Dusky Leaf-monkey (Trachypithecus obscurus)


MORPHOLOGY:
This species has a sacculated stomach to assist in the breakdown of cellulose found in plant cell walls. The dusky leaf-monkey has enlarged salivary glands to assist it in breaking down food. The intermembral index (humerus length + radius length/femur length + tibia length) is relatively high for this species as compared to other langur (Fleagle, 1976). The brachial index (radius length/humerus length) is relatively lower than other langur species (Fleagle, 1976). The scapular index (width of supraspinous fossa/width of infraspinous fossa) is relatively high as compared to other langur species (Fleagle, 1976). The dusky leaf-monkey has relatively short forelimbs and hindlimbs (Fleagle, 1976). The pectoralis minor, serratus anterior pars cranialis, and triceps brachii muscles are relatively larger in this species as compared to other sympatric langurs (Fleagle, 1977). The femur and the fibula are more robust in this species as compared to other langur species (Fleagle, 1976). The crural part of the flexor cruris lateralis muscle (biceps femoralis) has a significantly greater relative mass as compared to other langurs (Fleagle, 1977). Also the tensor fasciae femoris, gracilis, semimembranosus, and semitendinosus muscles are relatively larger as compared to other langurs (Fleagle, 1977). The average body mass for an adult male dusky leaf-monkey is around 8.30 kilograms, and for the female it is around 6.5 kilograms. The face of this species is black with white around the eyes and over the center of the mouth. The back and the limbs are gray, with the hands and feet being black in color. On the stomach region the hair color is creamy white. The newborn has a bright orange pelage color all over its body with pink skin underneath and light pink colored bare skin (Badham, 1967; Horwich, 1974). The infant pelage begins to darken at one month of age (Badham, 1967). Horwich (1974) found that black hairs begin to appear on the face at around seven weeks, and by twenty weeks the head and back are gray with only the body showing traces of light yellow. At ten moths the entire body is gray, but lighter than that of the adults (Horwich, 1974).

RANGE:
The dusky leaf-monkey lives in the countries of Burma, Malaysia and Thailand (Curtin, 1976, 1980; MacKinnon and MacKinnon, 1978; McClure, 1964; Aggimarangsee, 2002; Anonymous, 1993/1994). This species may also occur in the Gaoligongshan Region of Yunnan, China (Ma et al., 1995). This species prefers to live in closed primary forests, but is also found in old-growth secondary forests and urban forests (MacKinnon and MacKinnon, 1978, 1980; Curtin, 1980; Southwick and Cadigan, 1972). The dusky leaf-monkey spends most of its time in the upper canopy levels of the forest (MacKinnon and MacKinnon, 1978, 1980; Fleagle, 1978).

ECOLOGY:
The dusky leaf-monkey is a folivorous species, but it will also consume fruit and flowers. Young leaves constitute a high proportion (52%) of the diet for this species (Aldrich-Blake, 1978). Fruit consumed tends to be unripe (MacKinnon and MacKinnon, 1980). Dusky leaf-monkeys are able to take advantage of unripe fruit, which have chemical defenses, by the same means that they breakdown toxins in plant leaves, the bacteria found in their digestive system (MacKinnon and MacKinnon, 1980). Among fruit consumed includes figs (Ficus) of the following species: Ficus sumatrana, Ficus delosyce, Ficus stricta, Ficus cucurbitina, Ficus dubia, and Ficus aurantiacea (Lambert, 1990). The dusky leaf-monkey will consume both ripe and unripe figs (Lambert, 1990). During the dry season this species will consume Intsia palembanica, Koompassia excelsa, and Koompassia malaccensis (Curtin, 1980). Dusky leaf-monkeys will feed in the emergent trees, which is higher than other sympatric langurs (Curtin, 1976, 1980; Curtin and Chivers, 1978; Fleagle, 1976). During the day, the main group splits up and forms smaller foraging groups (Fleagle, 1988). The dusky leaf-monkey prefers to forage in the main canopy and the emergent layer of the forest (Fleagle, 1988, 1978). When feeding this species will sit on large branches and boughs (Fleagle, 1978). This species will spread foraging activity throughout the day so as not to overload their digestive processes that remove plant toxins and process leaf matter efficiently (MacKinnon and MacKinnon, 1980).

