Objectives: Magnitudes of morphological integration may constrain or facilitate craniofacial shape variation. The aim of this study was to analyze how the magnitude of integration in the skull of Macaca fascicularis changes throughout ontogeny in relation to developmental and/or functional modules.

Materials and Methods: Geometric morphometric methods were used to analyze the magnitude of integration in the macaque cranium and mandible in 80 juvenile and 40 adult M. fascicularis specimens. Integration scores in skull modules were calculated using ICV (Integration Coefficient of Variation of eigenvalues) based on a resampling procedure. Resultant ICV scores between the skull as a whole, and developmental and/or functional modules were compared using Mann-Whitney U tests.

Results: Results showed that most skull modules were more tightly integrated than the skull as a whole, with the exception of the chondrocranium in juveniles without canines, the chondrocranium/face complex and the mandibular corpus in adults, and the mandibular ramus in all juveniles. The chondrocranium/face and face/mandibular corpus complexes were more tightly integrated in juveniles than adults, possibly reflecting the influences of early brain growth/development, and the changing functional demands of infant suckling and later masticatory loading. This is also supported by the much higher integration of the mandibular ramus in adults compared with juveniles.

Discussion: Magnitudes of integration in skull modules reflect developmental/functional mechanisms in M. fascicularis. However, the relationship between ‘evolutionary flexibility’ and developmental/functional mechanisms was not direct or simple, likely because of the complex morphology, multifunctionality, and various ossification origins of the skull.

For this study, 80 juvenile and 40 adult M. fascicularis skeletal specimens were used. When they were alive, these specimens were used for biomedical research, and their skeletal remains are housed in the Department of Anthropology of the University at Buffalo, SUNY. Age groups were partitioned based on mandibular dental developmental stages, with juvenile specimens divided into two groups: with or without permanent canines. This division is based on the results of previous studies that showed changes in the skull tended to be most distinct around the eruption of permanent canines in M. fascicularis (Mouri, 1994, 1996). Adult specimens were defined according to the full eruption of entire mandibular dentition to occlusion and a fully fused spheno-occipital synchondroses. Thus, shape changes were analyzed among two juvenile groups and one adult group.

In order to conduct geometric morphometric analyses, crania and mandibles of M. fascicularis specimens were scanned with an HDI Advance high definition 3D scanner (LMI Technologies INC., Vancouver, Canada.). To capture the shape of objects, midline and bilateral landmarks were applied with reference to Smith & von Cramon-Taubadel (2015). Landmarks not defined by Smith & von Cramon-Taubadel (2015) were added due to the different cranial and mandibular shapes between juvenile and adult groups (Table 2). These landmarks were digitized in Landmark Editor (Wiley et al., 2005) by a single researcher (HJ). For intra-observer error assessment, one cranium and one mandible were digitized three times and error was calculated in millimeters for each landmark. This error assessment method calculates errors based on standard deviations among coordinates of landmarks on 3D models because each scan shares the same fixed coordinated plane (von Cramon-Taubadel et al., 2007).