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. 2011 Jun 30;51(2):260–270. doi: 10.1093/icb/icr066

A Preliminary Analysis of Correlations between Chewing Motor Patterns and Mandibular Morphology across Mammals

Christopher J Vinyard *,1, Susan H Williams †, Christine E Wall ‡, Alison H Doherty *, Alfred W Crompton §, William L Hylander ‡
PMCID: PMC3135828  PMID: 21724618

Abstract

The establishment of a publicly-accessible repository of physiological data on feeding in mammals, the Feeding Experiments End-user Database (FEED), along with improvements in reconstruction of mammalian phylogeny, significantly improves our ability to address long-standing questions about the evolution of mammalian feeding. In this study, we use comparative phylogenetic methods to examine correlations between jaw robusticity and both the relative recruitment and the relative time of peak activity for the superficial masseter, deep masseter, and temporalis muscles across 19 mammalian species from six orders. We find little evidence for a relationship between jaw robusticity and electromyographic (EMG) activity for either the superficial masseter or temporalis muscles across mammals. We hypothesize that future analyses may identify significant associations between these physiological and morphological variables within subgroups of mammals that share similar diets, feeding behaviors, and/or phylogenetic histories. Alternatively, the relative peak recruitment and timing of the balancing-side (i.e., non-chewing-side) deep masseter muscle (BDM) is significantly negatively correlated with the relative area of the mandibular symphysis across our mammalian sample. This relationship exists despite BDM activity being associated with different loading regimes in the symphyses of primates compared to ungulates, suggesting a basic association between magnitude of symphyseal loads and symphyseal area among these mammals. Because our sample primarily represents mammals that use significant transverse movements during chewing, future research should address whether the correlations between BDM activity and symphyseal morphology characterize all mammals or should be restricted to this “transverse chewing” group. Finally, the significant correlations observed in this study suggest that physiological parameters are an integrated and evolving component of feeding across mammals.

Morphologists have put forth significant effort describing patterns of evolutionary variation in the bones, muscles, and teeth of the mammalian masticatory apparatus (e.g., Gregory 1922; DuBrul and Sicher 1954; Maynard Smith and Savage 1959; Turnbull 1970; Scapino 1972, 1981; Hershkovitz 1977; Hylander 1979a; Lucas 1979; Radinsky 1981, 1985; Greaves 1982; Freeman 1988; Ravosa 1991; Biknevicius and Ruff 1992). Compared to these morphological descriptions, we know far less about variation across mammals in physiological measures during feeding, such as jaw-muscle activity, jaw movements, and tissue deformations. In fact, the consensus view of the number of distinct jaw-muscle motor patterns hypothesized to characterize mammalian feeding has increased in proportion to the number of species studied over the past 40 years.

As we continue to learn more about the physiology of feeding behaviors across mammals, we can begin to ask key questions about how feeding physiology evolved in this clade (Wall et al. 2011). At present, we lack a strong, phylogenetically-informed hypothesis of the primitive mammalian state for most physiological variables relating to feeding (Hiiemae 2000). Moreover, the lack of phylogenetic reconstructions have left us with little more than informed speculation about physiological states characterizing the ancestors of most mammalian clades. These shortcomings limit our understanding of the evolution of feeding physiology in mammals, specifically (1) when major changes took place, (2) the extent of convergence among lineages, and (3) the rates at which evolution occurred.

The relative lack of physiological data that are comparable across most mammalian clades in part reflects (1) the difficulty in collecting these data, (2) variation in methods of data collection used in different laboratories, and (3) the absence of a data repository that provides the research community centralized access. The goals of the Feeding Experiments End-user Database (FEED) are to address these second and third issues by providing a public database and by beginning a discussion of data collection standards to facilitate comparability across experiments and laboratories. As the physiological data in FEED accumulate, we anticipate being able to address several important questions regarding the physiology and evolution of mammalian mastication. This issue of ICB provides initial considerations of important evolutionary questions for mammalian feeding relating to (1) the homology and evolution of jaw-muscle morphology across mammals (Druzinsky et al. 2011), (2) the evolution of jaw-muscle motor patterns across mammals (Williams et al. 2011), (3) how the activity of mammalian jaw muscles compares to motor patterns across vertebrates (Konow et al. 2011), and (4) how variation in jaw-muscle activity relates to differences in masticatory morphology across mammals. We consider this final question in a preliminary fashion here.

