Body Size and Proportions and Pelage Color of Free-Ranging Macaca mulatta from a Zone of Hybridization in Northeastern Thailand

Abstract

Rhesus (Macaca mulatta) and long-tailed (M. fascicularis) macaques belong to the same species, and are parapatric within a zone that lies between 15° and 20° N on the Indochinese peninsula. Researchers have reported probable hybrids between the 2 species from that zone, but have not studied the extent of introgression. To test for phenotypic evidence of hybridization, we collected body mass, morphometrics (body size and proportions), and pelage color readings from free-ranging rhesus living close to the zonal boundary at Wat Tham Pa Mak Ho (WTPMH), Wang Saphun district, Loei province, northeastern Thailand (17°14′N, 101°47′E). Female WTPMH rhesus macaques (n =12) were 10–20% smaller, but with a greater relative tail length than the captive Chinese or Indian female rhesus. Female WTPMH were larger than the free-ranging long-tailed macaques, but with similar limb proportions and a shorter relative tail length. The WTPMH rhesus macaques also displayed the bipartite pelage color pattern typical of Macaca mulatta . The evidence suggests slight contribution of long-tailed macaques to the gene pool of the WTPMH population. Further sampling of other macaque populations within the zone and genetic analysis are essential to address better the question of hybridization. Determination of the distribution and range of biobehavioral variation of macaques within the zone is urgently needed, because their habitat is being rapidly destroyed by deforestation, and their demography and social structure are threatened by artificial disturbance.