Ontogeny of feeding function in the gray short-tailed opossum Monodelphis domestica: empirical support for the constrained model of jaw biomechanics
Elicia N. Thompson1,
Audrone R. Biknevicius2,* and
Rebecca Z. German3
1 Department of Biological Sciences, Ohio University, Athens, OH 45701,
USA
2 Department of Biomedical Sciences, Ohio University College of Osteopathic
Medicine, Athens, OH 45701, USA
3 Department of Biological Sciences, University of Cincinnati, Cincinnati,
OH 45221, USA
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Fig. 1. The constrained lever model of the masticatory apparatus illustrated on the
ventral skull of Monodelphis domestica. (A) The Region I-II boundary
(indicated by the star) is located where the trajectory from the
balancing-side temporomandibular joint (TMJ) intersects the working-side jaw
after passing through the midline muscle resultant (midline circle). Region I
and Region II are those parts of the dental array rostral and caudal to the
Region I-II boundary, respectively. (B) The in-lever and out-lever are
measured as the perpendicular distance from the interglenoid line to the
muscle resultant (LM) and to the bite point (shown here at the Region I-II
boundary, LRII), respectively. (C) Biting with Region I teeth. The triangle of
support (hatched area) is defined by the working- and balancing-side TMJs and
the bite point (shown here as the I2). Because a midline muscle
resultant is enclosed within the triangle of support, jaw adductor muscles may
contract maximally and bilaterally. (D) Biting with Region II teeth. The
triangle of support shrinks as bites are produced with teeth located caudal to
the Region I-II boundary (bite point shown here is the M4). The
muscle resultant vector must shift laterally toward the working side jaw in
order to remain within the triangle of support (illustrated by the arrow);
this accomplished via a reduction in the contractile force of the
balancing-side musculature.
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Fig. 2. Effect of changing craniofacial configurations on mechanical advantage. The
inset illustrates the simplification of the palate as a rectangle, the glenoid
fossae as ovals, and the midline muscle resultant as the filled circles. The
solid lines portray a baseline condition for comparison with the altered
condition (indicated by the broken lines), illustrating three different means
by which mechanical advantage may be enhanced: (A) elongation of the
masticatory muscle resultant lever arm (not effective; see Appendix I), (B)
widening interglenoid width and (C) palatal widening.
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Fig. 3. Ventral view of the skull of a Monodelphis domestica adult. (A)
Maximal bite forces were measured from three locations along the jaw: incisors
or canines (I,C), premolars (P) and molars (M) and deciduous premolar (dP;
juveniles only) located in Region II (RII). The first two locations are in
Region I. The oblique broken trajectory intersects the left jaw at the Region
I-II boundary. (B) Landmarks digitized on radiographs (see
Table 1). (C) Linear
measurements produced for the reduced major axis and Gompertz regressions:
IGW, interglenoid width; JL, jaw length; LM, lever arm of resultant adductor
muscle force (in-lever); LRII, lever arm to the Region I-II boundary
(out-lever); PW, palatal width. Stars indicate the bite point.
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Fig. 5. Scaling of (A) interglenoid and palatal widths and (B) lever arms to the
Region I-II boundary and the muscle resultant against jaw length. The solid
line represents isometry. All length measurements are in mm. The reduced major
axis regression slopes and 95% confidence intervals listed in
Table 3. Abbreviations as in
Fig. 3.
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Fig. 4. Box plots of bite forces collected from Monodelphis domestica
juveniles (J) and adults (A) at three locations along the dental arcade:
Region II (RII, including molars and, in juveniles, deciduous premolar),
premolar (P) and incisors or canines (I,C). Significant differences within the
sample: 1, P<0.02 between age groups within a single dentary
region; 2, P<0.001 between tooth positions within an age
group.
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Fig. 6. Growth in cranial length and width for M. domestica, with the
best-line fit by the Gompertz model: (A) interglenoid width, (B) palatal width
and (C) jaw length.
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© The Company of Biologists Ltd 2003