Researchers collectively recognize the significance of linking physiological data on feeding to masticatory apparatus morphology as these interactions jointly affect feeding performance. Linking these data are essential for understanding how form and function evolved because physiology puts morphology into action. While most analyses of jaw-muscle EMGs do relate observed motor patterns to morphology, in general these comparisons are appropriately focused on the species being studied. Our effort here is to broaden the scope of comparison to identify potential correlations between masticatory form and physiology across mammals in an explicitly phylogenetic framework.

Demonstrating correlations between masticatory form and physiology suggests that certain functional relationships are maintained across Mammalia. Although not typical of functional integration studies in the sense of demonstrating a joint impact on performance (Cheverud 1996), the correlated evolution of physiological and morphological traits across mammals, despite variation in diet and feeding styles, would suggest an integrated functional relationship during feeding. Conversely, the absence of mammal-wide correlations between form and physiology would not preclude functional associations within mammals, but more reasonably would suggest that these associations characterize specific groups of mammals sharing similarities in diet, feeding behaviors, and/or phylogenetic history. Simply describing these quantitative associations between physiological variables and morphology helps us to interpret patterns of morphological integration (sensu Olson and Miller 1958; Dullemeijer 1989) and evolution of the mammalian masticatory apparatus.

Potential associations between jaw-muscle motor patterns and masticatory morphology will also contribute to the on-going discussion of whether feeding motor patterns are conserved in different vertebrate groups, including mammals (Hiiemae 1978; 2000; Shaffer and Lauder 1985; Wainwright and Lauder 1986; Smith 1994; Weijs 1994; Alfaro and Herrel 2001; Alfaro et al. 2001; Herrel et al. 2001; Langenbach and van Eijden 2001; Wainwright and Friel 2001; Wainwright 2002; Vinyard et al. 2007; Williams et al. 2007; Crompton et al. 2008a, 2010). Given that conservation of motor pattern is typically assessed relative to levels of morphological variation, we argue that significant associations between jaw-muscle activity and masticatory shapes across mammals would provide evidence against the conservation of motor patterns in mammals. Alternatively, significant correlations would suggest that physiological parameters are an integrated and evolving component of feeding in mammals.

Hypotheses and exploratory analysis

We examine three hypotheses linking electromyographic (EMG) activity from the mammalian jaw muscles to the morphology of the mammalian jaw. Additionally, we conduct an exploratory analysis asking whether other jaw-muscle activity patterns (i.e., those not considered in our hypothesis tests) relate to measures of jaw robusticity across our limited sample. The data set we examine is decidedly nonrepresentative of Mammalia as it includes data from only a limited number of species representing relatively few mammalian clades. Thus, the main benefits of this preliminary analysis are to provide a template for future analyses of jaw-muscle activity and morphology using data in FEED as well as to develop hypotheses for future testing.

Hypothesis 1

Increased recruitment of the balancing-side jaw muscles during chewing is linked to relatively robust jaws. This hypothesis argues that increased production of force during mastication is disproportionately generated by the recruitment of balancing-side (or non-chewing-side) muscles and that the elevated loads are resisted by more robust jaws. The argument was originally formulated for primates based on bone strain and on jaw-muscle EMG data (Hylander 1979a, 1979b; Hylander et al. 1998, 2000). Similar hypotheses have been suggested for artiodactyls (Hogue and Ravosa 2001) and marsupials (Hogue 2008). We consider the correlation between working- to balancing-side (W/B) recruitment ratios and an estimate of jaw robusticity to test this hypothesis. This hypothesis, as well as others linking EMG activity and jaw robusticity, assume that peak EMG activity occurs during a quasistatic period of the chewing cycle in order to apply peak EMG data as a surrogate of muscle force (e.g., Weijs 1980; Hylander and Johnson 1989, 1993, Hylander et al. 2000, 2005; Ross et al. 2005; Vinyard et al. 2005, 2006).

