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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 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.