At Krau Game Reserve, Pahang, West Malaysia, MacKinnon and MacKinnon (1978, 1980) found that leaves were consumed the most (71.4%) in January/February and least (15.6%) in July when fruits became more important (83.5% of the diet). Flowers are important in the diet in the months of April (9.0%) and June (14.6%) (MacKinnon and MacKinnon, 1978, 1980). The most important food items are: Maranthes corymbosum (19.52%), Pentaspadon velutinum (7.18%), Koompassia malaccensis (6.00%), and Derris sp. (4.55%) (MacKinnon and MacKinnon, 1978, 1980). Dusky leaf-monkeys eat the leaves, unripe fruit, and flowers of Maranthes corymbosum (MacKinnon and MacKinnon, 1978, 1980).

At Krau Game Reserve, Curtin (1976) found that the diet of the dusky leaf-monkey consisted of new leaves (35.6%), mature leaves (22.5%), fruit (31.8%), flowers (6.8%), and seeds and beans (3.4%). 87 species were used as food sources at this site (Curtin, 1976, 1980). Important food species were: Pentaspadon velutinum, Maranthes corymbosa, Intsia palembanica, and Ficus sp. (Curtin, 1976, 1980; Curtin and Chivers, 1978). The new leaves of Intsia palembanica was found to be an important food source during the dry season (Curtin, 1976). Koompassia excelsa was an important source of leaves, new leaves in March and mature leaves in August and February (Curtin, 1980). The dusky leaf-monkey also foraged on Parkia javanica for leaves, with new leaves eaten in May and mature leaves eaten in January, March, and July (Curtin, 1980). During the dry season (January to April) the diet consisted of leaves (65%), fruit (39%), and flowers (12%) (Curtin and Chivers, 1978). During the intermediate season (May to August) the diet consisted of leaves (57%), fruit (39%), and flowers (4%) (Curtin and Chivers, 1978). Mature leaves that are high in secondary compounds were eaten more selectively, where mostly the leaf tips and petioles were consumed (Curtin and Chivers, 1978).

In comparison with the sympatric langur species the banded langur (Presbytis femoralis), the dusky leaf-monkey is more folivorous and consumes more mature leaves (Curtin, 1976, 1980). Dusky leaf-monkeys are also mainly folivorous throughout the year where the banded langur will become more frugivorous during certain months (Curtin, 1976, 1980). During the fruiting season these two species both fed on the fruit of Maranthes corymbosa and Ficus sp. which bore large fruit crops over a long period of time (Curtin, 1976, 1980). Dusky leaf-monkeys also consume fewer species than banded langurs (Curtin, 1976, 1980). Curtin (1976, 1980) found that dusky leaf-monkeys fed for fewer hours, fed in fewer trees, and fed for longer periods of time in each tree than the banded langur. Dusky leaf-monkeys groups will also spread out when foraging, whereas banded langurs will forage more as a tight unit, sometimes splitting into foraging subgroups (MacKinnon and MacKinnon, 1980).

MacKinnon and MacKinnon (1978, 1980) found that at Krau Game Reserve the mean group size was 10.3 individuals. Curtin (1980) found at Krau Game Reserve the average group size was 17 individuals. McClure (1964) found that at a diptocarp forest near Kuala Lumpur, Malaysia, group sizes were up to 15 individuals in size. This is a diurnal and an arboreal species. Groups will travel in wide circuits through the home range, making frequent passes through the core area (MacKinnon and MacKinnon, 1980). MacKinnon and MacKinnon (1980) found that when groups travel through the forest, they do not visit a given area frequently, which may be related to the consumption of mature leaves. During the day groups spent 46% of the time resting, 34% of the time feeding, and 20% of the time traveling (Raemaekers and Chivers, 1980). Groups have two feeding peaks during the day, in the early morning and in the late afternoon (Raemaekers and Chivers, 1980). Groups sleep together in the same or in adjacent trees and sleep in different places each night in the home range (MacKinnon and MacKinnon, 1980; Fleagle, 1978).

