Jaw Muscle EMG and Jaw Morphology in Treeshrews and Primates

Belanger's treeshrews, greater galagos and ring-tailed lemurs are more similar to each other in jaw-muscle EMG activity patterns as compared to this anthropoid sample. Given this broad resemblance, we find it worthwhile to ask whether treeshrew jaw morphology is also more similar to strepsirrhines than anthropoids. If so, then we can extend the associations among jaw-muscle activity patterns, inferentially internal jaw forces, and jaw form observed in comparisons of the primate suborders to include treeshrews (Hylander, 1979a,b; Hylander et al., 1998, 2000; Ravosa, 1991; Ravosa et al., 2000; Vinyard, 1999).

In Figure 5, we have added 8 treeshrew species to our previous comparisons of anthropoids to strepsirrhines with unfused symphyses. Treeshrews appear more similar to strepsirrhines than anthropoids in their relative corporal depth (Figure 5A), symphyseal area (Figure 5B) and condylar area (Figure 5C). Thus, treeshrews are more like strepsirrhines in both their jaw morphology and jaw-muscle activity patterns during chewing. This result further substantiates the link between jaw form and its load-bearing function during chewing. Based on the EMG and morphological comparisons, we predict that treeshrews will have relatively higher W/B corporal strain ratios than anthropoids during chewing.

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In Condyle - M1 Distance (mm)

In Condyle - M1 Distance (mm)

Figure 5. Comparison of load-resistance ability in the jaws of strepsirrhines with unfused symphyses, anthropoids, and treeshrews. (A) Plot of In corporal depth versus the In chewing moment arm (condyle—M1 distance) in 44 anthropoids, 47 strepsirrhines, and 8 treeshrew species.

(B) In Condyle- M1 Distance(mm)

(B) In Condyle- M1 Distance(mm)

Figure 5. (Continued) (B) Plot of ln symphyseal area versus ln chewing moment arm length. (C) Plot of ln condylar area versus ln chewing moment arm length. treeshrews appear more similar to strepsirrhines with unfused symphyses than either does to anthropoids in all three plots (5a-5c). This result suggests that anthropoids can resist relatively greater chewing forces for a given chewing moment arm length when compared to both strepsirrhines and treeshrews.

Figure 5. (Continued) (B) Plot of ln symphyseal area versus ln chewing moment arm length. (C) Plot of ln condylar area versus ln chewing moment arm length. treeshrews appear more similar to strepsirrhines with unfused symphyses than either does to anthropoids in all three plots (5a-5c). This result suggests that anthropoids can resist relatively greater chewing forces for a given chewing moment arm length when compared to both strepsirrhines and treeshrews.

Despite a highly plausible functional argument, we are careful to point out that this relationship is still only a correlation between jaw form and jaw loading patterns during chewing. Furthermore, this link may only be apparent at higher taxonomic levels and disappear when comparing more closely related species. It has been convincingly argued and empirically demonstrated that form and function need not share a law-like relationship in animals (Bock, 1977, 1989; Lauder, 1995, 1996). Without this invariant link, we must bear in mind that any inference of function from form in species lacking the appropriate in vivo data is a significant assumption that may not hold up on further analysis.

Establishing this correlation between jaw form and jaw-muscle activity during chewing across treeshrews and primates raises the question of whether we can extend these form-function relationships to include additional mammalian groups. While researchers have collected EMG data from the jaw muscles of other small mammals (e.g., Crompton et al., 1977; de Gueldre and de Vree, 1988; Dotsch and Dantuma, 1989; Kallen and Gans, 1972; Oron and Crompton, 1985), these data are not directly comparable to our results because of methodological differences in EMG data collection and analysis. More importantly, no one has systematically compared jaw morphologies of primates to those of other small mammals. Such comparisons would help us to better understand whether, and if so how, the functional morphology of strepsirrhine and anthropoid jaws differ from jaw forms in these other mammalian groups.

