A Multivariate Analysis of the Craniofacial Morphology of the Sulawesi Macaques

Albrecht, Gene Harrison

     Previous studies document seven allopatric phenotypically distinct populations of Celebesian macaques: brunnescens, hecki, maura, nigra, nigrescens, ochreata, and tonkeana. These populations, considered to be a species because of a lack of morphological intermediates, are recognized by a complex of external features; craniofacial differences have also been reported. The Celebesian macaques were apparently derived from a Pleistocene invasion from Borneo of an ancestral Macaca nemestrina stock; endemic differentiation, resulting in the stepped morphological gradients observable among living Celebesian macaques, occurred in three radiations corresponding to the arm-like peninsulas of the island. The geologic history of the Celebes (as an archipelago) potentially allowed for morphological and genetic divergence in isolation.
     This study investigates craniofacial variability with the intent of making biological, evolutionary, and functional inferences. The data consist of 24 linear measurements which describe the general size and shape of the skull in an extensive sampling of adult Celebesian macaques . . . ; non-Celebesian macaques were included for comparative purposes . . . Methods included regression analysis for description of allometric phenomena, and multivariate statistical procedures (canonical and principal components analysis) for the efficient summarization of morphometric relationships within and among populations.
     The craniometric results reveal differences among the Celebesian macaques comparable in magnitude to all of the widely dispersed and taxonomically diverse non-Celebesian macaques. Moreover, craniofacial variation among Celebesian macaques is dominated by shape differences in contrast to the size-related morphology of the non-Celebesian macaques.
     Craniofacial differences allow for partitioning of the Celebesian macaques into seven well-defined groups congruent with previously reported classifications based on geographic distributions and other morphological features. . . . The craniometric data demonstrate the derivation of the Celebesian macaques from a generalized non-Celebesian ancestor (data relating to a particular choice among the non-Celebesian macaques is equivocal). . . . Reanalysis of other morphological data accords with this hypothesis, but an additional component of mosaic morphological evolution suggests these primates experienced a complex history intermediate to the extreme possibilities of step-by-step colonization of peripheral islands and initial rapid dispersal throughout the Celebes with subsequent isolation and differentiation.
     Craniofacial variation among the non-Celebesian macaques is largely related to simple allometric phenomena involving the braincase, dentition, and facial skeleton. The biological significance of these allometric trends, which correspond to known allometric relationships for other primates and mammals, remains unknown. Sexual dimorphism in the Celebesian macaques is also explicable as allometric phenomena. The craniofacial dimensions of male Celebesian macaques generally follow the described craniometric relationships for non-Celebesian macaques; significant exceptions relate to progressive modification of the facial skeleton (especially the development of prominent supramaxillary ridges). The distinct morphologies of the Celebesian macaques . . . cannot be correlated with known factors such as feeding or habitat differences; a possible explanation is that the craniofacial modifications represent one more of a complex of externally visible features associated with species recognition (perhaps developed or intensified in response to selective pressures acting on populations, once separated, which are now in contact after partial genetic and morphological divergence).