MANDIBULAR CORPUS STRAIN IN PRIMATES - FURTHER EVIDENCE FOR A FUNCTIONAL-LINK BETWEEN SYMPHYSEAL FUSION AND JAW-ADDUCTOR MUSCLE FORCE

Previous work indicates that compared to adult thick-tailed galagos, a dult long-tailed macaques have much more bone strain on the balancing- side mandibular corpus during unilateral isometric molar biting (Hylan der [1979a] J. Morphol. 159:253-296). Recently we have confirmed in th ese same two species the presence of similar differences in bone-strai n patterns during forceful mastication. Moreover, we have also recorde d mandibular bone strain patterns in adult owl monkeys, which are slig htly smaller than the galago subjects. The owl monkey data indicate th e presence of a strain pattern very similar to that recorded for macaq ues, and quite unlike that recorded for galagos. We interpret these bo ne-strain pattern differences to be importantly related to differences in balancing-side jaw-adductor muscle force recruitment patterns. Tha t is, compared to galagos, macaques and owl monkeys recruit relatively more balancing-side jaw-adductor muscle force during forceful mastica tion. Unlike an earlier study (Hylander [1979b] J. Morphol. 160:223-24 0), we are unable to estimate the actual amount of working-side muscle force relative to balancing-side muscle force (i.e., the W/B muscle f orce ratio) in these species because we have no reliable estimate of m agnitude, direction, and precise location of the bite force during mas tication. A comparison of the mastication data with the earlier data r ecorded during isometric molar biting, however, supports the hypothesi s that the two anthropoids have a small W/B jaw-adductor muscle force ratio in comparison to thick-tailed galagos. These data also support t he hypothesis that increased recruitment of balancing-side jaw-adducto r muscle force in anthropoids is functionally linked to the evolution of symphyseal fusion or strengthening. Moreover, these data refute the hypothesis that the recruitment pattern differences between macaques and thick-tailed galagos are due to allometric factors. Finally, altho ugh the evolution of symphyseal fusion in primates may be linked to in creased stress associated with increased balancing-side muscle force, it is currently unclear as to whether the increased force is predomina tely vertically directed, transversely directed, or is a near equal co mbination of these two force components (ef. Ravosa and Hylander [1994 ] In Fleagle and Kay [eds.]: Anthropoid Origins. New York: Plenum, pp. 447-468). (C) 1998 Wiley-Liss, Inc.