Morphological and Body Color Variation in Thai Macaca fascicularis fascicularis North and South of the Isthmus of Kra

Long-tailed macaques (Macaca fascicularis fascicularis) are widely distributed in Southeast Asia and are morphologically and genetically (Tosi et al. in International Journal of Primatology 23:161–178, 2002) distinguishable on either side of the Isthmus of Kra (ca. 10.5°N). We compared the somatometry and body color of 15 local populations of long-tailed macaques in Thailand distributed over areas from 6.5°N to 16.3°N and also a Thai rhesus macaque population at 17.2°N. Limb proportions and body color variation follow the geographical trend. However, contrary to a previous report, body size does not decrease with latitude in the northern group and also in the southern (southerly distributed) rhesus macaque. Relative tail length (RTL) and color contrast in yellow between the back and thigh are the sole traits that distinctively separate the 2 groups: the southern group has a long relative tail length (RTL >125%) and small color contrast, whereas the northern group has a short RTL (<120%) and large color contrast. The southern rhesus macaques appear to have somatometric and body color traits that follow the geographical trend in long-tailed macaques, though they maintain their distinctive species-specific traits of shorter RTL (ca. 55%), shorter relative facial length, and a bipartite body color pattern. Researchers assume that the northern group of long-tailed macaques and the southern rhesus macaques had undergone partial introgression with each other. Montane refugia present during the glacial period are localities in which introgression occurred in long-tailed macaques.