Hybridization and its impact on the ontogenetic allometry of skulls in macaques.

The role of hybridization in morphological diversification is a fundamental topic in evolutionary biology. However, despite the accumulated knowledge on adult hybrid variation, how hybridization affects ontogenetic allometry is less well understood. Here, we investigated the effects of hybridization on postnatal ontogenetic allometry in the skulls of a putative hybrid population of introduced Taiwanese macaques (Macaca cyclopis) and native Japanese macaques (Macaca fuscata). Genomic analyses indicated that the population consisted of individuals with varying degrees of admixture, formed by male migration from Japanese to Taiwanese macaques. For overall skull shape, ontogenetic trajectories were shifted by hybridization in a nearly additive manner, with moderate transgressive variation observed throughout development. In contrast, for the maxillary sinus (hollow space in the face), hybrids grew as fast as Taiwanese macaques, diverging from Japanese macaques, which showed slow growth. Consequently, adult hybrids showed a mosaic pattern, that is, the maxillary sinus is as large as that of Taiwanese macaques, while the overall skull shape is intermediate. Our findings suggest that the transgressive variation can be caused by prenatal shape modification and nonadditive inheritance on regional growth rates, highlighting the complex genetic and ontogenetic bases underlying hybridization-induced morphological diversification.

Phenotypic plasticity in the mandibular morphology of Japanese macaques: captive-wild comparison.

Despite the accumulating evidence suggesting the importance of phenotypic plasticity in diversification and adaptation, little is known about plastic variation in primate skulls. The present study evaluated the plastic variation of the mandible in Japanese macaques by comparing wild and captive specimens. The results showed that captive individuals are square-jawed with relatively longer tooth rows than wild individuals. We also found that this shape change resembles the sexual dimorphism, indicating that the mandibles of captive individuals are to some extent masculinized. By contrast, the mandible morphology was not clearly explained by ecogeographical factors. These findings suggest the possibility that perturbations in the social environment in captivity and resulting changes of androgenic hormones may have influenced the development of mandible shape. As the high plasticity of social properties is well known in wild primates, social environment may cause the inter- and intra-population diversity of skull morphology, even in the wild. The captive-wild morphological difference detected in this study, however, can also be possibly formed by other untested sources of variation (e.g. inter-population genetic variation), and therefore this hypothesis should be validated further.

Skeletal determinants of tail length are different between macaque species groups.

Macaques (genus Macaca) are known to have wide variation in tail length. Within each species group tail length varies, which could be associated with a phylogenetic trend seen in caudal vertebral morphology. We compared numbers and lengths of caudal vertebrae in species of the fascicularis group, M. assamensis (sinica group), M. nemestrina (silenus group), and those obtained from reports for an additional 11 species. Our results suggest different trends in number and lengths. The caudal vertebral length profiles revealed upward convex patterns for macaques with relative tail lengths of ≥15%, and flat to decreasing for those with relative tail lengths of ≤12%. They varied between species groups in terms of the lengths of proximal vertebrae, position and length of the longest vertebra, numbers and lengths of distal vertebrae, and total number of vertebrae. In silenus and sinica group, the vertebral length is the major skeletal determinant of tail length. On the other hand, the vertebral number is the skeletal determinant of tail length in the fascicularis group. Tail length variation among species groups are caused by different mechanisms which reflect the evolutionary history of macaques.