Mentawai Macaque

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 (5). 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 (5). 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 (5). 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 (7).

The Mentawai Islands are of high conservation significance as they are home to an unusually high number of endemic species and subspecies (4) and are part of the Sundaland Biodiversity Hotspot (8). Three major protected areas cover 18% of the Mentawai Islands: Tai-tai Batti is 930 km², Muara Siberut is 80 km² and Gunung Nanu'ua is 80 km² (6).

The macaques belong to the subfamily Cercopithecinae, or the 'typical monkeys', and are members of the most widespread primate genus (Macaca), ranging from northern Africa to Japan. In common with other macaques, the Mentawai macaque has cheek pouches into which it stuffs its food using the back of the hand (5). The fur is dark brown on the back, and pale chestnut to pale ochre on the sides of the neck, the front of the shoulders and the underside (2) (5). The legs are brown and the arms reddish brown. The tail is sparsely furred and the cheeks have relatively short whiskers. The face is hair-free, revealing black skin that frames the brown eyes (5).

The Mentawai Islands became separate from the Sumatran mainland over half a million years ago, so the tropical forests there are not as diverse of those on the mainland. The forest is characterised by extremely large buttressed trees, known as dipterocarps, as well as woody climbers and epiphytes. Smaller trees exist at the ground level but the forest floor is sparsely vegetated. The Mentawai macaque forages in the canopy layer, between 24 and 36 metres above the ground. It moves up to the tallest trees to sleep, some reaching 45 metres (6). It is accustomed to high levels of rainfall and is also found in riverine and coastal swamp forests (5).

The Mentawai macaque is endemic to the Mentawai islands, 150 km off the west coast of Sumatra, Indonesia. Of the four islands of Mentawai, this species is found on North Pagai, South Pagai and Sipora (5).

The Mentawai macaque is classified as Critically Endangered (CR) on the IUCN Red List 2006 (1) and is listed on Appendix II of CITES (3). Although the IUCN Red List lists the Siberut macaque (M. p. siberu) and the Pagai macaque (M. p. pagensis) as subspecies of the Mentawai macaque, with both classified as Critically Endangered (CR), they are now universally recognized as distinct species (2) (4).

The forests of the Mentawai Islands survived intact far longer than the forests of the mainland, but a recent migration of mainland Indonesians to the islands has resulted in large tracts of land being cleared for cash crops and oil palm plantations (6). Not only does this logging result in a reduced area for the macaques to live, but it also destabilises the local ecosystem, causing water levels in the forest rivers to fluctuate much more extensively than in the past. The alternating flooding and very low water levels has also caused an increase in malarial mosquitoes (8). Commercial logging continues and further applications for logging and plantation concessions are pending. The shift away from traditional resource management and low-level forest exploitation to highly commercial development schemes is putting pressure on many of the Mentawai Islands' endemic species, including four primates found nowhere else in the world (9).