Presbytis thomasi

121.

Thomas's Langur

Presbytis thomasi

French: Langur de Thomas / German: Thomas-Langur / Spanish: Langur de Thomas

Other common names: North Sumatran Leaf Monkey, Sumatran Grizzled Langur, Thomas Langur, Thomas's Leaf Monkey

Taxonomy. Semnopithecus thomasi Collett, 1893,

Indonesia, Sumatra, Aceh, Langkat.

This species is monotypic.

Distribution. N Sumatra (Aceh Province), mostly N of the Wampu and Alas (= Simpang Kiri) rivers; a population has also been discovered S of the Alas at ¢.3° 50” N.

Descriptive notes. Head-body c.55 cm, tail 69-76 cm; weight 7-8 kg. There is no sexual dimorphism in size. Thomas’s Languris similar to the Javan Langur (P. comata), but differs slightly in having white patches above each eye and a vertical black stripe down the middle of its crest, with an additional black stripe on each side. Upperparts (including hands, feet, and face) are grayish-black, while underside is creamywhite with traces of yellow, the latter tone extending to inner surfaces of limbs, ventral surface of tail, and ears. Area surrounding muzzle is often pinkish.

Habitat. Primary and secondary lowland rainforest, swamp forest, lowland alluvial (riparian) forest, and fruit and rubber tree plantations. The highest densities of Thomas’s Langurs have been recorded in swamp forest, with densities somewhat lower in lowland alluvial and lowland forests. They are also found in mosaics of rubber plantations and primary hill dipterocarp forest, and they tolerate selectively logged forest. Thomas’s Langur prefers the understory. They occur at elevations of 1500-2400 m and in lower parts of the subalpine zones up to 3400 m in the Leuser Ecosystem. Rainfall is high where they occur (3288-4575 mm/ year), and forests are humid throughout the year. They were studied in a forest-plantation mosaic at Bohorok (1981-1984) and alluvial lowland rainforest at Ketambe Research Station, Aceh (1987-2001).

Food and Feeding. Thomas’s Langurs eat mainly young leaves, fruits and seeds, along with flowers, stems, toadstools, stalks of coconuts, soil, and animal prey (including insects and ground snails). Groups often raid crops. At Ketambe, Thomas’s Langur fed from 218 species of trees and lianas: 191 for fruits, 69 for leaves, and 28 for flowers. In forest-plantation mosaic at Bohorok, the number of species exploited was considerably reduced: 26 including cultivated banana (Musaceae) and seeds of the rubber tree (Hevea brasiliensis, Euphorbiaceae). Diets of groups of Thomas’s Langur at Ketambe comprised 36-1% fruits and seeds, 30-8%), young leaves, 13-4% mature leaves, 3-6% flowers, and 6% animal prey (including ants and snails), with the remainder, soil from termite mounds, algae, and other food items. At Bohorok, they ate more fruits (more than 50% of the diet) and leaves (¢.36%).

Breeding. Births of Thomas’s Langurs occur throughout the year. The natal coatis white, with a thick dark dorsal stripe. Females are considered adult at 60 months, about one year before the average age at which infants are born (74 months) and the youngest age at which a female in a study population gave birth. Interbirth interval averages 22 months (27 months counting only instances in which the previous infant survived). Females have a birth rate of c.0-44 infants/year. Mortality is high in the first year oflife (48% for male infants, 43% for females). Instances of infanticide have been observed. In the wild, one female lived for more than 20 years.

Activity patterns. Thomas's Langur is diurnal and largely arboreal. An activity budget recorded in one study was: 60-4% of their day resting, 31-7% feeding, 4-9% traveling, and 3% engaging in social activities. They sleep high up in tall trees that are clear of lianas.

Movements, Home range and Social organization. Thomas’s Langurslive in unimale— multifemale groups of 2-16 individuals, and they also form all-male groups. Two-male groups were recorded at Bohorok, and they were always larger (9-21 individuals). Both males and females disperse from their natal groups, and females also transfer among groups. Sometimes all females leave a group and transfer to another, resulting in an all-male group being left behind. Sometimes females leave a group to join a solitary male. Home ranges averaged 27 ha at Ketambe, but they were smaller at Borohok at 14 ha. The largest home range was 53 ha (at Ketambe). Home ranges tend to be larger in groups that have been stable for some years. Newly established groups have small home ranges, independent of the group’s size. At Ketambe, male group tenure was five days to 72 months, with a median of 60 months. Males are typically resident in only one mixed-sex group; establishing their residency at c¢.7 years old and maintaining their status until they are c.13 years old. Daily distance traveled was smaller at Bohorok (average 683 m/day) than at Ketambe (average 1068 m/day). Thomas’s Langurs have very distinct long calls, which distinguish them from all other langurs on Sumatra, especially neighboring Black-crested Sumatran Langur (P. melalophos). These calls show age, individualism, and contextual variation. Thomas’s Langurs utter different calls depending on whether or not other individuals are present and whether or not they give alarm calls. Loud calls are uttered in the morning before leaving their sleeping trees, during encounters with other groups, and as an alarm call after spotting a predator. Males give loud calls and chase each other when groups meet; females of each group generally ignore each other. The aggressive behavior is believed to result from the male’s defense of his females rather than defense of the territory itself.

Status and Conservation. CITES Appendix II. Classified as Vulnerable on The IUCN Red List. Thomas’s Langur is protected by Indonesian law. It is threatened mainly by habitat loss. Hunting does not seem to be a significant threat, but loss of primary habitat due to logging and conversion to oil palm plantations is probably displacing some populations. Thomas’s Langurs are sacred to the Batak people and consequently not hunted by them, although some marginal hunting occurs elsewhere in its distribution. Thomas’s Langurs occur in Gunung Leuser National Park, Sumatra.

Bibliography. Aimi & Bakar (1992, 1996), Groves (2001), Gurmaya (1986), Meyer, Hodges et al. (2012), Meyer, Rinaldi et al. (2011), Steenbeek (1996), Steenbeek & Assink (1998), Steenbeek & van Schaik (2001), Steenbeek, Assink & Wich (1999), Steenbeek, Piek et al. (1999), Sterck (1997), Sterck & Steenbeek (1997), Wich & Sterck (2003, 2010, 2011), Witch, Assink, Becher & Sterck (2002a, 2002b), Witch, Assink & Sterck (2004), Witch, Fredriksson & Sterck (2002), Witch, Koski et al. (2003), Witch, van der Post et al. (2003), Witch, Steenbeek et al. (2007), Wilson & Wilson (1976).