Hypothesis 2

Increased recruitment of the balancing-side deep masseter muscle (BDM) is related to symphyseal robusticity. This prediction follows from research demonstrating that primates with fused symphyses exhibit increased levels of BDM recruitment (Hylander et al. 1998, 2000, 2005, 2011; Vinyard et al. 2005, 2006). Moreover, quantitative comparisons suggest that relative symphyseal size may be correlated with activity levels of the BDM in primates with either fused or unfused symphyses (Vinyard et al. 2007). Similar relationships between BDM recruitment and symphyseal robusticity have been predicted for artiodactyls (Hogue and Ravosa 2001) and extended to mammals (Ravosa and Hogue 2004). In primates the elevated BDM recruitment is hypothesized to create increased lateral transverse bending, or “wishboning”, of the symphysis (Hylander and Johnson 1994), but outside of primates symphyseal fusion is not necessarily linked to the wishboning loading regime (Williams et al. 2008; Crompton et al. 2010). We consider the relationship between the relative activity level of the BDM and symphyseal robusticity across our mammalian sample.

Hypothesis 3

The timing of peak activity of the BDM is correlated with symphyseal robusticity. This predicted relationship is a corollary of Hypothesis 2. Primates with fused symphyses tend to exhibit a late-peaking BDM when most of the other jaw muscles are rapidly relaxing (Hylander and Johnson 1994; Hylander et al. 2000, 2011; Vinyard et al. 2005, 2006). This late peak of the BDM is linked to wishboning in primates (Hylander et al. 2000) and is observed in other mammals with fused symphyses (Williams et al. 2007; Crompton et al. 2010). We consider whether the relative timing of the BDM is correlated with relative symphyseal area across mammals.

Exploratory analysis

We also conduct an exploratory analysis to consider potential relationships between relative jaw robusticity and both jaw-muscle recruitment levels and relative timing of peak activity. In these comparisons, we focus on recruitment levels and relative peak timing for the balancing-side superficial masseter and temporalis.

Materials and methods

Sample

We compared quantitative data from jaw-muscle EMGs and jaw morphometrics for 19 species of mammals (Fig. 1). Even though we incorporated eutherians and metatherians, our sample is not evenly distributed throughout the mammalian phylogeny. The sample includes representatives from only six of the ∼29 orders of mammals (Wilson and Reader 2005) with more than half of our sample composed of primate species. While the poor phylogenetic coverage across mammals primarily highlights where more physiological data on feeding are needed, we urge caution in extrapolating any findings beyond our goals of preliminary data analysis and further hypothesis development.

Fig. 1.

Fig. 1

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Phylogeny of mammalian species (n = 19) with quantitative EMG and morphometric data used in the analysis. Phylogeny based on Bininda-Emonds et al. (2007) with branches depicted proportional to their lengths.

Data

Jaw-muscle EMGs were collected from published means for species as well as unpublished data for primates (Hylander et al. 2000, 2005, 2011; Vinyard et al. 2006, 2007; W. L. Hylander and C. J. Vinyard, unpublished data), treeshrews (Vinyard et al. 2005), ungulates (Williams et al. 2007; S. H. Williams, unpublished data), and marsupials (Crompton and Hylander 1986; Crompton et al. 2008a, 2010; A. W. Crompton, unpublished data) (Fig. 1). We collected no new data for this analysis and interested readers are referred to the original publications for details regarding individuals, experimental preparation, implantation of electrodes, data recording, and analyses of original data.

