JAW MORPHOLOGY AND FUNCTION IN LIVING AND FOSSIL OLD-WORLD MONKEYS

This allometric investigation on a sample of 29 cercopithecine and 22 colobine taxa augments the data and implications of prior work on subf amilial variation in mandibular form and function in recent Cercopithe cidae. To increase the size range encompassed by living cercopithecine s and colobines, I included many of the larger-bodied fossil specimens . These analyses serve to fill a gap in our understanding of size-rela ted changes in masticatory function and symphyseal morphology and curv ature in extant and extinct Old World monkeys. Results of subfamilial scaling comparisons indicate that for a given jaw length, colobines po ssess significantly more robust corpora and symphyses than those of ce rcopithecines, especially at smaller sizes. Following from previous wo rk, the most plausible explanation for why the subfamilies differ in r elative corporeal and symphyseal dimensions is that colobine mandibles experience elevated loads and greater repetitive loading during masti cation due, on average, to processing a diet of tough leaves and/or se eds. Although colobines have relatively larger symphyses, subfamilial analyses of symphyseal curvature demonstrate that they evince less sym physeal curvature vis-a-vis cercopithecines of a common size. Moreover , both subfamilies exhibit similar allometric changes in the degree of curvature, such that larger-bodied Old World monkeys have more curved symphyses than those of smaller taxa. Subfamilial scaling analyses al so indicate that colobines possess a shorter M(2) bite-point length re lative to masseter lever-arm length, but not versus temporalis lever-a rm length. Thus, as compared to cercopithecines, colobines can recruit less masseter-muscle force to produce similar bite forces during mast ication. In both clades, MZ bite-point length scales with positive all ometry relative to masseter lever-arm length, such that larger species are less efficient at generating molar bite forces. This seems especi ally important due to the lack of subfamily difference in M(2) bife-po int:temporalis lever-arm scaling (which is isometric across cercopithe cids). A consideration of extinct cercopithecids indicates that many o f the large-bodied papionins have more robust corpora, due perhaps to a diet which was of similar toughness to that of extant and extinct co lobines. However, the biomechanical arrangements of the masseter and t emporalis in all but one fossil cercopithecine and all of the fossil c olobine specimens are much as predicted for a subfamilial member of it s skull size. That most large-bodied papionins with tougher diets none theless maintain a less efficient jaw-muscle configuration may be due to stronger offsetting selection for increased relative canine size an d gape.