Macaques are easily identified by their long tails, shape of their muzzels and its colour which is generally greyish brown. They are highly sociable and noisy, often found in troupes of between 5 to 60. They live in a wide variety of habitats, including primary lowland rainforests, coastal forests of nipa palm and mangrove. They easily adjust to human settlements (i.e. Thomson, Hindhede road) and are even considered sacred at some Hindu temples especially so in Bali. In the past, Malay kampong villagers would keep Macaques as working pets for the purpose of plucking coconuts.
Although they are called "crab-eating Macaques", they don't only consume crabs but are considered opportunistic omnivores. Give them a hamburger and chances are, they'll take it. This is why Macaques at Lower Pierce and Bukit Timah Nature reserve get very excited when they see people because this usually translates into 'meal-time'. Hence, we have sign-boards and still penalties put up asa means of discouraging people from feeding the Macaques. But based on what I have seen so far, I think this has had had very little effect.
Studies have shown that Macaques will groom females in order to get sex. It is found that a femalte has a greater likelihood to engage in sexual activity with a male if he had recently groomed her, compared to males who had not. I should probably go warn my friend who allowed 3 young juveniles who appeared at his gate at Hindhede Road, to give him a 'manicure' when he offered them his hand.