Feeding systems are the energy-acquiring systems of humans and other primates. The teeth, jaws, and muscles, are the proximal interface between primates and their environments. Therefore, understanding the evolutionary forces that shape those components is essential to understanding the adaptation and evolution of primates. This work tests hypotheses and models relating diversity in the primate feeding system to size-related changes in food intake rate. It documents how shape and movements of the mandible, the size and architecture of the jaw muscles, and the amount of food an animal eats in a single bite change with body size to meet size-required changes in food intake rate of primates. These data will be collected using computed tomography (CT) scanning of primate mandibles, anatomical studies of primate chewing muscles, and behavioral studies of primates feeding in captivity. Mathematical models of the feeding system will be tested, modified and improved, then used to examine how feeding system designs in different evolutionary groups of primates balance trade-offs between advantages of bite force production, chewing speed and gape.
This research will create novel and important data sets that can be accessed in the future by other researchers interested in feeding biomechanics, bone biomechanics, and musculoskeletal systems in general. The investigators will continue to recruit under-represented minority and female undergraduates to receive training and mentoring in research and advice on their paths to graduate, medical, and other professional schools. These students will collaborate in all aspects of the work, including presentation and publication. The PIs will continue their outreach programs to local schools and their synergistic activities with other NSF-funded projects.
Goals Food intake rate is an important performance variable related to how primates meet their energy demands and spend their time. One of the things that determines food intake rate in primates is chewing frequency (i.e., how fast primates chew). In 2009, Ross et al. (Scaling of chew cycle duration in primates. American Journal of Physical Anthropology 138(1):30-44) advanced a biomechanical model to predict the scaling of chewing cycle duration in primates. This model included estimates of the rotational inertia of the mandible (i.e., how difficult it is to rotate the lower jaw up and down during chewing), jaw muscle architectural variables, and maximum ingested bite size (i.e., how much food primates can put in their mouths). The goals of the research at the University of Chicago were to gather data from a broad sample of primates on the rotational inertia and location of the instantaneous center of rotation of the mandible and use these data to evaluate the accuracy of the biomechanical model advanced by Ross et al. (2009). To accomplish these goals, we quantified the location of the center of mass (CoM) of the mandible in a broad size and diet range of primates. From a more restricted sample of primates, we also calculated the location of the finite helical axis (FHA) and quantified the rotational inertia of the mandible. Intellectual merit Our research on jaw kinematics revealed that the axis of rotation of the mandible during chewing in three species of primates is located below the jaw joint, near the level of the toothrow. Our research on the center of mass (CoM) of the primate mandible located the CoM in the midline near the back of the toothrow in all primates. Combined with assumptions about the location of the axis of rotation of the mandible (based on the results above), we were able to then calculate how the difficulty of opening and closing the mandble changes with primate size. We found that as primates get larger, the shape and size of their mandible changes in such as way as to make it more and more difficult for them to rotate their mouths open and closed. They must be compensating for this by changes in jaw muscle architecture and size, because larger primates chew faster than you ould expect based on the size related changed in the moment of inertia of the mandible. These results suggest that future understanding of the determinants of primate food intake rate (chewing speed) will depend on how well we understand the anatomy and function of primate jaw muscles. Broader impacts of our research Our data set on jaw movements during primate chewing is the largest ever collected and is significantly larger than that available for humans. Combined with published data on humans, it constitutes a significant advance in our understanding of how primates chew. It will enable detailed biomechanical modeling of primate feeding systems which will in turn allow functional interpretation of the fossil record of our close relatives and ancestors. In performing this research, several undergraduates, graduate students and postdocs were exposed to and trained in new research techniques, developing their interest and ability to contribute to scientific research and teaching in the future. Our lab also participated in outreach activities at local schools in Oak Park, Illinois, discussing the function and structure of the human body with middle school and elementary school students.
