A similing monkey is a nervous monkey

Long-tailed macaque (Macaca fascicularis) are native to Singapore. Although their original habitat was mangroves (you can still see them at Sungei Buloh Wetland Reserve, Sentosa, the Sisters Islands, Pulau Ubin and Pulau Tekong), you will most probably encounter them in the Western and Central Catchment areas as well as at Bukit Batok Nature Park.

Macaques communicate differently from humans. One of the most important thing to remember is that smiling may be a friendly gesture for you but for macaques, this gesture may be interpreted as aggression. So avoid smiling them, especially by widely showing your teeth, and even do not bother to create an eye contact since this also may be interpreted as aggression.

And of course, do not even try to approach the baby macaques because the parents are extremely protective and your smile on your face to the baby macaque would be an invitation for trouble.

This also means that it is not always a good thing if a monkey smiles at you. What is often perceived as a smile of happiness is actually a nervous monkey expressing discomfort. This is typically known as "fear grin".

A sign board in MacRitchie Reserve in Singapore warning people about smiling at long-tailed macaques. The gesture which looks like smiling to us is actually a nervous monkey showing its discomfort. They also interpret smiling as aggression.