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François' langur (Trachypithecus francoisi), also known as Francois' leaf monkey, Tonkin leaf monkey, or white side-burned black langur is a species of lutung and the type species of its species group. It is the least studied of the species belonging to the Colobinae subfamily.
The species is distributed from Southwestern China to northeastern Vietnam. The total number of wild individuals is unknown, but there are believed to be less than 500 left in Vietnam and 1,400–1,650 in China. There are about 60 langurs in captivity in North American zoos. The species is named after Auguste François (1857–1935) who was the French Consul at Lungchow in southern China.
Physical description 
François' langur is a medium sized primate with black silky hair. It has very distinct white sideburns that grow down from its ears to the corners of its cheeks. A morphological specialization of François' langur is its complex stomach, made up of four separate chambers. This is a necessary adaptation for the digestion of its folivorous diet.
This species shows sexual dimorphism in its size. Males have a head-body length of 55–64 cm (22–25 in), while females are only 47–59 cm (19–23 in) in length. Males likewise have longer tails of 82–96 cm (32–38 in) compared to the 74–89 cm (29–35 in) for females. Males are significantly heavier than females, weighing 6.5–7.2 kg (14–16 lb) compared to 5.5–5.9 kg (12–13 lb) for females. Infants weigh 0.45–0.50 kg (16–18 oz) at birth.
François' langur has large salivary glands to begin the digestion of tough leaf fibers. A more notable evolutionary adaptation seen in this langur is the sacculated stomach with two chambers. In the upper chamber, bacteria help to continue the breakdown of fibers started by the saliva. The upper chamber has a relatively normal pH, to create favorable conditions for bacterial growth. The lower chamber is similar to that of other mammals, in that it contains the acids that finish the breakdown of food components.
François' langur is diurnal and spends most of the day resting and foraging. One study investigated time distribution across activities in a disturbed environment, showing resting 35.41%, foraging 31.67%, traveling 14.44%, huddling 9.61%, playing 8.54%, and grooming 0.33%. Traveling, playing, grooming and huddling are more dependent on the season. Interestingly it has been found that grooming occurs in all seasons but spring. François' langur spends a greater part of its day travelling during the winter (20.12%) and huddling in the spring (14.62%).
François' langur lives in groups of four to twenty-seven langurs, but will usually be found in groups of around twelve. It lives in a matriarchal society where the females lead the group. Within the society, the females perform alloparenting, sharing parenting responsibilities with one another, and are philopatric to the group. Males within the group take no part in the raising of the young, the young males will leave the group before reaching sexual maturity. Young langurs are nursed for up to two years before being weaned, and once weaned the relationship amongst the relatives becomes that of any other member of a given group.
Over 50% of François' langur's diet is made up of leaves. It will also consume fruits (17.2%), seeds (14.2%), flowers, stems, roots, bark and occasionally minerals and insects from rock surfaces and cliffs. This langur consumes its favorite food, young leaves, at the highest rate during the dry season, April through September; between October and March when young leaves are less common, the langur supplements its diet with seeds, petioles, and stems.
François' langur is selective in its diet, in Nonggang Nature Reserve, China, it primarily eats the young leaves of ten different species of plant, only two of which are common within the reserve. Its diet includes Pithecellobium clypearia, Ficus nervosa, Garcinia pauncinervis, Sinosideroxylon pedunculatum, F. microcarpa, Miliusa chunni, Securidaca inappendiculata, Bauhinia sp., and Canthium dicoccum. Though these are the preferred plant species, it will still consume other plant species opportunistically. Another study on François' langur in a fragmented habitat found that it preferred on just four plant species: litse, Litsea glutinosa; seatung, Pittosporum glabratum; Cipadessa cinerascens; and Chinese desmos, Desmos chinensis. The study showed that the langur spent 61.6% of its feeding time on these four plant species, and 38.4% of its time on 36 other known species.
