Craniofacial adaptations to tree-gouging among marmosets

Many primates rely on exudates as dietary items, but comparatively few elicit exudates via tree-gouging. Marmosets are the only haplorhines to extensively utilize this behavior during feeding. Several studies have explored craniofacial adaptations to this behavior, but its morphological correlates are a matter of debate. Various studies suggest that gougers exhibit bite-force maximizing adaptations, load resistance adaptations, and/or jaw-gape maximizing characteristics. All of these seemingly incompatible biomechanical adaptations have previously been argued for marmosets. This study utilizes multivariate and univariate analyses to compare gouging and non-gouging callitrichids for 25 biomechanically relevant craniofacial variables to address this form-function debate. We show that marmosets differ from non-gouging callitrichids in few craniofacial characteristics. Specifically, three craniofacial features differentiate them from non-gougers: relatively longer basicrania, narrower palates, and shorter coronoid processes. We suggest that these characteristics are consistent with a mosaic model for gouging adaptations. In particular, we argue that: (a) shortening the coronoid processes facilitates relatively larger maximum jaw-gapes, (b) basicranial elongation facilitates the extended neck/head posture utilized by marmosets during gouging to maximize gapes, and (c) narrowing the palate serves to more effectively dissipate forces through the maxillary canines during substrate anchoring. Previous studies have documented some of these characters as typical of marmosets, but this combination has not been interpreted as core elements of the marmoset adaptive complex. Marmosets exhibit a unique anatomical repertoire that biomechanically adapts them to both increased jaw-gape and the force dissipation regime associated with tree-gouging. Comparisons among marmoset taxa may enlighten the evolutionary history of the features reported here.