Langurs and Leaf Monkeys

Langurs belong to the primate family Cercopitecidae, of which 13 species are represented in the genus Semnopithecus. This represents one of the largest and most diverse groups of colobine monkeys in Asia, with most species restricted to the south and southeast. Many species are distinguished by their vocalizations and the color of their fur, which ranges from a silver-grey in the common or Hanuman langur (Semnopithecus entellus), to the glossy black fur of the Ebony leaf monkey (S. auratus) from Vietnam and Indonesia, a glistening orange coloration and black face in the golden leaf monkey (S. geei), and a ruby face in the elegantly patterned purple-faced leaf monkey (S. vetulus) of Sri Lanka, which has a brownish coat and a distinct white-yellow throat patch. Many species have raised brow crests which are used to express messages such as anger or pleasure to other members of the same species.

These monkeys are extremely agile animals with long limbs and tails, which are not prehensile like those of South American monkeys. Most species measure from 16-32 in (40-80 cm), with a tail length that can reach up to 43 in (108 cm) in the Hanuman langur. Body weight ranges from 11-53 lb (5-24 kg), with most species weighing around 13-18 lb (6-8 kg). They are mostly arboreal, leaf-eating species, but some may spend a lot of time foraging on the ground. Most of these monkeys, however, are opportunistic feeders and will consume insects, fungi, and fruit when the opportunity arises. Many species also eat small amounts of soil—probably for its mineral content. Some species, such as the Hanuman langur eat sap and gum and even large quantities of certain fruit with high strychnine content that could kill other species.

Leaf monkeys have large stomachs that are divided into a number of sacs, like those of ruminants such as cattle and deer. The upper section is larger than, and separated from, the lower, more acidic, section. The upper part is where the fermentation of green foliage takes place with assistance from anaerobic bacteria. These bacteria allow the monkeys to break down cellulose (a major component of all leaves) and overcome the many toxins in the leaves, enabling them to feed from a wide range of trees, many of which are inedible to other monkeys. This not only enables leaf monkeys to feed off a wider selection of food items than other monkeys, but also guarantees them a more efficient digestion of low-quality leaves than any other primate. The slow fermentation and digestive processes also allow for a higher absorption rate of materials as they pass through the intestines.

The social behavior of langurs and leaf moneys varies considerably according to species as well as to age, sex, and ecological conditions. In Hanuman langurs for example, one of the best-studied species, each troop has its leaders and subordinate animals, but the roles are often not clearly defined. Most breeding groups consist of a single adult male. The core of the troop, which may number up to 70 animals, is composed of many related adult females. Females rarely leave the troop. Males, in contrast, leave shortly before they reach sexual maturity, at about three years of age—before they are able to threaten the dominant male of the troop and/or mate with females within their natal troop. Solitary males usually join bands of other nomadic males and may wander over large areas each year in search of breeding females and the opportunity to establish their own troops. Nomadic males therefore pose a constant threat to the dominant male: aggressive encounters and chases are common in such attempts to take over a territory and breeding females. If an intruding male is successful in his bid, the former leader will be evicted and the newcomer then takes over the troop and breeds with the females. Often such males will kill any young offspring sired by the former male, and by doing so, the females will come into breeding condition again relatively quickly and allow this new male to increase his breeding potential.

Not all langur or leaf monkey troops are as large as those of the Hanuman langur. Troop size in the purple-faced, hooded-black (S. johnii) and capped (S. pileatus) leaf monkeys, for example, is often just six to nine animals, while most other species have a range of 10-18 individuals. In addition to size, the composition and role of troop members also varies considerably. Some species may have more than one breeding male in the troop and, in the case of territorial species, the role of defending the home range may fall primarily to adult males or to all members of the troop. The size of individual territories varies enormously according to local habitat and food conditions, as well as the size of the troop. Large troops of Hanuman langurs, for example, may have a home range of more than 2,400 acres (1,000 ha), but this is an extreme case. More often, leaf monkeys and langurs occupy home ranges from 25 to 50 acres (10-200 ha), of which only part may be actively defended from other monkeys.

Young langurs learn to recognize their own mothers shortly after birth, which is important in a society where A spectacled langur (Trachypithecus obscurus) nursing her young. Photograph by Tom McHugh. The National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.
many curious arms reach out to touch and hold a newborn infant. Although the mother tries to remain apart from the rest of the troop with her infant, young monkeys will be given to temporary female babysitters to hold and groom. Mothers may even suckle offspring that are not their own. When they are weaned, young langurs usually retain some association with their mothers, even though she may by now have additional offspring. Young females, in particular, often assist the mother with bringing up her young.

Although langurs and leaf monkeys are all versatile animals and display a wide range of feeding habits, the populations of many species have been seriously reduced in recent decades as a result of habitat loss and destruction. Many of southeast Asia's tropical forests have been seriously affected by logging and subsequent clearance by agricultural settlers. Newly built logging roads have opened up the interior of many forests, allowing greater access to remote regions. As a result of new and spreading settlements along these roads, hunting wild animals for food and their glossy fur, particularly vocal and visible species such as leaf monkeys and langurs, has increased significantly in some areas. Some species, such as Hanuman's langur, are pests on agricultural crops when the crops have been planted up to the forest fringes. Although this species is considered sacred by Hindus in part of its range, this does not prevent its destruction in others.