This diurnal species occupies all levels of the forest canopy and also spends much of its time foraging on the ground. Fruits and seeds form the bulk of the diet, together with young leaves, buds, shoots, fungus and animal prey (including insects, river crabs and nesting birds). As an opportunistic feeder (4), however, this macaque also has a tendency to raid crops such as corn, papaya, oil palm and grain, earning it a reputation as a serious pest over much of its range (2).
The northern pigtail macaque lives in multi-male / multi-female groups of 5 to 40 (average 15 to 22), with around five to eight females to every male. Females remain with their natal group, which is structured by a matrilineal dominance hierarchy. By contrast, males disperse at puberty and remain solitary or peripheral to a group. Mating occurs year-round, although a reproductive peak occurs between January and May. Females have a 30 to 35-day reproductive cycle, and display an enormous, purplish-pink genital swelling at oestrous (2). These swellings provide a visual cue to males that the female is about to ovulate, and adult males rarely attempt to copulate otherwise (4). Mating is initiated by the male, whose courtship approach involves retracting the ears and pushing the lips forward (2). Since mates are usually familiar with each other within a group, cercopithecines (guenons, macaques and baboons) typically display only minimal courtship behaviour, confined to signals that indicate an immediate readiness to mate (4). Single offspring are born after a gestation period of 162 to 186 days, and the young are then nursed for 8 to 12 months. Juveniles reach sexual maturity at around four years (2).