Conclusions

The primary purpose of our chapter was to examine two long-standing hypotheses regarding circumorbital function, and then discuss the implications of these data for understanding adaptive transformation in skull form during primate origins. As greater galagos retain the primitive primate condition of a postorbital bar and an unfused symphysis, an understanding of masticatory function in such a representative strepsirhine is of considerable importance for interpreting circumorbital form in basal primates and other mammals (cf., Cartmill, 1970, 1972; Noble et al., 2000; Pettigrew et al., 1989; Ravosa et al., 2000a,b).

The presence of a significant strain gradient along the strepsirhine and anthropoid facial skull provides no support for the claim that mammalian cir-cumorbital structures are functional adaptations to counter routine masticatory stresses (Bouvier and Hylander, 1996a,b; Hylander and Johnson, 1992, 1997a,b; Hylander and Ravosa, 1992; Hylander et al., 1991a,b; Ravosa et al., 2000a,b,d; Ross and Hylander, 1996). Galago circumorbital principal-strain directions during unilateral mastication are close to 45° relative to the skull's anteroposterior axis, much as predicted by the facial torsion model. Contrary to Greaves' model, neither the postorbital bars nor the supraorbital tori are oriented 45° relative to the cranial long axis in primates (Hylander and Ravosa, 1992; Ravosa, 1991b,c). Furthermore, as galago masticatory forces during biting and chewing differ from those of the facial torsion model but nonetheless result in circumorbital strain directions much as predicted, it is likely inappropriate to model the skull of primates and other mammals as a simple hollow cylinder loaded in axial torsion (Hylander et al., 1991a,b; Ravosa et al., 2000a,b).

Analyses of several mammalian clades suggest that the presence of a bony postorbital bar is correlated with higher levels of orbital convergence and/or frontation (Noble et al., 2000; Ravosa et al., 2000a). Consideration of the interspecific and ontogenetic evidence suggests that relative increases in these two orbital parameters during primate origins appear linked, respectively, to a shift to nocturnal visual predation and increased encephalization. Therefore, support is provided for the NVPH regarding postorbital bar and orbital convergence, as well as for our emphasis on the role of encephalization and orbital frontation in postorbital bar formation. These and several other aspects of the cranial bauplan of basal primates underscore the importance of small size on postorbital bar function (i.e., relatively larger brain and relatively larger, more convergent orbits). In this regard, our study complements a prior suggestion that a suite of life-history features unique to basal primates is associated with small body size (Shea, 1987).

These analyses contribute to an understanding of the broader influence of orbital frontation on other aspects of primate circumorbital form (e.g., browridge formation - Hylander and Ravosa, 1992; Moss and Young, 1960; Ravosa, 1988, 1991b,c; Ravosa et al., 2000d; Shea, 1986; Vinyard and Smith,

2001). Further support for this structural relationship is provided by within-species and between-suborder comparisons of the primate ontogenetic data (see an earlier section). Such postnatal growth data clearly demonstrate the importance of structural and allometric affects on orbital orientation in the evolution of basal primate and basal anthropoids. The interspecific and, especially, ontogenetic comparisons underscore the importance of controlling for size in assessing the functional and phyletic significance of forward-facing orbits. Indeed, selection for higher levels of convergence at small body sizes has to overcome the tendency for smaller sister taxa to develop more divergent orbits due to the presence of relatively large eyes. Such countervailing factors are presumably further pronounced if a given morphological transformation is coupled with a shift in activity cycle (i.e., the evolution of a relatively large-eyed nocturnal descendant from a smaller-eyed diurnal ancestor). In anthropoids, it has been argued that the origin of pronounced levels of convergence was linked to a shift to diurnality at small body sizes (Cartmill, 1980)—a pattern variably supported in interspecific analyses (Ross, 1995). Much stronger support for this prediction is demonstrated by the negative correlation between orbital convergence and relative orbit size during the postnatal development of six diverse strepsirrhines, and by the relatively elevated levels of convergence throughout anthropoid ontogeny (Table 4).

In evaluating the galago experimental data vis-à-vis the NVPH, we identify two factors underlying why basal primates may have evolved a rigid postorbital bar: loading asymmetry along the facial skull and negative scaling of ocular on orbital size. On the other hand, a recent experimental study suggests that visual acuity in domestic cats with postorbital bars may be compromised during bilateral tetanic bilateral stimulation of the jaw adductors (Heesy et al., this volume). Therefore, while the presence of a bony bar is linked to increased orbital convergence and/or frontation, it is conceivable we have yet to identify an adequate functional explanation for such a relationship, much as is the case for the postorbital bar of large-bodied taxa such as bovids. Furthermore, the presence of intraspecific variation in postorbital bar formation among certain felids, herpestids, and pteropodids (Noble et al., 2000) underscores the need for laboratory and field studies of alert organisms so as to properly address arguments regarding visual acuity, orbital orientation, and postorbital bar development. Indeed, additional testing of this and other functional models in a broader variety of clades would greatly improve our understanding of the evolutionary morphology of the masticatory apparatus and circumorbital region in primates and other mammals.

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