Date Published: July 24, 2011
Publisher: Hindawi Publishing Corporation
Author(s): Jonathan M. G. Perry, Adam Hartstone-Rose, Rachel L. Logan.
We reconstructed the jaw adductor resultant in 34 primate species using new data on muscle physiological cross-sectional area (PCSA) and data on skull landmarks. Based on predictions by Greaves, the resultant should (1) cross the jaw at 30% of its length, (2) lie directly posterior to the last molar, and (3) incline more anteriorly in primates that need not resist large anteriorly-directed forces. We found that the resultant lies significantly posterior to its predicted location, is significantly posterior to the last molar, and is significantly more anteriorly inclined in folivores than in frugivores. Perhaps primates emphasize avoiding temporomandibular joint distraction and/or wide gapes at the expense of bite force. Our exploration of trends in the data revealed that estimated bite force varies with body mass (but not diet) and is significantly greater in strepsirrhines than in anthropoids. This might be related to greater contribution from the balancing-side jaw adductors in anthropoids.
Knowing the magnitude and orientation of the force produced by a muscle is critical to understanding muscle function. These variables can be used to better understand the properties of foods and how they relate to food-processing anatomy and behavior. They can also be used to provide more informed inferences about feeding behavior in fossils. The magnitude of muscle force can be estimated from the physiological cross-sectional area (PCSA) of the muscle. Orientation can be estimated from the positions of the muscle attachments. In a complex system where several muscles work together to perform a single action, knowing the vector of each muscle is critical to understanding function and adaptation in the system as a whole. The jaw adductors comprise such a system and the forces might be used as signals for dietary adaptation.
We used published data on jaw adductor PCSA for all taxa. These data come from two sources: one for the strepsirrhines and tarsiers [30, 32, 33] and one for the anthropoids . Other data on primate chewing muscle PCSA have been collected by others using slightly different methods [34–36]. We have not used those data here because they pertain mostly to large catarrhines which are mainly absent from our current sample.
The primary goal of this study was to test three predictions regarding the jaw adductor resultant in primates. We also analyzed the variation in resultant orientation in the context of diet. Our secondary goal was to make a preliminary examination of the scaling of estimated bite force, using data on PCSA and skull shape. We also analyzed estimated bite force in the context of diet and major taxonomic group. In the following sections, we discuss the degree to which our observations did or did not conform to the three predictions. Then, we discuss variation in estimated bite force.
Our data on primates suggest that the resultant vector of the jaw adductors passes significantly posterior to the 30% point, the location where average bite force across the teeth is maximized. Furthermore, the most posterior molar is located well anterior to that 30% point. These observations suggest that bite force is reduced in primates relative to the previously hypothesized optimum, perhaps as an extra precaution against TMJ tension and/or to allow for increased gape.