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First published online March 2, 2007
Journal of Experimental Biology 210, 1046-1063 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02733
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Modulation of mandibular loading and bite force in mammals during mastication

Callum F. Ross1,*, Ruchi Dharia2, Susan W. Herring3, William L. Hylander4, Zi-Jun Liu3, Katherine L. Rafferty3, Matthew J. Ravosa5 and Susan H. Williams6

1 Organismal Biology and Anatomy, University of Chicago, 1027 E. 57th Street, Chicago, IL 60637, USA
2 Stony Brook School of Medicine, Health Sciences Center Level 4, Stony Brook, NY 11794-8434, USA
3 Department of Orthodontics, School of Dentistry, University of Washington, Seattle, WA 98195-357446, USA
4 Department of Biological Anthropology and Anatomy, Duke University Lemur Center, Durham, NC 27710, USA
5 Department of Pathology and Anatomical Sciences, University of Missouri School of Medicine, One Hospital Drive – Medical Sciences Building, Columbia, MO 65212, USA
6 Department of Biomedical Sciences, Ohio University College of Osteopathic Medicine, 228 Irvine Hall, Athens, OH 45701, USA


Figure 1
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  		Fig. 1. Diagram illustrating two ways of modulating strain magnitude and bite force during the power stroke of a chewing cycle. (A) Strain profiles, modified to illustrate the hypotheses. (B) Triangles illustrating the variables measured in this study. Note that these triangles only describe strain profiles in loading (i.e. prior to peak strain). The dark lines and triangle are low-magnitude events, while the lighter (red) lines are higher-magnitude events. (Top) Magnitude can be increased by increasing loading time, while load rate is kept constant. (Bottom) Magnitude can be increased by increasing loading rate, while keeping loading duration constant. Combinations of these strategies are possible.

 

Figure 2
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  		Fig. 2. Illustration of variables extracted from the strain data. Plot of {epsilon}1 magnitude recorded from lateral aspect of the mandibular corpus of owl monkey 1 in experiment 9 (data from C.F.R. and W.L.H., recorded at Duke University). Three chews ipsilateral to the strain gage are shown. The strain profile between power strokes does not return to zero because of strain in the mandible during opening. The following data were extracted from each power stroke: time (open circles) and magnitude (black circles) of peak strain, and time at which 5% of peak strain was reached in loading (grey circles). The duration of loading was calculated as the time from 5% of peak to strain to peak strain; the loading rate was calculated as peak strain magnitude divided by duration of loading. Cycle time for each cycle was estimated as the duration of time from the preceding peak to the following peak, divided by 2. In the case of the middle cycle in this figure, cycle time=(T3–T1/2)

 

Figure 3
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  		Fig. 3. Bivariate plots of {epsilon}1 magnitude in microstrain (µ{epsilon}) against loading time (in s) (A) and loading rate in µ{epsilon} s–1 (B). Data recorded during Experiment 71 on Eulemur fulvus eating apple (black circles), grapes (grey circles) and raisins (open circles). Note that there is not a significant correlation between strain magnitude and loading time (A), but there is a significant correlation between strain magnitude and loading rate (B). Although these data are not presented here, it is clear that these patterns of relationship (or lack thereof) also apply within different food types, although the nature of the relationship (i.e. the slope) may vary across food types.

 

Figure 4
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  		Fig. 4. Bivariate plots of {epsilon}1 magnitude in microstrain (µ{epsilon}) against loading time (in s) (A) and loading rate in µ{epsilon} s–1 (B). Data recorded during Experiment 9 on Aotus eating a range of foods. Note that there is not a significant correlation between strain magnitude and loading time (A), but there is a significant correlation between strain magnitude and loading rate (B). It is of interest that these patterns of relationship (or lack thereof) apply within most, but not all, of the different food types. Variations in these relationships within food type are not consistent across experiments.

 

Figure 5
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  		Fig. 5. Bivariate plots of {epsilon}1 magnitude in microstrain (µ{epsilon}) against loading time (in s) (A) and loading rate in µ{epsilon} s–1 (B). Data recorded during Experiment 103 on Sus eating pig chow. The data are labeled by chewing sequence. Note that, across all chews, there is not a significant correlation between strain magnitude and loading time (A), but there is a significant correlation between strain magnitude and loading rate (B). Analyses of data within chewing sequences reveal that these patterns are also seen within sequences.

 

Figure 6
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  		Fig. 6. Plots illustrating analysis of data from Experiment 16 on Capra eating hay. (A) Bivariate plot of {epsilon}2 magnitude in microstrain (µ{epsilon}) against loading time (in s). (B) Bivariate plot of {epsilon}2 magnitude in microstrain (µ{epsilon}) against loading rate in µ{epsilon} s–1. Regression equation of {epsilon}2 magnitude against loading rate in µ{epsilon} s–1: {epsilon}2 magnitude=–117.07+0.11x{epsilon}2 load rate. (C,D) Partial regression plots from multiple regression of {epsilon}2 magnitude in microstrain (µ{epsilon}) against loading time (in s) and loading rate in µ{epsilon} s–1. (E) Plot of residual {epsilon}2 magnitude (i.e. variance not explained by the regression in B) against load time (s). (F) Bivariate plot of loading rate in µ{epsilon} s–1, against loading time (in s). There is not a significant correlation between strain magnitude and loading time (A), but there is a significant correlation between strain magnitude and loading rate (B). Partial regression plots illustrate relationship between dependent variable ({epsilon}2 magnitude) and one independent variable, while holding the other variable constant. These partial regression plots suggest close relationships between strain magnitude and each independent variable when controlling for the other because, as quantified here, strain magnitude must be nearly completely explained by a combination of load rate and load time. (F) Increases in loading rate are accompanied by increases in loading time, reinforcing the conclusion that load time and load rate are both strategies employed to increase strain magnitude. However, examination of bivariate plots A and B reveals that load time explains little of the variance in strain magnitude. Once the effect of strain rate is accounted for, there is a weak relationship between residual strain magnitude and load time, as illustrated in E, with load time explaining much less of the variance in strain magnitude than load rate. The data from this experiment consist of two separate chewing sequences. The data from the two sequences are indicated by separate symbols, showing that the effects revealed across the whole experiment also obtain within chewing sequences.

 




© The Company of Biologists Ltd 2007