Research Article: Allometry and Interspecific Differences in the Facial Cranium of Two Closely Related Macaque Species

Date Published: May 25, 2011

Publisher: Hindawi Publishing Corporation

Author(s): Tsuyoshi Ito, Takeshi Nishimura, Masanaru Takai.


Interpreting evolutionary history of macaque monkeys from fossil evidence is difficult, because their evolutionary fluctuations in body size might have removed or formed important morphological features differently in each lineage. We employed geometric morphometrics to explore allometric trajectories of craniofacial shape in two closely related species, Macaca fascicularis and M. fuscata. These two species exhibit a single shared allometric trajectory in superoinferior deflection of the anterior face, indicating that the differences in this feature can be explained by size variation. In contrast, two parallel trajectories are demonstrated in craniofacial protrusion, indicating that even if they are comparable in size, M. fuscata has a higher and shorter face than M. fascicularis. The degree of facial protrusion is most likely a critical feature for phyletic evaluation in the fascicularis group. Such analyses in various macaques would help to resolve controversies regarding phyletic interpretations of fossil macaques.

Partial Text

Macaques are medium-sized cercopithecine monkeys, which are currently distributed widely in the southern, southeastern, and eastern regions of Asia, and in a restricted area of northern Africa [1]. It is considered that this group probably arose as early as the Late Miocene in northern Africa and spread to Eurasia by the beginning of the Pliocene [2, 3]. Dispersal to eastern Eurasia occurred by the Late Pliocene, and macaques subsequently accomplished successful adaptive radiation in the southern, southeastern, and eastern regions of Asia [2]. The living species are usually classified into four species-groups, that is, sylvanus, silenus, sinica, and fascicularis groups ([2, 4, 5], and see also [6]).

The samples used in the present study comprised 645 adult specimens with full eruption of the third molars, including 393 specimens of M. fascicularis (151 females and 242 males) and 252 specimens of M. fuscata (103 females and 149 males). The specimens are stored in the National Museum of Natural History (Washington, USA), the American Museum of Natural History (New York, USA), the Museum of Comparative Zoology of Harvard University (Cambridge, USA), the Field Museum of Natural History (Chicago, USA), the Museum für Naturkunde of the Humboldt University (Berlin, Germany), the Zoologische Staatssammlung München (Munich, Germany), the Natural History Museum (London, UK), the Hakusan Nature Conservation Center (Hakusan, Japan), and the Primate Research Institute of Kyoto University (Inuyama, Japan).

The centroid size of  M. fuscata is significantly larger than that of M. fascicularis (t = −27.7, P <  .001). PC1 and PC2 account for 31.4% and 22.6% of the total variance, respectively, and they are distinct with respect to the PCs enlisted in Table 3. The shape variations represented by the first two PCs depend on the size in M. fascicularis and M. fuscata. These two allometric scaling patterns found in our study are commonly observed across varied clades of the papionin monkeys, for example, in the studies on Chinese macaques [15], baboons [16], and the entire Papionini tribe [17]. However, the two shape variations observed in this study have distinct allometric trajectories with respect to each other. We demonstrated that two closely related macaque species, M. fascicularis and M. fuscata, exhibit a single shared allometric trajectory in the supero-inferior deflection of the anterior face, and two parallel trajectories with respect to craniofacial protrusion. The interspecific difference in the latter feature is not explained by the evolutionary modification in body size. The craniofacial protrusion is one of the most reliable characteristics for phyletic evaluation of a given fossil specimen relative to living taxa, at least within the members of the fascicularis group. Future efforts with respect to allometric analyses in various macaques are expected to provide critical references for solving continuing controversies about the phyletic relationships between fossil macaques and living taxa.   Source: