Taxonomic and sex differences in sagittal cresting among gracile and robust capuchin monkeys

Sagittal crests are observed among some primate species, including early extinct hominins, however the majority of research investigating sagittal cresting among extant primates has been confined to catarrhines. Sagittal cresting has not been well-investigated among capuchin monkeys, and understanding whether there are taxonomic differences in the frequency and pattern of sagittal cresting among gracile and robust capuchin species, or whether sagittal crest expression is only confined to the males of some species, may yield important insights in a comparative context, to better understand the underlying basis for the frequency and pattern of sagittal cresting among australopithecine species. In the research presented here, I investigate whether there are interspecific differences in the frequency and pattern of sagittal cresting, and sexual dimorphism in cresting frequency among six capuchin species, representing three gracile capuchin species (Cebus albifrons, Cebus capucinus and Cebus olivaceus) and three robust capuchin species (Cebus apella, Cebus macrocephalus and Cebus libidinosus). I collected sagittal cresting data for 279 dentally mature cranial specimens using 3D models. There are interspecific differences in the frequency of sagittal cresting among capuchin species, with four out of the six species investigated (C. capucinus, C. apella, C. macrocephalus and C. libidinosus) showing sagittal crests. There are significant sex differences in the frequency of sagittal cresting in C. capucinus and robust capuchin species (C. apella, C. libidinosus, C. macrocephalus). I further show that there are interspecific differences in the pattern of sagittal cresting among the four species that exhibit sagittal crests. Sagittal cresting in C. capucinus occurs in the posterior region of the neurocranium, in contrast to the robust capuchin species, whose sagittal crests mainly extend from anterior to bregma, to the posterior cranial region at the midline. The underlying reasons for interspecific and sex differences in the frequency and pattern of sagittal cresting among species are yet to be elucidated, and may be associated with dietary, habitat or socioecological differences among capuchin groups.