Sexual dimorphism and facial growth in papionin monkeys

Sexual dimorphism in the primate face has been studied intensively lately, but a number of issues remain controversial. For example, some studies have indicated that facial sexual dimorphism arises through ontogenetic scaling, whereas others have found it to be a consequence of both ontogenetic scaling and divergence in male and female growth trajectories. To shed further light on primate facial sexual dimorphism, geometric morphometric methods were applied to crania representing five papionin genera: Cercocebus, Lophocebus, Macaea, Mandrillus and Papio. A first set of analyses focused on the pattern and degree of facial sexual dimorphism exhibited by adult specimens. A second set of analyses concentrated on the ontogeny of facial sexual dimorphism in infant-to-adult age series. The first set indicated that the five genera exhibit significant facial sexual dimorphism. These analyses also revealed that the genera share several features of sexual dimorphism. Males are distinguished from females in having a more prognathic mid-face, a relatively more inferiorly and anteriorly positioned prosthion, relatively increased subnasal height and relatively broader zygomatic roots. They are also differentiated from females in their maxillae, which relative to the zygomatics are narrow but vertically deep, especially in the posterior aspect. Also, the first set of analyses indicated that Macaca is the most distinctive of the genera in terms of sexual dimorphism. The distinctive features of the Macaca sexual dimorphism are that males exhibit relatively wider nasal apertures and premaxillae than females, together with a relative increase in breadth across the inferior zygomatic roots. The second set of analyses demonstrated that in all the genera, sexual dimorphism arises partly through ontogenetic scaling and partly through a late divergence in male and female growth trajectories. The analyses also indicated that the relative contribution of these processes to sexual dimorphism varies among the genera. Ontogenetic scaling is by far the most important process in Lophocebus and Papio. Late divergence between male and female growth trajectories contributes proportionately more in Cereocebus and Mandrillus, although ontogenetic scaling still plays the major role. The relative contribution of late divergence between male and female growth trajectories is greatest in Macaca, in which it accounts for about half of the facial differences between adult males and females. In all five genera, ontogenetic scaling results in increased prognathism and greater relative maxillary size in males compared to females, whilst the late divergence involves the male mid-face becoming proportionately larger and deeper than the female mid-face, and the male posterior maxilla coming to lie more inferiorly with respect to the zygomatic root than its female counterpart.