Callitrichids, an ecologically diverse radiation of small-bodied primates inhabiting Central and South America, include three groups, Goeldi’s monkeys (Callimico), tamarins (Saguinus, Leontopithecus), and marmosets (Callithrix, Cebuella). Marmosets differ from other callitrichids in that they actively stimulate tree exudate flow by biting trees with their anterior teeth. This type of biting behavior has been defined as gouging. Seasonally, exudates comprise a significant part of the marmoset diet suggesting the possibility of natural selective and/or functional adaptive changes in masticatory apparatus form related to this feeding behavior.
Comparative histomorphometrics are utilized to determine whether loading of the TMJ at wide jaw gapes elicits modifications of articular cartilage depth, area or proteoglycan density in two habitual gouging species, common (Callitrix jacchus) and pygmy marmosets (Cebuella pygmaea), compared with nongouging cotton-top tamarins (Saguinus oedipus) and squirrel monkeys (Saimiri sciureus). Histological comparisons found no difference in articular cartilage form along the posterior mandibular condyle between gouging and non-gouging species. Alternatively, the articular cartilage of the anterior glenoid of gougers was relatively larger and deeper compared with non-gouging species.
Allometric and heterochronic analyses of callitrichid growth patterns are analyzed to determine size-related shape changes and the timing of development of cranial features associated with wide gape. Analyses of marmoset and tamarin growth patterns indicate that cranial features associated with wide gapes exhibit changes in size-related shape and developmental timing consistent with increased jaw opening ability throughout postnatal ontogeny compared to non-gouging tamarins. The results are consistent with the hypothesis that marmosets exhibit morphological adaptations for tree gouging at large jaw gapes early in ontogeny.
Masticatory apparatus features of neonatal marmosets and tamarins are analyzed to determine if morphological differences observed between adults are present at birth, suggesting potential adaptations for tree gouging at wide gapes. Few differences in neonatal jaw-muscle architecture, mandibular cross-sectional geometry or symphyseal strength are found, indicating that at least some postnatal changes in the marmoset masticatory apparatus facilitating wide gapes arise from phenotypic plasticity related to gouging.
The mosaic nature of modifications of the marmoset masticatory apparatus suggest the existence of selective pressure providing morphological adaptations for gouging trees at wide gape.