Developmental aspects of sexual dimorphism in hominoid canines

We examined the histology of canine teeth in extant hominoids and provided a comparative database on several aspects of canine development. The resultant data augment the known pattern of differences in aspects of tooth crown formation among great apes and more importantly, enable us to determine the underlying developmental mechanisms responsible for canine dimorphism in them. We sectioned and analyzed a large sample (n = 108) of reliably-sexed great ape mandibular canines according to standard histological techniques. Using information from long- and short-period incremental markings in teeth, we recorded measurements of daily secretion rates, periodicity and linear enamel thickness for specimens of Pan troglodytes, Gorilla gorilla, Pongo pygmaeus and Homo sapiens. Modal values of periodicities in males and females, respectively, are: Pan 7/7; Gorilla 9/10; Pongo 10/10; and Homo 8/8. Secretion rates increase from the inner to the outer region of the enamel cap and decrease from the cuspal towards the cervical margin of the canine crown in all great ape species. Female hominoids tend to possess significantly thicker enamel than their male counterparts, which is almost certainly related to the presence of faster daily secretion rates near the enamel-dentine junction, especially in Gorilla and Pongo. Taken together, these results indicate that sexual differences in canine development are most apparent in the earlier stages of canine crown formation, while interspecific differences are most apparent in the outer crown region. When combined with results on the rate and duration of canine crown formation, the results provide essential background work for larger projects aimed at understanding the developmental basis of canine dimorphism in extant and extinct large-bodied hominoids and eventually in early hominins.