We considered both the relative level of peak muscle recruitment and the relative timing of peak activity for the jaw muscles using these previously collected EMGs. Relative muscle recruitment was compared using working- to balancing-side ratios (W/B Ratios) (see Hylander et al. 1992, 2000 for more information). To compare EMG amplitudes across electrodes (and muscles), EMG activity for an electrode is first scaled so the largest peak activity throughout an experiment equals 1.0 and all remaining peaks are rescaled accordingly (≤1.0). To generate a W/B ratio for a muscle pair, scaled activity of a muscle when it is acting on the working- (or chewing-) side is divided by scaled activity of its balancing-side counterpart. W/B ratios were considered for the superficial masseter, deep masseter, temporalis, and various combined averages for these jaw muscles (Table 1). Timing of peak muscle activity is quantified by taking the difference in time of peak activity for a muscle relative to the time of peak activity of the working-side superficial masseter (as a reference muscle) (Hylander and Johnson 1994; Hylander et al. 2000). Because the length of the chewing cycle changes with body size (Fortelius 1985; Druzinsky 1993; Gerstner and Gerstein 2008; Ross et al. 2008), we scaled the timing of peak muscle activity by the total amount of time between the first and last jaw adductors to reach peak activity during a chewing cycle. Future work should consider variation in timing relative to duration of the chewing cycle, but this variable was not available for most species in this analysis. We considered scaled peak activity for the balancing-side deep masseter, superficial masseter, and temporalis muscles (Table 1).

Table 1.

EMG and morphometric variables

Variable Description Hypothesis
EMG Variables
    Total combined W/B ratio (Total W/B ratio) Average of temporalis, superficial, and deep masseters W/B ratios 1
    Superficial masseter and temporalis W/B ratio (SM–TM W/B ratio) Average of superficial masseter and temporalis W/B ratios 1
    Deep masseter W/B ratio (DM W/B ratio) Ratio of WS deep masseter scaled activity to BS deep masseter scaled activity 2
    Superficial masseter W/B ratio (SM W/B ratio) Ratio of WS superficial masseter scaled activity to BS superficial masseter scaled activity Exp
    Temporalis W/B ratio (TM W/B ratio) Ratio of WS temporalis scaled activity to BS temporalis scaled activity Exp
    BS deep masseter timing (BDM time) Time of peak BDM activity relative to the peak of working-side superficial masseter (WSM)/length of peak jaw-adductor activity 3
    BS superficial masseter timing (BSM time) Time of peak BSM activity relative to the WSM/length of peak jaw-adductor activity Exp
    BS anterior temporalis timing (BAT time) Time of peak BAT activity relative to the WSM/length of peak jaw-adductor activity Exp
    BS posterior temporalis timing (BPT time) Time of peak BPT activity relative to the WSM/length of peak jaw-adductor activity Exp
Morphometric Variables
    Symphysis area shape (SymphArea) (½ AP symphysis length × ½ SI symphysis depth × 3.14)0.5/Condyle-M1 length 2–3, Exp
    Corpus area shape (CorpArea) (½ SI corpus depth at M1 × ½ ML corpus width at M1 × 3.14)0.5/Condyle-M1 length Exp
    Condyle width Maximum ML width of mandibular condyle/Condyle-M1 length –
    Jaw robusticity shape (JawRobust) [(Symphysis area shape + Corpus area shape + Condyle width shape) /3]/Condyle-M1 length 1, Exp
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Note. W/B ratio, working/balancing ratio (see text); BS, balancing-side (i.e., non-chewing-side); Exp = Exploratory analysis.

We measured the jaws of conspecifics for comparison to EMG data. Typically, species’ means represent measurements of six to ten individuals with approximately equal sampling of males and females. We considered three measures related to the jaw’s overall robusticity and load resistance ability (Table 1). All measurements were divided by condyle – M1 (Cond-M1) distance, as a biomechanical standard representing the load arm during chewing (Vinyard 2008) to provide a dimensionless measure for comparison with dimensionless EMG data.

