Thomas's Leaf-monkey (Presbytis thomasi)


MORPHOLOGY:
This species has a sacculated stomach to assist in the breakdown of cellulose. In the forestomach there are microbes that break down cellulose into fatty acids by anaerobic fermentation (Davies et al., 1988). Adult males have canines that are larger than the canines of the adult female (Steenbeek et al., 1999a). The incisors of this seed predator are relatively larger as compared to the more folivorous members of the closely related genus Trachypithecus (Ungar, 1996b). 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). Thomas's leaf-monkeys have a pelage coloration that is gray on the backside and limbs and white on the ventral side (Wilson and Wilson, 1976). The tail is pale colored on the ventral side and bicolored gray on the dorsal side (Wilson and Wilson, 1976; Aimi and Bakar, 1992). The hands and feet are black in coloration (Aimi and Bakar, 1992). There is a blackish vertical stripe on the crest of the head with white patches on either side of the crest (Wilson and Wilson, 1976). The face is colored light gray with blackish colored moustache (Wilson and Wilson, 1976). The chin is white and the muzzle is flesh-colored in this species (Aimi and Bakar, 1992). The neonate of this species has a pelage coloration that is white (Wilson and Wilson, 1976). Adult males have an average body mass of 6.67 kilograms and females 6.69 kilograms (Fleagle, 1999).

RANGE:
Thomas's leaf-monkey is found on the island of Sumatra, Indonesia. This species occurs in Aceh Province north of the rivers Simpangkiri and Wampu (Wilson and Wilson, 1976). The range has recently been found to extend to the left bank (south) of the Simpangkiri River (Aimi and Bakar, 1996). This species is found to live in rubber plantations (Kunkun, 1986). This species lives in primary and secondary forests (Kunkun, 1986; Wilson and Wilson, 1976).

ECOLOGY:
Thomas's leaf-monkey is primarily a folivorous species (Ungar, 1995), although in some parts of its range it may be more frugivorous (Kunkun, 1986). Fruits, insects, flowers, stems, and soil are also consumed (Ungar, 1995). Toadstools and the stalks of coconuts are occasionally fed on by this species (Kunkun, 1986). This species has also been found to feed upon gastropods such as ground snails (Steenbeek, 1999). Fruits consumed by this species tend to have high pH levels, be unripe, are large, and have hard rinds or husks (Ungar, 1995). Examples of fruits eaten by this species include: Gnetum cf. latifollum, Paranephelium nitidum, and Quercus sp. (Ungar, 1995). Large seeds of dry fruits of the species Dysoxylum spp., Cnestis platantha, and Scorodocarpus borneensis are consumed by Thomas's leaf-monkeys (Ungar, 1995). Most of the fruit consumption consisted of eating the seeds of fruits with dry pericarps (Ungar, 1995). Fruits with high tannin levels are consumed because the forestomach microbes are able to break down the tannins (Ungar, 1995). Thomas's leaf-monkeys will avoid fruits with fleshy sugar-rich pericarps having a low pH because the bacteriocidal resins in the pericarps would kill forestomach microbes and a low pH level could cause acidosis of the forestomach (Ungar, 1995). Kunkun (1986) found that the following species were the staple food for Thomas's leaf-monkeys: Samanea saman, Hevea brasiliensis, Mikania chordata, Artocarpus integer, Sandoricum koetjape, Musa sp., and Durio zibethinus. This species will use the front teeth to pierce through the hard husks of fruit or through thick bunches of leaves (Ungar, 1996b). Most water is obtained from the food consumed, but individuals have been observed to drink water from tree holes and small rivers (Kunkun, 1986).

