Mentawai Island Leaf-monkey (Presbytis potenziani)
This species has a sacculated stomach to assist in the breakdown of cellulose. The incisors are narrow and the molars have sharp, high crests (Oates and Davies, 1994). This species has a dental formula of 2:1:2:3 on both the upper and lower jaws (Ankel-Simons, 2000). The jaw is deep and the face is short and broad (Oates and Davies, 1994). The pollex (thumb) is reduced in this species (Davies, 1991). The orbits are widely spaced and the hindlimbs are longer as compared to the forelimbs (Oates and Davies, 1994). Little sexual dimorphism occurs in this species (Tilson and Tenaza, 1976). The mean body mass for adult males was found to be 6.5 kilograms and for adult females it was 6.4 kilograms (Tilson and Tenaza, 1976). The species has a small sagittal crest. The infant has a white pelage coloration (Tilson, 1976). After a few days the pelage of the infant's face becomes darker, and after two to three weeks the entire pelage darkens beginning at the dorsal midline and the head (Tilson, 1976).
This species has two subspecies each having their own pelage coloration:
- Presbytis potenziani potenziani: The pelage color of this subspecies is described as being either rufous-white or yellow (Chasen and Kloss, 1927). On the crown and hindlimbs the hair is gray or grayish-white in color (Chasen and Kloss, 1927). The most basal two-thirds of the lower side of the tail is grayish-white in color (Chasen and Kloss, 1927).
- Presbytis potenziani siberu: The dorsal side, limbs, and tail is glossy black (Chasen and Kloss, 1927). The forehead, sides of the face, and the throat are colored white with the hairs of the throat tipped black (Chasen and Kloss, 1927). The ventral side is tawny or ochraceous-tawny in coloration (Chasen and Kloss, 1927). This subspecies has a white pubic patch (Chasen and Kloss, 1927).
The Mentawai Island leaf-monkey is found on the Mentawai Islands that are off of the West coast of Sumatra. This species lives in lowland forests. On the island of Siberut, this species occurs in primary and secondary forests (Watanabe, 1981). On the island of South Pagai this species was reported in swamp forests and near human settlements (Tenaza, 1987). On both Pagai islands this species occurs in disturbed areas (Fuentes and Ray, 1995/1996). On North Pagai Island this species lives in primary, secondary, swamp, and Barringtonia forests and in garden areas (Fuentes, 1996).
The two subspecies of this species have different ranges:
- Presbytis potenziani potenziani: This subspecies is found on Sipora, North Pagai, and South Pagai Islands of the Mentawai Island chain, Indonesia (Brandon-Jones, 1993).
- Presbytis potenziani siberu: This subspecies is found on the island of Siberut of the Mentawai Island chain, Indonesia (Brandon-Jones, 1993).
The Mentawai Island leaf-monkey is primarily a folivorous species. Food sources come from climbers/lianas, or vines (48%), trees (45%), and epiphytes, shrubs, or other plant sources (7%) (Fuentes, 1996). This species will eat leaves, fruits/seeds, or flowers (Fuentes, 1996). Leaves account for 55% of the diet, fruits and seeds 32%, and flowers, bark, and sap 13% (Fuentes, 1996). Young leaves are consumed more than mature leaves (Fuentes, 1996). Ficus spp., Dipterocarpusspp., and Shorea sp. are important plant species consumed (Fuentes, 1996). Feeding for this species primarily occurs in the upper part of the canopy, at heights >20 meters (Fuentes, 1996).
This is a diurnal and an arboreal species. Group sizes range from 2 to 5 individuals. Tenaza (1987) found that the mean group size for this species was 3.3 individuals. Fuentes (1996) found that on North Pagai Island the average group size for this species was 3.6 individuals. Groups will move little during heavy rainfall (8 millimeters per hour) (Fuentes, 1996). Daily activity begins with group members waking before the sun rises and the male moves away from the group to give a morning loud call that is like the male warning call (Fuentes, 1996). After the loud call the group moves to feeding sources, then a mid-day resting period occurs after a couple hours of feeding (Fuentes, 1996). Another feeding peak follows the rest period, then the group moves towards the sleeping trees for the night (Fuentes, 1996). The Mentawai Island leaf-monkey mostly (75%) occurs at heights 20 meters and higher (Fuentes, 1996). Sleeping sites are found in the core area (Tilson and Tenaza, 1976). This species sleeps in tall, emergent trees (Tilson and Tenaza, 1982). Fuentes (1996) found that sleeping trees are about 35 meters in height, members of the family Dipterocarpaceae, have moderate to heavy climber loads, and are not emergents. This species will sleep in the mid-upper levels of these trees (Fuentes, 1996). Resting sites are commonly located in large emergents or upper canopy trees that have heavy climber cover (Fuentes, 1996). During group resting period this species will emit contact calls (Fuentes, 1996). Core areas are usually found on major ridge crests, on the boundaries between Kloss' gibbons (Tilson and Tenaza, 1976). The reason for this is so that the Mentawai Island leaf-monkey will have an area to retreat to after engaging in hostile interactions with Kloss' gibbons; Kloss' gibbons spend less time at the boundaries, so this species would rarely encounter one there (Tilson and Tenaza, 1982). Also to avoid feeding competition with Kloss' gibbons, this species will leave the sleeping site earlier in the day so as to gain additional feeding time (Tilson and Tenaza, 1982).
