This lone male clearly wants to range alone as he is trying to scare us off by displaying aggressive behaviors. Aggression in macaques, and in this example long-tailed macaques (Macaca fascicularis), can be identified via the intense direct stare, raised eyebrows and round “open threat face”. 

[Photo and caption by Josephine Hubbard]

Reference

Van Hooff, J. A. R. A. M. (1967). The Facial Displays of the Catarrhine Monkeys and Apes. In D. Morris (Ed.), Primate ethology (pp. 7-68). New Brunswick, NJ, US: AldineTransaction.