LOCOMOTION:
The dusky leaf-monkey moves through the forest primarily by quadrupedal locomotion (Fleagle, 1976, 1978, 1988; MacKinnon and MacKinnon, 1980). Fleagle (1978) found that when traveling the dusky leaf-monkey moves quadrupedally (50.6%), by leaping (40.2%), and by climbing (9.2%). Fleagle (1978) also found that when feeding, movement is done quadrupedally (68.6%), by leaping (15.0%), and by climbing (16.4%). Quadrupedal walking and running is a horizontal progression that uses the four limbs in regular sequences (Fleagle, 1977, 1978). During quadrupedal locomotion support and propulsion is provided by both the forelimbs and hindlimbs (Fleagle, 1977, 1978). This quadrupedalism is shown in the morphology of limbs, feet, hands, and pelvis (Fleagle, 1976). The asymmetrical shape of the proximal talar facet, the relatively long olecranon process of the ulna, a more pronounced pronator crest, and a broad hamate bone reflect this arboreal quadrupedalism (Fleagle, 1976). The dusky leaf-monkey also moves by leaping, which is moving between discontinuous supports using the hindlimbs as propulsion (Fleagle, 1977, 1978). When leaping, landing involves both the forelimbs and hindlimbs (Fleagle, 1977, 1978). The leaps of the dusky leaf-monkey are shorter than those of the sympatric banded langur (Fleagle, 1978). This species will also climb trees, where this is a continuous progression in the vertical plane, with the forelimbs used to pull the body up and the hindlimbs used to push the body (Fleagle, 1977, 1978). When moving through the trees, broad firm substrates are preferred (Curtin, 1976, 1980; Fleagle, 1976). Fleagle (1978) suggests that since the dusky leaf-monkey can consume unripe fruits and leaves that other species cannot digest, this permits a more sedentary habit.

SOCIAL BEHAVIOR:
The dusky leaf-monkey can have a social system of two types: multimale-multifemale or unimale (Curtin and Chivers, 1978). This is a territorial species where home range boundaries rarely overlap (Curtin, 1976; MacKinnon and MacKinnon, 1980). Territories might be maintained because of the territorial honks emitted by the adult male (Curtin, 1980). Solitary individuals are known to occur of both sexes (Curtin, 1976; Curtin and Chivers, 1978). Adult males do not interact with many members of the group; their roles are confined to predator detection, maintenance of group cohesion during travel, and defense of territorial boundaries (Curtin, 1976, 1980). Males will not become aggressive with intruders; rather hide with the rest of the group (Curtin, 1980). As compared to the banded langur, the dusky leaf-monkey has a higher frequency of social interactions within the group (Curtin, 1980). Infants will be transferred between females, and sometimes one adult female will watch as many as four infants while the other females rest and feed (Curtin, 1976, 1980; Badham, 1967). Juveniles will be associated more with an adult female than with members of their own peer group, which is different from banded langurs (Curtin, 1980).

Aggression is low amongst group members (Curtin, 1976, 1980). When aggression does occur amongst group members, there is a high rate of reconciliation (Arnold and Barton, 1997, 2001a). Affiliative dyadic relationships, and not kinship and rank, are associated with conciliatory tendencies (Arnold and Barton, 1997, 2001a). Ventro-ventro hugging (embracing) is an important behavior during reconciliation (Arnold and Barton, 1997, 2001a). Former opponents will engage in hugging as will victims with a third party who with they have a strong bond (Arnold and Barton, 1997, 2001b). Social grooming was the most popular first contact behavior after an agonistic encounter (Arnold and Barton, 2001b). The intensity of aggression was not found to have an effect on the likelihood of reconciliation to occur (Arnold and Barton, 2001a). Reconciliation was found to occur within three minutes after an agonistic encounter (Arnold and Barton, 2001a). Reconciliation can be initiated by the recipient of aggression or the aggressor (Arnold and Barton, 2001a).

