Morphology and somatometric growth of long-tailed macaques (Macaca fascicularis fascicularis) in Singapore

Crab-eating, or long-tailed, macaques [Macaca fascicularis (Raffles, 1821)] have been studied extensively throughout their distribution in South and South-east Asia. Despite this extensive body of research, the island population of long-tailed macaques from Singapore remains virtually undescribed. In the present study, we compare the morphometric variability and patterns of growth observed in a population sample from Singapore with a composite sample from Thailand, north of the Isthmus of Kra. The results of our analyses indicate that there are statistically significant differences between the two populations in adult size and shape. For both males and females, the Singapore population is smaller than the Thai population. Relative to body length, the Singapore macaques exhibit significantly longer tails, and, relative to cranial length, they exhibit significantly more narrow faces than the Thai macaques. Although levels of sexual dimorphism for most morphometric traits are very similar, indicating similar levels of male-male competition for females, the Singapore males exhibit a significantly larger testicular volume relative to body weight, suggestive of an alternative male reproductive strategy. In addition to adult somatometric size and shape, comparisons of growth patterns relative to age and body size reveal significant differences between the two population samples. Combined, these results suggest either that statistically significant differences in adult morphology and patterns of growth can occur in presumably reproductively cohesive subspecies, or the Singapore macaques may be taxonomically distinct.