Group size in Thomas's leaf-monkeys varies from 3 to 21 individuals (Kunkun, 1986). This is a semi-arboreal and a diurnal species (Kunkun, 1986). Individuals do occasionally come down to forage for mature rubber seeds and durian fruits (Kunkun, 1986), although Ungar (1996a) found that this species will feed on the ground on average once per day. The most common activity for this species is resting, followed by feeding and then moving (Kunkun, 1986). There are three major feeding peaks: in the early morning, noon, and in the late afternoon (Kunkun, 1986). All-male groups will split up into subgroups when foraging (Steenbeek et al., 2000). Thomas's leaf-monkey will be found in the lower level of the canopy, the understory, and the ground most often (Ungar, 1996a). Sleeping sites at night tend to be in tall trees and near the tops of the trees (Kunkun, 1986). Midday sleeping sites tend to be in trees with a dense cover of leaves and twigs (Kunkun, 1986). Potential predators of this species include the clouded leopard, Neofelis nebulosa, the tiger, Panthera tigris, and the reticulated python, Python reticulatus (Sterck, 1997).

LOCOMOTION:
Thomas's leaf-monkey moves through the forest quadrupedally (Fleagle, 1988). This species also moves through the forest by leaping (Reed, 1999). For this species arboreal quadrupedalism makes up 20.5% of locomotion, climbing 12%, and leaping 67.5% (Reed, 1999).

SOCIAL BEHAVIOR:
Thomas's leaf-monkey has a unimale social system and a polygynous mating system that also have all-male groups (Kunkun, 1986). Some groups may have two adult males; usually one is the son of the other (Kunkun, 1986). Adult male tenure in a unimale group lasts on average 70 months (Steenbeek, 1997; Steenbeek et al., 1999a). There are three phases to the tenure of a male in a unimale group: early phase, middle phase, and late phase (Steenbeek, 1999). The early phase is characterized by the females coming together with a new adult male to form a new group (Steenbeek, 1999). In the middle phase, the female begin to bare young (Steenbeek, 1999). The final phase sees females leaving the male to form a new group with a new male and an all-male group is left behind (Steenbeek, 1999). Copulations between females of a unimale group and extra-group males will occur in the early and late phases, when the resident male is weakest, but not during the middle phase (Steenbeek, 1999). Males will herd their females more often in the early phase as compared to the middle phase during encounters with other groups (Steenbeek, 1999). During the early and late phases, extra-group males will provoke the adult male of a unimale group more than during the middle phase (Steenbeek, 1999). Males of all-male groups will only provoke adult males of unimale groups during the early and late phases and not during the middle phase (Steenbeek, 1999). All-male groups are also provoked by extra-group males and by males of unimale groups (Steenbeek, 1999). Members of an all-male group receive more aggression than males in unimale groups and they tended to rest lower in the forest canopy to avoid detection from extra-group males (Steenbeek et al., 2000). Adult males in a unimale group will expel juvenile males, but sometimes juvenile males will stay in the group and assume a subordinate position to the father (Steenbeek, 1997; Steenbeek et al., 2000). A juvenile male has greater advantages staying in a unimale group as opposed to an all-male group where they receive more aggression directed at them (Steenbeek, 1997). Juveniles also gain fighting experience by remaining with the father in a unimale group (Steenbeek, 1997). All-male groups are composed of adult males, subadults, and juveniles (Kunkun, 1986). In the all-male group, embracing, mounting, and social grooming frequently occurs between group members (Kunkun, 1986). When a unimale group lost its male, the group split into a group of adult females and a group of all juvenile males, and after some time the group of females accepted a male from an all-male group (Steenbeek, 1996).