Groups will split-up during the day to form subgroups that forage and then reunite during the evening (Curtin, 1976, 1980). The most common subgroup is an adult female and a juvenile (Curtin, 1976, 1980). Subgroups tend to be large in the early morning and smaller in the mid and late afternoon (Curtin, 1976, 1980). Subgroups remained in contact through visual means and also by knowing familiar travel routes through the forest (Curtin, 1976, 1980). Ranging patterns of a group are conservative by nature, which helps in predicting where a subgroup will be at a certain time (Curtin, 1980). The ranging patterns might also explain why there is little home range overlap between neighboring groups (Curtin, 1980). Subgroups may be adaptive for feeding on a limited and dispersed food supply such as leaves (Curtin and Chivers, 1978). Trees may be dispersed throughout the forest, and new leaves may replace themselves at a slow rate (Curtin and Chivers, 1978).

Social play in the dusky leaf-monkey includes wrestling, sham-biting, jumping on or over, chasing, fleeing, and tail pulling (Arnold and Barton, 2001a). Play face will occur when engaging in social play (Arnold and Barton, 2001a).

The dusky leaf-monkey is sympatric with the banded langur, the pig-tailed macaque (Macaca nemestrina), the long-tailed macaque (Macaca fascicularis), the siamang (Hylobates syndactylus), and the white-handed gibbon (Hylobates lar) in a diptocarp forest near Kuala Lumpur (McClure, 1964; MacKinnon and MacKinnon, 1980). Curtin and Chivers (1978) found that this species associates with siamangs, feeding in the same or adjacent trees and using the same pathways when traveling. Associations between dusky leaf-monkeys and siamangs were found to be most frequent at dawn and dusk (Curtin and Chivers, 1978).

VOCAL COMMUNICATION:
honk: This call is emitted by adult males and is used to demarcate the groupís territory (MacKinnon and MacKinnon, 1980). This call is a honking type, two-phase noise that sounds like "chengkong" (Curtin, 1980). In extreme circumstances this call may be three or four phased (Curtin, 1980). This call will be alternated with coughing calls and soft warning call when a disturbance is detected (Curtin, 1980). Females will emit a muffled version of this call (Curtin, 1980). When used as a territorial call, an arboreal display also occurs (Curtin, 1980). This call is heard most often during the early morning (Curtin, 1980). Alarm honks and territorial honks are similar in structure, but group members will flee when hearing an alarm honk (Curtin, 1980).

soft warning call: This call sounds like "whoo" and often precedes the loud call (Curtin, 1980).

aa-aa: This vocalization is found to accompany embracing (Arnold and Barton, 2001b). This call is staccato in nature (Arnold and Barton, 2001b).

OLFACTORY COMMUNICATION:

VISUAL COMMUNICATION:
presenting: This is when one individual has the anogenital region oriented towards another (Arnold and Barton, 2001a). The tail may be raised and/or the head may look back over the shoulder of the sender of this signal (Arnold and Barton, 2001a). This behavior is used to communicate submission (Arnold and Barton, 2001a).

mounting: This is when an individual will mount another who is on four limbs and has the rump raised into the air (Arnold and Barton, 2001a). The mounter will stand behind with or without grasping with the feet the recipient's ankles (Arnold and Barton, 2001a). This behavior occurs with no intromission or thrusts (Arnold and Barton, 2001a). This behavior serves to communicate domination (Arnold and Barton, 2001a).

tongue flick: This is when an individual will stare at another while flicking the tongue in and out of the mouth (Arnold and Barton, 2001a). This is done in a rapid movement (Arnold and Barton, 2001a). This behavior serves to communicate threat on the part of the sender (Arnold and Barton, 2001a).

lunge: This is another threat behavior, where an individual will move swiftly towards another, of a distance of less than two meters (Arnold and Barton, 2001a).

play initiation: This is where an individual will shake or quickly twist the head in the horizontal plane (Horwich, 1974). This signal serves to solicit play on the part of the sender (Horwich, 1974). Infants will perform this during self play by themselves (Horwich, 1974).

