A comparative assessment of external morphological traits between Macaca munzala, Tawang and Macaca assamensis, Goalpara population in Northeast India.

The present study aimed to conduct a comparative assessment of the external morphology, relative tail length and colour variation (RGB additive colour model) of Macaca munzala, Tawang and Macaca assamensis, Goalpara (southern Brahmaputra population), using a non-invasive photogrammetry method. The study revealed that the relative tail length of M. munzala (0.43 ± 0.03) is similar to that of M. assamensis assamensis, a subspecies of M. assamensis. On the other hand, the relative tail length of M. assamensis, Goalpara (0.61 ± 0.04) is similar to M. assamensis pelops, the other subspecies of M. assamensis. A total of 12 external morphological traits were observed and similarities found between the two studied populations. The study also found that the species-specific traits of M. munzala such as "dark patch of the crown whorl", "dark patch of hair at the temporal side", "spectacle appearance around eyes" and "whip-like tail appearance" in the infant and "stocky tail" are present in M. assamensis, Goalpara as well. Likewise, the "chin and cheek whiskers" are found in adult female individuals of both species. The comparative assessment of the dorsal coat colour (RGB value) revealed a darker brown dorsal coat in M. munzala (R 123.14 ± 11.62; G 107.71 ± 10.37, B 89.43 ± 9.21) than in M. assamensis, Goalpara (R 136 ± 23.57, G 112 ± 15.63, B 97.83 ± 13.04). The comparative assessment of facial skin colour among the adult individuals showed that the male M. munzala has darker brown facial skin than that of M. assamensis, whereas the female M. munzala shows darker reddish facial skin compared to M. assamensis, Goalpara. The species-specific traits of M. leucogenys were also compared, and the traits "chin and cheek whiskers", "darker dorsal coat" and "round penile" appearance have a striking resemblance to those of the M. munzala population of Tawang. Thus, the external morphological traits that are being used to distinguish these macaque species are highly variable even within the same species, and there is an urgent need to identify more precise species-specific morphological traits.