1Department of Anatomy & Neurobiology, NEOMED, 2Department of Evolutionary Anthropology, Duke University
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The ability to open the jaw widely has significant functional consequences for primate feeding and social behaviors. We measured maximum jaw-opening ability, or maximum gape, in 765 adult male and female anesthetized/cadaveric individuals representing 63 primate species to examine patterns of allometry in this performance measure. Reduced-major axis (RMA) regressions of combined sex samples indicate that maximum gape scales with slight negative allometry (slope=0.95) relative to body mass0.333 and slight positive allometry relative to jaw length (slope=1.11) across primates, although isometry is not ruled out in either case. Relative to basion-nasion length, maximum gape scales with positive allometry (slope=1.4). Maximum gape scales with negative allometry relative to antero-posterior condyle length (slope=0.89) and near isometry relative to glenoid length (slope=0.99). The functional implications of these scaling patterns suggest that relative increases in gape tend to follow from relative increases in condylar curvature (as estimated by AP condyle length) and relatively decreased contributions from jaw length across primates. While anthropoids and strepsirrhines show no differences in scaling patterns relative to body mass, maximum gape in anthropoids is transposed above strepsirrhines when regressed against either jaw length or basion-nasion length. This transposition along with the positive allometry and tendency for anthropoids to be larger than strepsirrhines all contribute to the relatively larger gapes of anthropoids compared to strepsirrhines when evaluated against craniofacial measures. Documenting the allometric relationships between maximum gape and other masticatory measures will provide a baseline for interpreting variation in this performance measure in specific primate groups.
This work was funded by grants from the National Science Foundation (BCS-0138565, BCS-0720025, BCS-0552285, BCS-0420133, BCS-0412153).