TACTILE COMMUNICATION:
social grooming: This is when one individual grooms another and is used to reinforce the bonds between individuals.

embrace: (hugging) This is when two individuals will sit ventro-ventrally (chests touching) and with both arms around each other (Arnold and Barton, 2001a). Kneading of the fur will often accompany this behavior (Arnold and Barton, 2001a). This behavior serves to communicate reconciliation and to consol the recipient (Arnold and Barton, 2001a, b). The aa-aa will usually accompany this behavior (Arnold and Barton, 2001b).

hold bottom: This is when one individual will sit and hold the hindquarters of another individual that is sitting or standing (Arnold and Barton, 2001a). This behavior serves to communicate domination on the part of the sender (Arnold and Barton, 2001a).

jumpkick: This is when one individual will jump in front of another and kick the individual one or more times (Arnold and Barton, 2001a). This behavior serves to communicate aggression (Arnold and Barton, 2001a).

hit: This is when individual will strike another with force (Arnold and Barton, 2001a). This behavior serves to communicate aggression (Arnold and Barton, 2001a).

grab: This is when one individual will grasp another with one or both hands, restraining the individual (Arnold and Barton, 2001a). This behavior serves to communicate aggression (Arnold and Barton, 2001a).

grappling: This is when two individuals are in conflict with each other, grabbing, pulling, wrestling, and kicking each other continuously (Arnold and Barton, 2001a). This behavior serves to communicate aggression (Arnold and Barton, 2001a).

REPRODUCTION:
The dusky leaf-monkey gives birth to a single offspring. During estrus the vulva of the female swells (Hrdy and Whitten, 1987). Menstruation in females was found to last three weeks and last two to three days in captivity (Badham, 1967). The gestation period for this species in captivity is about five months (Badham, 1967). Burton (1984) suggests that based on testicular histology, the dusky leaf-monkey is capable of breeding throughout the year.

Badham (1967) found that in captivity copulation occurred on the first day after menstruation, and lasted for two days after that. Mounting occurs from the rear or dorso-ventral in the dusky leaf-monkey (Badham, 1967).

Horwich (1974) found that for the first twenty days there are six main maternal behavioral patterns:
For the first twenty days the infant will spend most of the time on the nipple of the mother, often sleeping while the mother manipulates it (Horwich, 1974).

Infants start to climb away from the mother at about one month of age, and at this time will begin to take solid food (Badham, 1967). After about twenty days, the infant will starts to develop such behavioral patterns as mouthing, scratching, and locomotor skills (Horwich, 1974). Self-play behavior begins during this time period, represented by play jumping, which are a series of 3-4 jumps in the same place, pushing off with the hindlimbs (Horwich, 1974). Mothers also begin to repulse their infants during this time (Horwich, 1974). Repulsions include: (1) push or pull from the nipple (2) biting the infant (3) tongue flicking towards the infant (Horwich, 1974).

After about seventy days the infant becomes more interested in socializing with other group members (Horwich, 1974). The frequency of nipple contact varied between high and low, suggestive of anxiety on the part of the infant (Horwich, 1974). Social play with other group members begins during this time (Horwich, 1974). Social grooming, with the infant as the groomer, begins after seventy days also (Horwich, 1974). Horwich (1974) found that at 240 days the infant becomes increasingly distant from the mother. After this time, the infant will start presenting to the adult male, with the male pushing the infant to present correctly (Horwich, 1974).

REFERENCES:
Aggimarangsee, N. 2002. Preliminary observations on an isolated dusky langur (Trachypithecus obscurus) population at Khao Lommuak in Southern Thailand. (abstract) XIXth Congress of the International Primatological Society. Beijing, China. 0269.

Aldrich-Blake, F.P.G. 1978. Dispersion and food availability in Malaysian forest primates. In Recent Advances in Primatology, Vol. 1: Behaviour. eds. D.J. Chivers and J. Herbert. Academic Press: London.

Anonymous. 1993/1994. Kaser Doo Wildlife Sanctuary, Burma. Asian Primates. Vol. 3(3/4), 11-12.

Arnold, K. and Barton, R.A. 1997. Post-conflict behaviour in spectacled langurs (Trachypithecus obscurus). (abstract) Primate Eye. Vol. 62, 4.

Arnold, K. and Barton, R.A. 2001a. Postconflict behavior of spectacled leaf monkeys (Tracypithecus obscurus). I. Reconciliation. International Journal of Primatology. Vol. 22(2), 243-266.

