Does intraspecific variation in social systems explain reported differences in the social structure of the proboscis monkey (Nasalis larvatus)?

The social structure of proboscis monkeys (Nasalis larvatus) was investigated in the Brunei Bay area of Brunei Darussalam (Northern Borneo). Intraspecific variability in social systems had been suggested to occur in this species, with social structure varying with habitat differences. Prior reports for this area suggested that groups contained several males, females, and offspring and were unstable. Contrary to these reports, I found that the monkeys were organized into groups containing one male, several females, and offspring. These groups associated at sleeping sites at the river’s edge, thus forming a secondary level of organization. This is the same social system reported in Sarawak, Malaysia and Kalimantan, Indonesia in different habitat types. It appears that methodological differences may account for the discrepancies between reports. I have found no evidence to suggest that intraspecific variation in social systems occurs in this species.