Intragroup aggression tends to occur between females at small food sources (Sterck, 1996; Sterck and Steenbeek, 1997). A dominance hierarchy amongst females may be seen during aggression at small food patches where higher ranking females displaced lower ranking ones (Sterck and Steenbeek, 1997). Sterck and Steenbeek (1997) suggest that the dominance ranks are probably age-graded. Aggression rate of females was found to decrease with increasing group size and the presence of the resident male also served to decrease aggression (Sterck and Steenbeek, 1997). Thomas's leaf-monkey is a territorial species, with the ranges of groups tending to overlap (Kunkun, 1986). Interactions between groups occur only at areas of overlap (Kunkun, 1986). Type 1 and type 3 vocalizations are emitted by alpha males during inter-group interactions (Kunkun, 1986). Aggression also occurs between unimale groups and all-male groups, with the alpha male of the unimale group emitting type 1 calls during the encounters (Kunkun, 1986). Females never participate in aggression or attacks, but males of the all-male groups will attack females with infants (Steenbeek et al., 1999a; Sterck, 1997). When a male is absent from a group, females will defend their infants from attacks from extra-group males (Steenbeek, 1996). Aggression between males at territory boundaries is a result of mate defense and not a result of territorial guarding (van Schaik et al., 1992). Aggressive encounters sometimes occur between Thomas's leaf-monkeys and silvered leaf-monkeys, Trachypithecus cristatus (Kunkun, 1986).

Both males and females will disperse from their natal groups, and sometimes adult females will disperse, without dependent young, to avoid infanticide because the new adult male is a better protector (Sterck, 1999, 1997, 1998). Females will transfer with a male from an all-male group when fighting is occurring (Sterck, 1997). Females who transfer groups were found to have a longer interbirth interval than females who did not transfer (Sterck, 1997). Vigilance amongst group members, both in unimale groups and all-male groups, depends on the risks associated with predation and infanticide (Steenbeek et al., 1999a). Play can occur between the juveniles of two separate groups without interference from adults (Steenbeek, 1999).

VOCAL COMMUNICATION:
type 1 call: This call sounds like kak-kak-kak....ngkung-ngkung-ngkung, in which there are 9-17 kaks and 4-8 deep, resonant nasal sounds (ngkung) (Kunkun, 1986). This call is emitted by alpha males and is heard during inter-group relations (Kunkun, 1986). This call is also heard in the presence of a potential predator (Steenbeek et al., 1999b). This call may be a demonstration to an opponent group or to communicate territorial defense (Kunkun, 1986). This call may be accompanied by rapid movements of running and jumping (Kunkun, 1986; Steenbeek et al., 1999b). This is also referred to as a loud call. The mean duration of this call is 3.85 seconds (Steenbeek and Assink, 1998). This species gives on average 3.44 bouts of this call per day with an average of 3.14 calls per bout (Steenbeek et al., 1999b). Individual males can be recognized based on this call (Steenbeek and Assink, 1998). Males in a unimale group will decrease their frequency of emitting this call at dawn with decreased strength, which occurs at the latter part of their tenure (Steenbeek et al., 1999b).

type 2 call: This call sounds like choom-choom-choom, which are boom cough sounds (Kunkun, 1986). This call is emitted by the alpha male and rarely by the beta male, and is heard during inter-group relations and during animal disturbances (Kunkun, 1986). This call serves to communicate warning and aggression (Kunkun, 1986).

type 3 call: This call sounds like wek-wek-wek or kek-kek-kek and is described as a squeak call similar to ducks (Kunkun, 1986). This call is emitted by juveniles and is heard during inter-group relations (Kunkun, 1986). This serves as a demonstration call (Kunkun, 1986).

type 4 call: This call sounds like chup-chup-chup, and is a cough-like sound (Kunkun, 1986). This call is heard by males in an all-male group and is heard in interactions between all-male groups and unimale groups (Kunkun, 1986). This call serves to communicate aggression towards the alpha male of the unimale group (Kunkun, 1986).

type 5 call: This call sounds like kut-kut-kut and is described as being similar to the call of hens (Kunkun, 1986). This call is emitted by adult females and is heard during chasing episodes (Kunkun, 1986). This call serves to communicate aggression (Kunkun, 1986).