Arnold, K. and Barton, R.A. 2001b. Postconflict behavior of spectacled leaf monkeys (Trachypithecus obscurus). II. Contact with third parties. International Journal of Primatology. Vol. 22(2), 267-286.

Badham, M. A note on breeding the spectacled leaf monkey Presbytis obscura at Twycross Zoo. International Zoo Yearbook. Vol. 7, 89.

Burton, F. 1995. The Multimedia Guide to the Non-human Primates. Prentice-Hall Canada Inc.

Burton, G.J. 1984. Testicular histology of the dusky leaf monkey (Presbytis obscura) as it relates to birth pattern in peninsular Malaysia. International Journal of Primatology. Vol. 5(2), 183-195.

Curtin, S.H. 1976. Niche separation in sympatric Malaysian leaf-monkeys (Presbytis obscura and Presbytis melalophos). Yearbook of Physical Anthropology. Vol. 20, 421-439.

Curtin, S.H. 1980. Dusky and banded leaf monkeys. in Malayan Forest Primates: Ten Years' Study in Tropical Rain Forest. ed. D.J. Chivers. Plenum Press: New York.

Curtin, S.H. and Chivers, D.J. 1978. Leaf-eating primates of peninsular Malaysia: The siamang and the dusky leaf-monkey. In The Ecology of Arboreal Folivores. Ed. G.G. Montgomery. Smithsonian Institution Press: Washington, D.C.

Fleagle, J.G. 1976. Locomotor behavior and skeletal anatomy of sympatric Malaysian leaf-monkeys (Presbytis obscura and Presbytis melalophos). Yearbook of Physical Anthropology. Vol. 20, 440-453.

Fleagle, J.G. 1977. Locomotor behavior and muscular anatomy of sympatric Malaysian leaf-monkeys (Presbytis obscura and Presbytis melalophos). American Journal of Physical Anthropology. Vol. 46, 297-308.

Fleagle, J.G. 1978. Locomotion, posture, and habitat utilization in two sympatric, Malaysian leaf-monkeys (Presbytis obscura and Presbytis melalophos). In The Ecology of Arboreal Folivores. Ed. G.G. Montgomery. Smithsonian Institution Press: Washington, D.C.

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

Horwich, R.H. 1974. Development of behaviors in a male spectacled langur (Presbytis obscurus). Primates. Vol. 15(2-3), 151-178.

Hrdy, S.B. and Whitten, P.L. 1987. Patterning of Sexual Activity. In Primate Societies. eds. B.B. Smuts, D.L. Cheney, R.M. Seyfarth, R.W. Wrangham, and T.T. Struhsaker. University of Chicago Press.

Lambert, F. 1990. Some notes on fig-eating by arboreal mammals in Malaysia. Primates. Vol. 31(3), 453-458.

MacKinnon, J.R. and MacKinnon, K.S. 1978. Comparative feeding ecology of six sympatric primates in West Malaysia. In Recent Advances in Primatology, Vol. 1: Behaviour. eds. D.J. Chivers and J. Herbert. Academic Press: London.

MacKinnon, J.R. and MacKinnon, K.S. 1980. Niche differentiation in a primate community. in Malayan Forest Primates: Ten Years' Study in Tropical Rain Forest. ed. D.J. Chivers. Plenum Press: New York.

McClure, H.E. 1964. Some observations of primates in climax diptocarp forest near Kuala Lumpur, Malaya. Primates. Vol. 5(3-4), 39-58.

Ma, S., Han, L., Lan, D., Ji, W., and Harris, R.B. 1995. Faunal resources of the Goaligongshan Region of Yunnan, China: Diverse and threatened. Environmental Conservation. Vol. 22(3), 250-258.

Raemaekers, J.J. and Chivers, D.J. 1980. Socio-ecology of Malayan forest primates. in Malayan Forest Primates: Ten Years' Study in Tropical Rain Forest. ed. D.J. Chivers. Plenum Press: New York.

Southwick, C.H. and Cadigan, Jr., F.C. 1972. Population studies of Malaysian primates. Primates. Vol. 13(1), 1-18.

Last Updated: October 15, 2003.
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