The Mentawai Island leaf-monkey moves through the forest quadrupedally (Fleagle, 1988). This species mostly run on the tops of large branches and climbs up and down the boles of trees and climbers (Fuentes, 1996). This species also leaps and this type of locomotion mostly occurs between large branches of adjoining trees in the middle and upper parts of the forest canopy (Fuentes, 1996). Bipedal walking was observed in a few instances when they fed on climber leaves overhead (Fuentes, 1996). Infants were also observed to move bipedally during solitary play (Fuentes, 1996). Arm-swinging also will occur when feeding on climber growth on the underside of large tree boughs (Fuentes, 1996).
The Mentawai Island leaf-monkey has a monogamous mating and social system (Watanabe, 1981; Tenaza, 1987; Fuentes, 1994b). The basic group is composed of an adult pair and their offspring (Watanabe, 1981). Solitaries do occur in this species (Kawamura and Megantara, 1986). There is dispersal by both males and females from their natal group, with the females tending to leave at adulthood (Fuentes, 1994a). The ranges of groups will overlap, but a group will stay out of a neighboring group's core area (Watanabe, 1981; Fuentes, 1996). If a neighboring group would come into another's core area then the resident adult male and the intruding male would engage in aggressive, loud vocalizations and visual displays (Tilson and Tenaza, 1976). Members of the group will remain close together during feeding, resting, and traveling (Fuentes, 1996). The Mentawai Island leaf-monkey will respond to alarm calls given by Kloss' gibbon, Hylobates klossii (Tenaza, 1987). Mentawai island leaf-monkeys have been found to be subordinate to Kloss' gibbons, being supplanted from feeding areas when approached (Tilson and Tenaza, 1982). Infant parking has been reported in this species where a female will put down an infant and then forage (Fuentes, 1994a; cited in Fuentes and Tenaza, 1995). The Mentawai Island leaf-monkey has been found to form associations with pig-tailed langurs, Nasalis concolor, and pig-tailed macaques, Macaca nemestrina (Rowe, 1996).
The major predator of the Mentawai Island leaf-monkey are humans, Homo sapiens (Tenaza, 1987).
male warning call: This call consists of 1 to 3 harsh notes and may be heard up to 500 meters in distance (Kawamura and Megantara, 1986). Males will emit this in a duet fashion with females (Kawamura and Megantara, 1986).
female alarm call: This is a high-pitched call that sounds like shrill meows (Fuentes, 1994a).
duetting: This call has a frequency range between 300 and 1500 hertz (Haimoff, 1986). This call is done by the adult male and female in alternating fashion at or near dawn (Haimoff, 1986; Tilson and Tenaza, 1976). This call lasts about five minutes (Haimoff, 1986). This call consists of vocalizations by the male and female then a period of silence that is interspersed with soft grunts (Haimoff, 1986). This call may maintain and reinforce the pair-bond between the adult pair of the group (Haimoff, 1986). This call may also serve as an intergroup spacing call (Tilson and Tenaza, 1976).
distraction display call: This is a loud call emitted by an adult male during the distraction display (Tilson and Tenaza, 1976).
harsh cough: This is given by the adult male and is emitted when his mate is feeding in a tree (Tilson and Tenaza, 1976). This call may function to warn the female that he is about to begin the distraction display (Tilson and Tenaza, 1976).
contact call: This call is given by group members during resting periods (Fuentes, 1996). This call is hiccup-grunt type call (Fuentes, 1996).
distraction display: This is performed by adult males in the presence of observing man, Homo sapiens (Tilson and Tenaza, 1976). This display consists of a loud vocalization followed by branch bouncing, while running through the forest canopy (Tilson and Tenaza, 1976). While running the vocalization is emitted every 20-30 meters (Tilson and Tenaza, 1976). Sometimes the male will make a complete circle while doing this display, then enter the tree in which the rest of his family is hiding, then go back and start another display directed towards the stimulus (Tilson and Tenaza, 1976). The male may emit harsh cough to warn the female before he begins the display (Tilson and Tenaza, 1976). This call serves to draw the attention of the human away from the rest of the family group (Tilson and Tenaza, 1976).
social grooming: This is when one individual grooms another and is used to reinforce the bonds between individuals.
The Mentawai Island leaf-monkey gives birth to a single offspring once every other year (Tilson and Tenaza, 1976). The birth season occurs from July to August (Tilson and Tenaza, 1976). During estrus the clitoris of the female turns red.
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Last Updated: October 12, 2003.
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