Macaques, are members of the macaque genus, Macaca, 23 species of monkeys, found across Asia and Africa. There are two macaque species in mainland Sumatra, as well as two further endemic species in the Mentawai islands.

Macaques generally weigh around 10 kilograms, with males larger than females, and most are fruit-eaters. Their faces are the most distinctive feature, appearing inquisitive, and having prominent ears.

Macaques live in female-bonded societies, that is to say that female pig-tailed macaques will stay with their group even into adulthood, with female macaques with more siblings in the group earning higher status. Both female and male macaques exist within a social structure of higher- and lower-ranking females, and higher- and lower-ranking males. The higher-ranking females will have more female offspring, as they are better able to care for them, while higher-ranking males enjoy the typical male privilege of mating with more females.

Female macaques, being smaller than the males, reach sexual maturity earlier, typically around three years, while males take around two years longer, and will continue to grow in size. Gestation is around six months, and macaques can give birth every 12 to 18 months. Life expectancy is around 25-30 years.

The pig-tailed macaques

The pig-tailed macaques are divided into four species (originally considered to be subspecies of one pig-tailed macaque species, but now considered separate species with a recent common ancestor):

• The Southern pig-tailed macaque, Macaca nemestrina, found in Sumatra, Borneo, Malaysia, and southern Thailand. It is the species found in Bukit Lawang.
• The Northern pig-tailed macaque, Macaca leonina, found across south-East Asia, north of southern Thailand.
• The Pagai island macaque, Macaca pagensis, found on the southern Mentawai islands, Sumatra.
• The Siberut macaque, Macaca siberu, found in Siberut island, Mentawai 

Pig-tailed macaques are named for their short, hairless, pig-like tails. Males weigh up to 14kg, with females only half that weight.

A pig-tailed macaque group consists of 15-40 macaques of mixed sex. The group may divide into a number of subdivisions during the day for feeding,  before reuniting into the group at night. Males may leave their group to join another group and seek dominance there. Females achieve dominance by bonding with female siblings.

Pig-tailed macaques range over large areas, of up to 10 square kilometres, staying almost exclusively in the trees, although mostly in the lower and middle canopy. Pig-tailed macaques mostly eat fruit, but also eat insects, seeds, leaves, eggs, baby birds, and other foods.Pig-tailed macaques are considered to be direct competitors of the lar gibbon (white-handed gibbon) for food.

In both Sumatra and southern Thailand, Southern pig-tailed macaques are trained by humans to pick coconuts.Unlike dog training, pig-tailed macaque training is not based on a reward system, but rather punishment avoidance, this is believed to reflect the pig-tailed macaque dominance hierarchy. The coconuts are picked by the monkey twisting the stem and then biting through it. The monkey is taught to pick only yellow, ripe coconuts, and not green, unripe ones.

Pig-tailed macaques habitually live up to the edge of forests, and as such agricultural land bordering forest is subject to raids by pig-tailed macaques. This brings them into conflict with humans. They live from 0 to 2000 metres of altitude, but prefer lower altitudes.

The long-tailed macaque

The long-tailed macaque, also known as the crab-eating macaque, Macaca fascicularis is the most common South-East Asian monkey, and are one of the most widely-ranging monkey species. They are known from temples in Thailand and Bali. They are also one of the most common monkey species used for experimentation. There are ten or more subspecies of long-tailed macaque, with the nominate, Macaca fascicularis fascicularis found across the range, and subspecies generally restricted to islands, where they are separate from the main population.

Despite their name, their diet consists of a majority of fruit, along with small birds, eggs, lizards, and other items. They also commonly feed on human waste, being particularly likely to raid garbage bins.

Female long-tailed macaques weigh around 4kg, with males around 6kg. Groups are mixed-sex, but with more females, and consist of around thirty monkeys.