Wl. Hylander et al., MANDIBULAR CORPUS STRAIN IN PRIMATES - FURTHER EVIDENCE FOR A FUNCTIONAL-LINK BETWEEN SYMPHYSEAL FUSION AND JAW-ADDUCTOR MUSCLE FORCE, American journal of physical anthropology, 107(3), 1998, pp. 257-271
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.