Cj. Vinyard et Mj. Ravosa, ONTOGENY, FUNCTION, AND SCALING OF THE MANDIBULAR SYMPHYSIS IN PAPIONIN PRIMATES, Journal of morphology, 235(2), 1998, pp. 157-175
In vivo study of mastication in adult cercopithecine primates demonstr
ates a link between mandibular symphyseal form and resistance to ''wis
hboning,'' or lateral transverse bending. Mechanical consideration of
wishboning at the symphysis indicates exponentially higher stresses al
ong the lingual surface with increasing symphyseal curvature. Lengthen
ing the anteroposterior width of the symphysis acts to resist these hi
gher loads. Interspecific adult cercopithecine allometries show that b
oth symphyseal curvature and symphyseal width exhibit positive allomet
ry relative to body mass. The experimental and allometric data support
an hypothesis that the cercopithecine mandibular symphysis is designe
d to maintain functional equivalence-in this case dynamic strain simil
arity-in wishboning stress and strain magnitudes across adult cercopit
hecines. We test the hypothesis that functional equivalence during mas
ticatory wishboning is maintained throughout ontogeny by calculating-r
elative stress estimates from morphometric dimensions of the mandibula
r symphysis in two cercopithecine primates, Macaca fascicularis and M.
nemestrina. Results indicate no significant differences in relative s
tress estimates among the two macaque ontogenies and an interspecific
sample of adult papionin primates. Further, relative stress estimates
do not change significantly throughout ontogeny in either species. The
se results offer the first evidence for the maintenance of functional
equivalence in stress and strain levels during postnatal growth in a h
abitually loaded cranial structure. Scaling analyses demonstrate signi
ficant slope differences for both symphyseal curvature and width betwe
en the ontogenetic and interspecific samples. The distinct interspecif
ic cercopithecine slopes are realized by a series of ontogenetic trans
positions in both symphyseal curvature and width. Throughout papionin
ontogeny, symphyseal curvature increases with less negative allometry,
while symphysis width increases with less positive allometry versus t
he interspecific pattern. As symphyseal curvature and width are invers
ely proportional to one another in estimating relative stresses, funct
ionally equivalent stress levels are maintained both ontogenetically a
nd interspecifically, because the relatively slower rate of allometric
increase in symphyseal curvature during growth is compensated for by
a slower rate of allometric increase in symphyseal width. These result
s indicate the primacy of maintaining functional equivalence during gr
owth and the need for ontogenetic data in understanding the evolutiona
ry processes that affect form-function relations as well as the inters
pecific patterning of adult form across a clade. (C) 1998 Wiley-Liss,
Inc.