ONTOGENY, FUNCTION, AND SCALING OF THE MANDIBULAR SYMPHYSIS IN PAPIONIN PRIMATES

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.