Underbite and the scaling of facial dimensions in colobine monkeys

This investigation was concerned with the dimensions of the facial skeleton and the incidence of underbite in colobine monkeys. Standard craniofacial dimensions and the notation of the presence or absence of underbite were recorded for skulls of 308 adult, wild-caught colobines belonging to 10 species. Underbite was noted in all species examined, ranging in frequency from 10% in Colobus polykomos to 100% in 2 species of Presbytis. Univariate statistics indicate that species with high frequencies of underbite tend to have shorter, broader maxillary dental arches, shorter faces and rostrums, and longer mandibular dental arches. A multivariate discriminant analysis supported the univariate statistics. Interspecific allometric analysis of facial measurements against a constructed size variable indicated that maxillary dimensions scale in a strongly positive manner in relation to size. However, mandibular dimensions tend to scale isometrically. This pattern of facial scaling indicates that the tendency toward underbite should increase as overall size decreases. This would explain why smaller species have higher frequencies of underbite than larger species and why females often have a higher incidence of underbite than conspecific males. Zingeser has suggested that the high incidence of underbite in colobine and some cebid monkeys is an adaptation to folivory. The results of this study support an alternative hypothesis that the high incidence of underbite in colobine monkeys is related to the pattern of facial scaling with size in combination with relaxed selection on the anterior dentition.