Habitat and distribution 
The preferred habitat of François' langur is a karst topography; limestone cliffs and caves of tropical and subtropical zones. By living on these limestone cliffs, the langur is at an advantage when it comes to sleeping arrangements. It sleeps either on ledges or in caves, with its preference being in the cave. François' langur has also been known to find sleeping sights in areas where the terrain is above 60 °F (16 °C), within evergreen forests. By living and sleeping in these limestone caves and cliffs, far from flat land, the langur has greatly reduced its rate of predation. It exhibits cryptic behavior and becomes very vigilant upon entry to the cave for final resting as a tactic to avoid any predators. In addition to this it also demonstrates a loud call to declare its territorial spacing. François' langur will also choose its sleeping habitat depending upon foraging availability. It will choose sleeping sites that are close to potential foraging sites, to maximize energy and reduce travel costs. It is important to note that sleeping sites are not located in the heart of foraging sites, but are within reasonable proximity, as the preferred nesting and foraging sites do not completely line up with one another. When it does go to forage, it tends to travel along the same route and returns to the same sleeping site consecutive nights to avoid predation. François' langur has been known to have approximately 6-10 regularly used sleeping sites that are used at various points throughout the year as water and food resources fluctuate.
François' langur has a restricted range of areas in which it can inhabit. It is primarily found in Southwest China and northern Vietnam. The majority of scientific studies of François' langur in the wild take place in the Nonggang Nature Reserve and the Fusui Nature Reserve in Guangxi Province, China. The average home range size of this species is 19 hectares (230,000 sq yd) and its day range size is 341–577 square metres (3,670–6,210 sq ft). In general, the low quality of its folivorous diet leads to nutritional stress, a smaller home range size and reduced daily travel time. The largest group of langurs reported numbered 500-600 individuals, and was found in the Mayanghe National Nature Reserve. The average group size is approximately 4-27. The Fusui Nature Reserve reported in 2009 that François' langur population had declined 73% in the previous 5 years, thus lessening their distribution even more. Recent census numbers have concluded that it is now limited to 14 localities in 10 different counties.
Conservation status 
The population of François' langur has been on a steady decline for the past 30 years. Of the many factors threatening the survival of François' langur today, hunting has had one of the largest impacts. In Nonggang, where François' Langur is most prevalent, the natives believe that the langur has medicinal values, and have hunted them to make wine out of their bones, which they believe could cure fatigue and rheumatism. In Guangxi province there has been an estimated 90% decline in numbers since the 1980s, a 2002-2003 survey found 307 individuals in 14 populations remained. In 1983, the estimated population of François' langur was 4,000-5,000. In the 1970s, hunting records recorded more than 1,400 langurs killed and in the 1980s more than 1,500 langurs were killed.
Another threat to François' langur is the destruction of its habitat. The langur lives on limestone cliffs and when farmers look to cultivate their land they will light fires on the lower slopes. Limestone is particularly susceptible to fire; therefore this practice not only destroys its habitats but also causes major food shortages for the langur because its diet is primarily folivorous. The primary predators of François' langur are both terrestrial and aerial. The clouded leopard is a potential predator to the langur but the clouded leopard's numbers are so low that they are not its greatest threat. Aerial predators such as the Crested Serpent Eagle and the Mountain Hawk-eagle are a greater threat to François' langurs of Nonggang, especially to their young.
The actions being taken towards the conservation of this species and its habitat is still minimal. Its current population size is less than 2,500 individuals. A plan to protect the forest and ban hunting, called the Conservation Action Plan, was drafted in 1996 but has still yet to be implemented. In order to protect the langur, not only does protection from hunting need to be implemented but its habitats must be protected as well. In 2003, the National Forestry Bureau acknowledged the rapid decline in François' langur and agreed to increase law enforcement in this area to help protect the langur from hunters. In addition, the Asia Developmental Bank has begun helping the residents that live in close proximity to the habitats of the langur build biogas facilities to reduce the fuel wood collection and thus possibly reduce the number of fires. And finally, a current project is underway by the Global Environmental facility to protect the Nonggang National and Dmingshan Natural Reserves and the langurs living within.
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