Canine reduction in the miocene hominoid Oreopithecus bambolii: behavioural and evolutionary implications

The degree of canine size sexual dimorphism and relative canine size, which have been related to levels of agonistic behaviour amongst living anthropoid primates, together with relative molar size, are evaluated in the fossil hominoid Oreopithecus bambolii from the Late Miocene of Italy. Although Oreopithecus displays a significant degree of canine height sexual dimorphism, using allometric techniques and body mass estimates for fossil species, it is shown that Oreopithecus males are microdont (smaller postcanine as well as canine teeth than expected) when compared to most living hominoids and its putative ancestor Dryopithecus. Canine reduction in Oreopithecus includes both crown height and, especially, basal area, and most closely resembles the condition found in the pygmy chimpanzee Pan paniscus. Interestingly, it had been previously proposed that Oreopithecus displays, like pygmy chimpanzees, a paedomorphic cranial morphology resulting in a reduction of facial prognathism, which could be related to microdontia in both taxa. Independent canine reduction in several anthropoid lineages (including hominids and P. paniscus) has been related to a relaxation of the selection pressure favouring canine use as a weapon. Although changes in socio-sexual behaviour, as documented in P. paniscus, cannot be currently discarded in Oreopithecus, canine reduction could be also alternatively (although not exclusively) interpreted as an aspect of generalized microdontia. The latter is best considered an adaptive readjustment required by the paedomorphic reduction of prognathism and the resulting lack of space to accommodate the adult dentition. This mechanism of canine reduction highlights the significance of developmental constraints in evolution and had not been previously suggested for any anthropoid primate.