Comparative phylogenetic analysis

We used Mesquite (Ver. 2.73; Maddison and Maddison 2010) to prune the Bininda-Emonds et al. (2007) mammalian phylogeny to the 19 species of interest in our analysis (Fig. 1). We utilized the phylogenetic position of Papio hamadryas for EMG data from P. anubis as well as Macaca fascicularis for EMG data on Macaca fascicularis and Macaca fuscata (Macaca sp.). We calculated phylogenetic independent contrasts (PIC) for EMG and morphometric data using the PDAP: PDTree module in Mesquite (Midford et al. 2005). Prior to estimating correlations, we confirmed that absolute values of standardized PIC data for each variable were not correlated with the standard deviation of branch lengths as a test of whether phylogenetic branch lengths reasonably fit the data from our species (Garland et al. 1991, 1992; Diaz-Uriarte and Garland 1996, 1998). Pearson product-moment correlations were calculated (α = 0.05) between standardized PIC data for EMG and jaw morphometric variables to test Hypotheses 1–3 and explore additional potential relationships between these physiological and osteological data.

Results

Hypothesis tests

Hypothesis 1 predicts that increased recruitment of the balancing-side jaw muscles during chewing is linked to relatively robust jaws. We found no relationship between our estimate of jaw robusticity shape (see Table 1 for definition of this variable) and either the average W/B ratio across all jaw-muscles (R = −0.32, P = 0.19) or the average W/B ratio of the superficial masseter and temporalis (R = −0.29, P = 0.23) (Table 2 and Fig. 2a). Hypotheses 2 and 3 predict that symphyseal area shape (see Table 1) will be correlated significantly with increased recruitment and the relative peak timing of the BDM, respectively. Symphyseal area shape is significantly negatively correlated with both the DM W/B ratio (R = −0.53, P = 0.03) and the relative timing of peak BDM activity (R = −0.67, P = 0.005) across our sample (Table 2 and Fig. 2b, c). These significant negative correlations suggest that mammals with relatively large symphyses also tend to exhibit increased recruitment of their BDM relatively late during the power stroke of mastication.

Table 2.

Product-moment correlations (R) among phylogenetically independent contrasts (PIC) for EMG variables and morphometric shapes for hypothesis tests and exploratory comparisons

PIC Variables Correlation (R) P-value Hypothesis
Hypothesis tests
    Total W/B ratio—Jaw Robust −0.32 0.19 1
    SM−TM W/B ratio—Jaw Robust −0.29 0.23 1
    DM W/B ratio—SymphArea −0.53 0.03 2
    BDM Time—SymphArea −0.67 0.005 3
Exploratory results
    SM and TM W/B ratio—CorpArea −0.40 0.09
    SM and TM W/B ratio—SymphArea −0.37 0.12
    SM W/B ratio—JawRobust −0.22 0.35
    SM W/B ratio—CorpArea −0.41 0.08
    TM W/B ratio—JawRobust −0.24 0.35
    TM W/B ratio—CorpArea −0.29 0.25
    BSM Timing—JawRobust −0.22 0.35
    BSM Timing—CorpArea 0.07 0.77
    BAT Timing—JawRobust 0.29 0.26
    BAT Timing—CorpArea −0.01 0.96
    BPT Timing—JawRobust −0.13 0.61
    BPT Timing—CorpArea −0.30 0.25
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Note. Bold values significant at α < 0.05.

W/B ratio, working/balancing ratio (see text); BS, balancing-side (i.e., non-chewing-side).

Fig. 2.

Fig. 2

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Plot of standardized phylogenetically independent contrasts (PIC) comparing (a) the average W/B ratio across jaw muscles (Total W/B ratio) to robusticity of the jaw, (b) the deep masseter (DM) W/B ratio to symphyseal area shape, (c) the relative timing of the balancing-side deep masseter to symphyseal area shape, and (d) the superficial masseter (SM) W/B ratio to corpus area shape across our mammalian sample. Product-moment correlations (R) and associated P-values (P) are reported for each plot. See Table 1 for variable definitions.

Exploratory analysis

Our exploratory analysis found no significant correlations between jaw shapes and the W/B ratios of the superficial masseter and temporalis muscles (Table 2 and Fig. 2d). Similarly, we observed no significant correlations between jaw shapes and the relative timing of these muscles when they acted as balancing-side muscles. The correlation between the superficial masseter W/B ratio and corpus area shape (see Table 1) approached significance (R = −0.41, P = 0.08) suggesting that future comparisons with larger sample sizes should reconsider this potential relationship (Table 2 and Fig. 2d).

