MITRED LEAF MONKEY
Geographic Distribution and Habitat
Mitred leaf monkeys, also known as yellow-handed mitred langurs and Sumatran surilis, are langurs found on the island of Sumatra, Indonesia, and on the small island of Pulau Pini in the Batu Archipelago, just off the coast of Sumatra. They make their homes mostly in lowland rainforests nearby rivers, but occasionally reside at higher altitudes. The forest understory is their preferred habitat; however, mitred leaf monkeys sometimes inhabit the highest elevations of the rainforest canopy.
Their foraging range encompasses one-third to a half-mile (500 to 800 m) of territory.
Size, Weight, and Lifespan
Slender bodies and long tails characterize mitred leaf monkeys. They weigh about as much as a large house cat, between 13 and 16 lbs (5.8 to 7.4 kg), with males weighing slightly more than females. Their tails, however, are significantly longer than that of a pussycat: 21 to 32 in (53 to 81 cm), and their head-to-body length ranges from 16.5 to 23 in (42 to 59 cm).
Average lifespan in captivity is 16 years.
With distinctive, bare rings around their eyes (giving the impression of “bags” beneath the lower eye), poorly developed or absent brow ridges, prominent nasal bones, narrow lips, and a tufted crest of hair on the crown of their head, this primate has earned the questionable comparison to an old woman. In fact, the species’ scientific name, presbytis, is derived from the Greek word for “old woman” and refers specifically to the monkey’s wizened facial features; “mitre,” a headband worn by women in ancient Greece, further promotes this comparison, regardless of the individual monkey’s gender.
At birth, mitred leaf monkeys usually have white fur coats, with a dark stripe extending down the back and bicolored tail and across the shoulders to form a cross-shape pattern. As adults, their fur coats vary in color from brownish-red, to pale orange, to blackish-gray and white, with paler coloring on the stomach, chest, and limbs. Their “old woman’s” brow is typically colored red, orange, or white.
Mitred leaf monkeys are herbivores, feeding exclusively on plant-based foods. Up to 197 tree species and 55 different plants provide their daily sustenance. As their name suggests, leaves comprise a portion of their diet, but only about one-third. These monkeys love fruits, which make up about 50 percent of their diet; the remainder of their diet includes seeds, nuts, and flowers. Water is provided through their frugivorous (fruit-based) meals; they also drink the dew from plants and the rainwater that collects in tree hollows.
Behavior and Lifestyle
An arboreal species, mitred leaf monkeys spend most of their time in trees. Their slender bodies and long tails are adapted for balance. Elongated hands and strong, developed fingers help them to swing from tree to tree in the rainforest understory, while their small, rudimentary thumb hitchhikes a free ride. Long forelimbs and relatively long hind limbs assist them in leaping and walking or running on all fours. They are mostly active during the daytime and early evening hours.
Mitred leaf monkeys are descendant of African colobine (that is, leaf-eating) monkeys, who entered Asia from north of the Himalayas around 15 million years ago.
Mitred leaf monkeys are equipped with three stomach chambers, one being a fermentation chamber that is adapted to digesting leaves, in a similar manner to ruminants such as cows.
Important contributors to healthy ecosystems, mitred leaf monkeys disperse the seeds of their plant-based diet through their feces.
The mitred leaf monkey is an Old World monkey and is currently considered the parent species to four subspecies. These subspecies are based on differences in coloration of fur coat (or “pelage”), distinctions in vocalizations, and their geographic location.
Mitred leaf monkeys live in groups, known as troops, of 12 to 18 individuals. Troops appear to be male-centric, often headed by a single male who mates with all the females. Other groups include multiple females and several “harem” males, who keep a lookout for threats to the group. All-male troops and lone males are another demographic; however, these males appear intent on altering their social situation by infiltrating male-female troops and absconding with some of the females to head their own troop. Lone males usually have a tougher time in this quest, as they are chased away by the harem males of an existing troop and banished to a resource-poor area of the forest.
Territorial behavior is influenced by the availability of resources. Troops that have a smaller home range are usually more territorial than troops with larger home ranges. Males use vocalizations to identify their troop’s territory, emitting distinctive, low-pitched territorial announcements at dawn and dusk, and at intervals throughout the night
But it is the females who determine their troop’s movement, which can lead to encounters with other mitred leaf monkey troops.
Harem males sound an alarm call, distinct from a territorial call, as these encounters can be contentious with the males becoming physically aggressive to perceived interlopers. Apart from setting the stage for these potential encounters, females play no role in mediating conflicts with outside troops.
Males also sound the alarm whenever a predator approaches; they will do their best, through loud cries and leaps, to lure the predator away from the troop. Predators to mitred leaf monkeys include birds of prey and snakes.
Female mitred leaf monkeys reach sexual maturity at about four years of age; males become sexually mature between four and five years of age. The species has no specific breeding season; a peak in the number of births corresponds to a seasonal overabundance of food. Females almost always give birth to a single offspring (twins are rare). Infants become independent at an early age, and juvenile males leave their birth group when they are between 5 and 10 months of age.
In other species of leaf monkeys, birth mothers and other females in the group help care for infants; however, this behavior has not been observed in mitred leaf monkeys, and the role (if any) of male mitred leafy monkeys in raising young is unknown.
Mired leaf monkeys are important forest cultivators. Their frugivorous diet and dispersal of seeds through their feces help to replenish their habitat. In this way, they are vital to the health of the ecosystem and tropical biodiversity.
The mitred leaf monkey is listed as Endangered by the International Union of Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
Extensive loss of habitat, particularly due to the conversion of forestland into palm oil plantations, poses the most significant threat to this monkey’s survival. Although mitred leaf monkeys are somewhat adaptable to changes in their environment, palm oil plantations have led to a decline in the species’ population. Indonesia remains a major supplier of palm oil and is the world’s number-one supplier of plywood.
Poaching and the illegal pet trade pose additional threats. Furthermore, some farmers regard the species as a crop pest; mitred leaf monkeys have been known to dig up and dine on sweet potatoes.
International trade of this species is prohibited by the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival.
Indonesian national law affords mitred leaf monkeys some protection; additionally, monkeys living in five protected areas of Indonesia’s national parks do not have the threat of habitat loss looming over them.
Written by Kathleen Downey, May 2016