The mechanics of anterior tooth use in primates

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The prevalence of tree exudate feeding (e.g., tree gums and saps) or gummivory among primates suggests that this feeding behavior has significantly influenced primate evolution. Some gummivorous primate species actively elicit tree exudate flow by mechanically damaging trees with their anterior teeth. This type of biting behavior is defined as gouging. This project will record jaw forces and jaw gapes (or the magnitude of jaw opening) during gouging in the South American common marmoset (Callithrix jacchus). Anthropologists previously have hypothesized that tree gouging involves large jaw forces or large jaw gapes relative to those experienced during chewing and incision. These hypotheses have been put forth without any attempt to measure the jaw forces or jaw gapes during gouging. The proposed research will generate novel information about the possible mechanical effects that gummivory might have on primate skull form and function. Based on preliminary data, gouging appears to require relatively large jaw gapes, but not necessarily relatively large jaw forces. This finding contradicts earlier ideas about this behavior and emphasizes the under-appreciated significance of jaw gapes in primate biting. With few exceptions, in vivo studies of primate jaw and tooth function (i.e., studies of living animals performing a behavior) have not focused on anterior tooth use or biting. This shortcoming persists despite several researchers noting the likely significance of biting for primate morphology and ecology. This proposal is important because it will be one of the first in vivo studies of jaw mechanics during a biting behavior in primates and therefore will contribute new insights into how biting might affect the form and function of primate skulls. The in vivo data collected in this project will play a key role in studying adaptations for gouging in the marmoset head. Using recently collected field data, the substrates that marmosets gouge in the lab will be made to mimic the trees that marmosets gouge in the wild. By matching the lab substrates to wild ones, this project can more realistically study jaw performance during gouging. This in turn provides improved tests of adaptive hypotheses for this behavior. Furthermore, the in vivo data are a core reference for comparisons of skull functional morphology, quantitative genetics of skull form, jaw-muscle fiber architecture and jaw trabecular structure between marmosets and non-gouging tamarins. These analyses will provide an extensive characterization of tree gouging adaptations in marmosets and serve as a model approach for other anthropologists for combining lab, field and skull morphometric methods in studying primate adaptations. Another important goal of this project involves training students through integrating education and research. The promotion of student learning and training through active participation will be emphasized in this project. Because in vivo research skills can only be mastered through interactive apprenticeship and these opportunities are relatively rare, this project will provide a significant educational resource for biological anthropology. Finally, the inclusive project studying skull function and ecology of tree gouging in marmosets links researchers from U.S. and Brasilian universities fostering interaction and intellectual exchange between them.