Mentawai macaque groups are flexible, but usually consist of between 5 and 25 individuals. Larger groups may split up into subunits to forage, and also to sleep. However, they are also seen contentedly foraging in mixed-species groups with the Mentawai Island leaf-monkey, Presbytis potenziani. The Mentawai macaque group tends to consist of a single adult male amongst a group of mature females and their young. Solitary males occasionally try to usurp ageing male group leaders in order to mate with the females, and meet with aggression to establish dominance (4).
Walking on all fours in the search for food, Mentawai macaques forage for the fruits of several trees, including two species of fig, and may stray from the forests to raid gardens and coconut groves. The group’s movements are coordinated by the male with a series of high-pitched cries. In the evenings, the group will always return to the forest where they seek a new sleeping tree every night to settle down with their subgroup. The group watches for predators, notably the crested serpent eagle (Spilornis deela sipora) and pythons (Python reticulatus) and any alarm will result in a short, gruff bark (4).
Females signify their fertility and willingness to mate by displaying their swollen and reddened genitals. Courtship is not elaborate since mates are usually known to each other, but females will crouch before males to initiate copulation (4). After a gestation of five to six months, a single infant is born during the night, clinging to its mother’s belly immediately. The mother eats the placenta and licks the infant clean before morning. She will retain a close bond with her daughters into adulthood and with sons until they reach sexual maturity and leave the group (6).