type 6 call: This call sounds like krr-krr-krr and is a low snore-like sound (Kunkun, 1986). This call is emitted by the alpha male of the unimale group and is heard in situations of herding the females by the male (Kunkun, 1986). This call serves to communicate warning and aggression (Kunkun, 1986).

type 7 call: This call sounds like chaark-chaark-chaark and is described as being a husky and low-tonal sound (Kunkun, 1986). This call is emitted by a lower ranking subadult male and is heard when the individual is approached by a higher-ranking male (Kunkun, 1986). This call serves to communicate submission (Kunkun, 1986).

type 8 call: This call sounds like chiit-chiit-chiit and is described as a slight husky squeak sounded call (Kunkun, 1986). This call is emitted low-ranking juvenile males and is heard when the individual is approached by a higher-ranking male (Kunkun, 1986). This call serves to communicate submission (Kunkun, 1986).

type 9 call: This call sounds like shiit-shiit-shiit and is described as being a squeak sound sharper than a type 8 call (Kunkun, 1986). This call is emitted by all members of the group and is during type 1 vocalization (Kunkun, 1986).

type 10 call: This call sounds like chekok or kok and is single sound (Kunkun, 1986). This call is emitted by subadult and juvenile males and is heard in situations of seeing an eagle-owl, Bubo sp., passing over (Kunkun, 1986). This call serves to communicate surprise (Kunkun, 1986).

type 11 call: This call sounds like check-check-check and is described as being a low hack sound (Kunkun, 1986). This call is emitted by all individuals of a group except infants and is heard when an eagle-owl passes over (Kunkun, 1986). This call serves to communicate fearfulness (Kunkun, 1986).

type 12 call: this call sounds like graakk and is described as being a single deep short resembling the sound of a hog (Kunkun, 1986). This call is emitted by juveniles, the alpha male, and subadult females and is heard in situations of play-fighting, fighting, and chasing (Kunkun, 1986). This serves to communicate a state of pain (Kunkun, 1986).

type 13 call: This call sounds like uungk-uuungk-uuungk and is described as being a tonal and nasal sound (Kunkun, 1986). This call is emitted by infant and juvenile males and is heard in the arousal of high tension (Kunkun, 1986). This call serves to communicate worry (Kunkun, 1986).

OLFACTORY COMMUNICATION:

VISUAL COMMUNICATION:

TACTILE COMMUNICATION:
social grooming: This is when one individual grooms another and is used to reinforce the bonds between individuals. This behavior may reduce tension between group members (Kunkun, 1986).

embracing: This behavior occurs between males in an all-male group (Kunkun, 1986). This behavior may reduce tension between group members (Kunkun, 1986).

mounting: This behavior occurs between males in an all-male group (Kunkun, 1986). This behavior may reduce tension between group members (Kunkun, 1986).

REPRODUCTION:
Thomas's leaf-monkey gives birth to a single offspring. Births in this species can occur any time during the year (Kunkun, 1986).

REFERENCES:
Aimi, M. and Bakar, A. 1992. Taxonomy and distribution of Presbytis melalophos group in Sumatera, Indonesia. Primates. Vol. 33(2), 191-206.

Aimi, M. and Bakar, A. 1996. Distribution and deployment of Presbytis melalophos group in Sumatera, Indonesia. Primates. Vol. 37(4), 399-409.

Ankel-Simons, F. 2000. Primate Anatomy: An Introduction. Academic Press: San Diego.

Davies, A.G., Bennett, E.L., and Waterman, P.G. 1988. Food selection by two south-east Asian colobine monkeys (Presbytis rubicunda and Presbytis melalophos) in relation to plant chemistry. Biological Journal of the Linnean Society. Vol. 34, 33-56.

Fleagle, J. G. 1988. Primate Adaptation and Evolution. Academic Press: New York.

Fleagle, J. G. 1999. Primate Adaptation and Evolution. Academic Press: San Diego.

Kunkun, J.G. 1986. Ecology and behavior of Presbytis thomasi in northern Sumatra. Primates. Vol. 27(2), 151-172.