Discussion

Implications for the function and evolution of chewing motor patterns across mammals

Our results provide little support for mammal-wide associations between jaw robusticity and those aspects of superficial masseter or temporalis activity patterns considered here. Given the variation in diet, feeding behavior and head form across mammals, the lack of association is not surprising. The absence of a correlation may reflect a reduced level of variance in superficial masseter and temporalis activity across mammals compared to jaw shapes as might be predicted by a conservation of motor pattern hypothesis. Unfortunately, it is currently unclear how to determine whether variance levels differ significantly between EMG and morphological variables. Alternatively, our results may reflect different functional relationships between jaw form and motor patterns among the clades represented in our sample. As an initial consideration, these different functional relationships would likely be tied to evolutionary shifts in diet, feeding behaviors, and/or form of the masticatory apparatus.

One potential exception to the lack of association noted above involves a possible relationship between the superficial masseter W/B ratio and the relative size of the corpus. Activity of the superficial masseter likely contributes to bending, twisting, and shearing (on the working-side) of the corpus during mastication (Hylander 1979a; Crompton 1995; Williams et al. 2009). It is unclear how variation in orientation and attachment location of the masseter affects these different potential loading regimes among mammals. If, however, elevated levels of balancing-side superficial masseter activity consistently track elevated loads in the balancing-side corpus, then a larger sample of mammals may reveal an association between balancing-side recruitment levels and relative area regardless of potentially different loading regimes.

We observed support for hypotheses linking relative symphyseal area both to BDM recruitment levels and relative timing. The significant correlations suggest that symphyseal loads created by elevated BS recruitment (i.e., lower W/B ratios) and relatively late peak activity of the BDM are typically resisted by relatively larger symphyses. In primates, this relationship between symphyseal morphology and BDM activity has been linked to wishboning in species with fused or relatively robust symphyses (Hylander et al. 2000, 2005, 2011; Ravosa et al. 2000; Vinyard et al. 2007). Not all mammalian taxa that exhibit the late-acting BDM, however, appear to wishbone their symphyses. Specifically, Williams et al. (2008) demonstrated that alpacas transversely twist their symphyses during chewing and the BDM, particularly its vertical component, may contribute to this loading regime. It remains to be seen whether, and if so, how, variation across mammals in the size, orientation, and position of the deep masseter causes different symphyseal loading regimes such as twisting or bending. Because our morphometric measure is relative size, we can hypothesize that elevated loading requires increases in symphyseal area to effectively resist these increased loads. Analyses focused on morphologies related to resisting specific symphyseal loading regimes would likely reveal different patterns. One potentially important similarity shared across taxa regardless of symphyseal loading regimes is the relatively late activity of the BDM. If the BDM is peaking at a time when many other jaw muscles are relaxing or not active, then the timing of peak activity may effectively isolate the load-related impact of this muscle during chewing.

One of the immediate challenges for interpreting the results of this study is the inconsistent sampling across mammals. Our sample is biased towards mammals that exhibit relatively large amounts of transverse movements during chewing. Many of the species examined here can be included in Weijs’s (1994) transverse chewing group—in several cases, a “herbivorous” morphotype. A late peak activity of the BDM has evolved multiple times in mammals that exhibit significant transverse movements during chewing, including rabbits (Weijs and Dantuma 1981), pigs (Herring and Scapino 1973; Huang 1994), alpacas (Williams et al. 2007), horses (Williams et al. 2003), anthropoid primates (Hylander and Johnson 1994; Hylander et al. 2000), sifakas (a lemuriform strepsirrhine) (Hylander et al. 2011), and koalas (Crompton et al. 2010). Thus, it remains to be seen whether our results apply broadly to other mammalian groups, including rodent and carnivoran morphotypes, or more specifically to this transverse chewing group of mammals.

