François' langur (Trachypithecus francoisi), also known as Francois' leaf monkey, the Tonkin leaf monkey, or the white side-burned black langur is a species of lutung and the type species of its species group. It is one of 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 fewer than 500 are believed to be left in Vietnam and 1,400–1,650 in China. About 60 langurs are 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 , while females are only in length. Males likewise have longer tails of compared to the for females. Males are significantly heavier than females, weighing compared to for females. Infants weigh at birth.
Infants are born with bright orange coloured fur, which fades to black throughout the period of infancy lasting several months. It is not known why their coats are so conspicuous, but current hypotheses suggest it may elicit attention, protection and caregiving by adults.
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.
Behavior
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. Grooming has been found to occur 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 4 to 27 langurs, but will usually be found in groups around 12. It lives in a matriarchal society where the females lead the group. Within the society, the females share parenting responsibilities with one another, and are philopatric to the group. Males within the group take no part in the raising of the young, and the young males leave the group before reaching sexual maturity. Young langurs are nursed 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 also consumes 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 10 different species of plants, 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 still consumes 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.