longer feeding sequence durations and an increased frequency of anterior ingestion, posterior ingestion, and chewing during a feeding sequence. Simulated discontinuous data from the first and last halves of the feeding sequences closely replicated continuous results. ConclusionsOur results indicate younger capuchins have reduced oral food processing efficiency compared to adults through increased duration, behavioral frequencies, number of chews, and behavioral patterns. Further, our continuous and discontinuous comparisons support the use of discontinuous feeding behaviors from the first and last halves of the feeding sequence. We caution that researchers should be careful to capture infrequent behaviors when using discontinuous data. 

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 ObjectivesCortical bone geometry is commonly used to investigate biomechanical properties of primate mandibles. However, the ontogeny of these properties is less understood. Here we investigate changes in cortical bone cross‐sectional properties throughout capuchin ontogeny and compare captive versus wild, semi‐provisioned groups. Tufted capuchins (Sapajusspp.) are known to consume relatively hard/tough foods, while untufted capuchins (Cebusspp.) exploit less mechanically challenging foods. Previous research indicates dietary differences are present early in development and adultSapajusmandibles can resist higher bending/shear/torsional loads. Materials and methodsThis study utilized microCT scans of 22Cebusand 45Sapajusfrom early infancy to adulthood from three sample populations: one captiveCebus, one captiveSapajus, and one semi‐provisioned, free‐rangingSapajus. Mandibular cross‐sectional properties were calculated at the symphysis, P₃, and M₁. If the tooth had not erupted, its position within the crypt was used. A series of one‐way ANOVAs were performed to assess differences between and within the sample populations. ResultsMandible robusticity increases across ontogeny for all three sample populations.Sapajuswere better able to withstand bending and torsional loading even early in ontogeny, but no difference in shear resistance was found. Semi‐provisioned, free‐rangingSapajustend to show increased abilities to resist bending and torsional loading but not shear loading compared to captiveSapajus. DiscussionThis study helps advance our understanding of the primate masticatory system development and opens the door for further studies into adaptive plasticity in shaping the masticatory apparatus of capuchins and differences in captive versus free‐ranging sample populations. 

Abstract In 1974, Sue Herring described the relationship between two important performance variables in the feeding system, bite force and gape. These variables are inversely related, such that, without specific muscular adaptations, most animals cannot produce high bite forces at large gapes for a given sized muscle. Despite the importance of these variables for feeding biomechanics and functional ecology, the paucity of in vivo bite force data in primates has led to bite forces largely being estimated through ex vivo methods. Here, we quantify and compare in vivo bite forces and gapes with output from simulated musculoskeletal models in two craniofacially distinct strepsirrhines:Eulemur, which has a shorter jaw and slower chewing cycle durations relative to jaw length and body mass compared toVarecia. Bite forces were collected across a range of linear gapes from 16 adult lemurs (suborder Strepsirrhini) at the Duke Lemur Center in Durham, North Carolina representing three species:Eulemur flavifrons(n = 6; 3F, 3M),Varecia variegata(n = 5; 3F, 2M), andVarecia rubra(n = 5; 5F). Maximum linear and angular gapes were significantly higher forVareciacompared toEulemur(p = .01) but there were no significant differences in recorded maximum in vivo bite forces (p = .88). Simulated muscle models using architectural data for these taxa suggest this approach is an accurate method of estimating bite force‐gape tradeoffs in addition to variables such as fiber length, fiber operating range, and gapes associated with maximum force. Our in vivo and modeling data suggestVareciahas reduced bite force capacities in favor of absolutely wider gapes compared toEulemurin relation to their longer jaws. Importantly, our comparisons validate the simulated muscle approach for estimating bite force as a function of gape in extant and fossil primates. 

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. 

Orienting a food item held in the hand to withdraw and optimally place it in the mouth for eating (withdraw-to-eat) is mediated by vision in catarrhine anthropoids and by nonvisual strategies in strepsirrhine primates. The present study asks whether vision contributes to the withdraw-to-eat movements in a platyrrhine anthropoid Cebus imitator, a member of a monophyletic primate suborder whose stem group diverged from catarrhines about 40 million years ago. Cebus imitator’s gaze and hand use for foraging for fruit is examined in its fine branch niche, the terminal branches of trees. Video of reach, grasp and withdraw-to-eat movements with associated gaze were examined frame-by-frame to assess food manipulation and its sensory control. Cebus imitator uses vision and touch to reach for and grasp food items with precision and whole hand grasps. They use vision to orient food items held in-hand into a precision grip and their withdraw-to-eat is assisted with a vertically oriented hand. The conjoint use of vision, a precision grasp, and hand posture and a central representation of object control likely originated in stem anthropoids and was derived from the staged evolution of the visual manipulation of food and other objects. 

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.