The overall similarity of treeshrew and strepsirrhine jaw-muscle activity patterns during chewing as compared to anthropoids mirrors observed differences in the morphology of their mandibular symphyses. Treeshrews, greater galagos and ring-tailed lemurs have relatively weak, unfused mandibular symphyses as compared to the fused symphyses of the anthropoid sample (Beecher, 1977b). Although we have described the symphysis as either fused or unfused throughout this chapter and we can reasonably interpret the relative strength and stiffness of the joint using such categorical terms, this characterization masks a continuous range of variation in the mechanical properties of the symphysis for resisting loads in various directions (Beecher, 1977a,b, 1979). We presently lack such mechanical information for primate mandibular symphyses.

Beecher (1977a) estimated the relative stress-resisting ability of the symphyses of several strepsirrhine species with either unfused or partially fused symphyses. We have scaled his estimates by the moment-arm length for chewing at the M1 to compare these relative symphyseal strength estimates to the average W/B ratios for the masseters and temporalis in treeshrews, greater galagos and ring-tailed lemurs (Figure 6). We also have included the representative anthropoids in this plot by making the assumption that their fused symphyses are relatively stronger than the unfused symphyses in these strepsirrhines and treeshrews. Variation in the anthropoid values reflects the area of their symphyses scaled by this same chewing moment-arm length.

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Owl monkey

Lemur o

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Galago

Average jaw muscle W/B ratio

Figure 6. Plot of relative symphyseal resistance ability versus the average jaw-muscle W/B ratios for treeshrews and primates. There is a near stepwise relationship between increasing resistance ability and decreasing W/B ratios, both within anthropoid species and among treeshrews and strepsirrhines. This association suggests that increasing balancing-side muscle recruitment may be related to symphyseal strength among primates and treeshrews. Relative symphyseal resistance ability for strepsirrhines and treeshrews is based on data from Beecher (1977a) that has been scaled by the chewing moment arm length. In anthropoids, this estimate is calculated by dividing (symphyseal area) by the chewing moment arm length. The broken line along the y-axis signifies that the values above and below the break cannot be compared except at a categorical level. Thus, we assume that anthropoids have greater resistance ability than treeshrews and strep-sirrhines with unfused symphyses. The x-axis is the average of the species means of W/B ratios for the four jaw muscles from Table 4.

There appears to be a consistent stepwise relationship between these two variables both across all taxa as well as within those species with fused and unfused symphyses. In fact, the rank correlation between these two variables is -0.92 across the seven groups (It is important to remember that there is a break in the y-axis between anthropoids and the remaining species in their relative strength estimates. A rank correlation is appropriate here if we assume that for a given size fused symphyses are stronger than unfused ones). This association provides further evidence linking relative balancing-side muscle forces to the strength of the mandibular symphysis. Furthermore, this plot fuels speculation that jaw-muscle force production and symphysis form may share a quantitative relationship across primates and treeshrews. We are currently measuring the strength and stiffness of primate and treeshrew symphyses under various loading regimes. We intend to use these quantitative strength estimates to determine how tightly these symphyseal properties are correlated with jaw-muscle EMG patterns. If the results of these analyses corroborate the pattern in Figure 6, then we suggest this correlation offers strong evidence linking symphyseal form, and hence strength, to relative balancing-side jaw-muscle forces during chewing.

Regardless of the outcome of these future tests, there is a categorical relationship between symphyseal fusion and jaw loading during chewing. This association between jaw-muscle EMG data and symphyseal morphology allows us to speculate on chewing behaviors in the earliest euprimates. If the earliest primates were small, had mobile unfused symphyses, and ate insects and fruits, then they probably chewed more like treeshrews and strepsir-rhines than living anthropoids. More specifically, they likely recruited relatively less muscle force from their balancing-side jaw muscles during chewing and they did not elicit large amounts of transverse muscle force from their balancing-side deep masseters late in the power stroke. As our interpretation is based on the correlation between jaw form and jaw-muscle functions it arguably remains valid even if treeshrews turn out to not be the sister taxa of primates (see Kupfermann et al., 1999; Liu et al., 2001; Madsen et al., 2001; Murphy et al., 2001a,b; Scally et al., 2001). What is important to our argument is that treeshrews share broadly similar diets and jaw form with the earliest euprimates.

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