Implications for conservation of motor pattern during feeding across mammals

The absence of significant correlations between jaw shapes and EMG data for the superficial masseter and temporalis may support an hypothesis of conservation of motor pattern for these muscles. Realistically, support requires additional assessment to confirm relatively low levels of variation in these EMG measures compared to jaw morphology. Alternatively, the significant correlations between deep masseter EMG variables and symphyseal shape refute a conservation of motor pattern hypothesis as outlined here. The significant associations suggest that deep masseter motor patterns are evolving with symphyseal shape across mammals. Future studies also need to consider how variation in fiber architecture and fiber type relate to muscle function during chewing across mammals as this functional variation may have significant consequences for the evolution of motor patterns in mammals.

Most hypotheses of conservation of motor pattern in mammals have focused on eutherian mammals (Hiiemae 1978, 2000; Weijs 1994; Langenbach and van Eijden 2001). Recent work (Crompton et al. 2008a,b, 2010; Crompton 2011) on herbivorous marsupials clearly refute the hypothesis of motor pattern conservation in this group. Among metatherians, wombats (Lasiorhinus latifrons) are strikingly autapomorphic (Crompton et al. 2008b, 2010). They employ a highly-derived motor pattern involving very little balancing-side muscle recruitment during chewing that differs from other mammals studied to date (Crompton et al. 2008b). Kangaroos exhibit a second, derived motor pattern, while the koala motor pattern converges on herbivorous ungulates with fused symphyses (Crompton et al. 2010). Collectively, these results reject a mammal-wide conservation of motor pattern hypothesis and suggest that marsupial herbivores may exhibit relatively greater variation in motor patterns compared to eutherians mammals (Crompton et al. 2010).

FEED forward: predictions for future analyses

Comparisons of physiological data between marsupial and placental mammals as well as clades within these infraclass groupings will become more informative as we continue to contribute physiological data sets into a centralized database. Based on our preliminary results, we predict that the balancing-side deep masseter muscle will exhibit significant associations with relative symphyseal size across mammals. This mammal-wide association may not be observed in the details of symphyseal morphology related to resisting specific loading regimes, as our prediction is not tied to a single shared loading regime across mammals. For other jaw muscles, our results suggest that we are more likely to identify functional relationships within specific groups of mammals that share similarities in diet, feeding behavior, and/or form of the masticatory apparatus.

In the introduction we described a general trend over time in the number of distinct jaw-muscle motor patterns thought to characterize mammalian feeding. Initially, Hiiemae (1978) suggested a single basic pattern that was subsequently modified to include a few patterns found in specific subgroups (e.g., Weijs 1994), whereas recent hypotheses incorporating data from additional species suggest a continuous range of covariation between motor patterns and morphology (e.g., Vinyard et al. 2007; Crompton et al. 2010). Our results for the deep masseter support this predicted continuous covariation across mammals and we further predict that other jaw-muscle activity patterns will exhibit significant associations with jaw form in specific groups of mammals. The continued development of FEED will provide the infrastructure needed to effectively test these hypotheses across mammals. We argue that the complexity of mastication, often requiring precise occlusion and intricate jaw movements during the breakdown of food and formation of the bolus, will dictate these quantitative relationships between physiology and form in mammals compared to non-masticating vertebrates.

Funding

Funds for the symposium were provided by a National Science Foundation grant (IOS-1050313 to S.H.W., C.J.V., C.E.W., and R.Z.G.); the Society for Comparative and Experimental Biology Divisions of Vertebrate Morphology, Comparative Physiology and Biochemistry, Neurobiology, and Systematic and Evolutionary Biology. We thank these groups for their support. Funding for original data collection provided by the National Science Foundation. (BCS-0552285, BCS-0412153, BCS-0138565, BCS-0241652, BCS-0094522, BNS-9100523, IOS-0520855, and SBR-9420764); National Institutes of Health (DE04531, DE05595, and DE05663); and Australian Research Council.

Acknowledgments

We thank two anonymous reviewers and the editor for their comments that improved this article.

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