Oates, J.F. and Davies, A.G. 1994. What are colobines? in Colobine Monkeys: Their Ecology, Behaviour and Evolution. eds. A.G. Davies and J.F. Oates. Cambridge University Press: Cambridge.

Reed, K.E. 1999. Population density of primates in communities: Differences in community structure. in Primate Communities. eds. J.G. Fleagle, C. Janson, and K.E. Reed. Cambridge University Press: Cambridge.

van Schaik, C.P., Assink, P.R., and Salafsky, N. 1992. Territorial behavior in southeast Asian langurs: Resource defense or mate defense? American Journal of Primatology. Vol. 26, 233-242.

Steenbeek, R. 1996. What a maleless group can tell us about the constraints on female transfer in Thomas's langurs (Presbytis thomasi). Folia Primatologica. Vol. 67, 169-181.

Steenbeek, R. 1997. Causes and consequences of the multi-male phase in wild Thomas langur groups. Primate Report. Vol. 48-2, 35.

Steenbeek, R. 1999. Tenure related changes in wild Thomas's langurs I: Between-group interactions. Behaviour. Vol. 136, 595-625.

Steenbeek, R. and Assink, P. 1998. Individual differences in long-distance calls of male wild Thomas langurs (Presbytis thomasi). Folia Primatologica. Vol. 69, 77-80.

Steenbeek, R., Piek, R.C., van Buul, M., and van Hooff, J.A.R.A.M. 1999a. Vigilance in wild Thomas's langurs (Presbytis thomasi): The importance of infanticide risk. Behavioral Ecology and Sociobiology. Vol. 45, 137-150.

Steenbeek, R., Assink, P., and Wich, S.A. 1999b. Tenure related changes in wild Thomas's langurs II: Loud calls. Behaviour. Vol. 136, 627-650.

Steenbeek, R., Sterck, E.H.M., De Vries, H., and van Hooff, J.A.R.A.M. 2000. Costs and benefits of the one-male, age-graded, and all-male phases in wild Thomas's langur groups. in Primate Males: Causes and Consequences of Variation in Group Composition. ed. P.M. Kappeler. Cambridge University Press: Cambridge.

Sterck, E.H.M. 1996. Females, foods, and fights: Comparing the ecology and social behaviour of the sympatric long-tailed macaque and Thomas langur. (abstract) Primate Report. Vol. 44, 46-47.

Sterck, E.H.M. 1997. Determinants of female dispersal in Thomas langurs. American Journal of Primatology. Vol. 42, 179-198.

Sterck, E.H.M. 1998. Female dispersal, social organization, and infanticide in langurs: Are they linked to human disturbance? American Journal of Primatology. Vol. 44, 235-254.

Sterck, E.H. 1999. Variation in langur social organization in relation to the socioecological model, human habitat alteration, and phylogenetic constraints. Primates. Vol. 40(1), 199-213.

Sterck, E.H.M. and Steenbeek, R. 1997. Female dominance relationships and food competition in the sympatric Thomas langur and long-tailed macaque. Behaviour. Vol. 134, 749-774.

Ungar, P.S. 1995. Fruit preferences of four sympatric primate species at Ketambe, northern Sumatra, Indonesia. International Journal of Primatology. Vol. 16(2), 221-245.

Ungar, P.S. 1996a. Feeding height and niche separation in sympatric Sumatran monkeys and apes. Folia Primatologica. Vol. 67, 163-168.

Ungar, P.S. 1996b. Relationship of incisor size to diet and anterior tooth use in sympatric Sumatran anthropoids. American Journal of Primatology. Vol. 38, 145-156.

Wilson, C.C. and Wilson, W.L. 1976. Behavioral and morphological variation among primate populations in Sumatra. Yearbook of Physical Anthropology. Vol. 20, 207-233.

Last Updated: June 21, 2007.
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