Folivore/frugivores. Unripe fruit and young leaves are less frequently available but preferred. Young leaves have less fiber and higher quantities of nutrients than mature leaves. Ripe fruit and its seed are usually not eaten. (Unripe fruit is lower in simple sugars. Ripe fruit can cause gastric distress)
Not true ruminants, but large, morphologically complex 4-chambered stomachs requires lots of roughage.
Process plant material from a wide variety of trees (Nhat (1994) documented 50 species) by fermentation. Plant cell walls are composed of cellulose that cannot be digested by vertebrates. Doucs maintain colonies of cellulolytic microorganisms (bacteria) in the Fore stomach which is used to ferment the polysaccharides making up cellulose
Glands secrete enzymes into the lumen of the GI tract to digest proteins, lipids and simpler carbohydrates.
Doucs are able to exploit mature foliage during food shortages (Nutritional value and protein of mature leaves are low, fiber is high).
Up to 20% of the body weight may be eaten as foliage and the animals often appear to have bloated stomachs
82% of intake is leaves and petioles (at least 75% are young leaves), 14% is fruits and seeds (mainly unripe), 4% flowers,
A recent study of food from five stomachs (Nhat) showed that (1) The quantity of food consumed every day varied from 450 to 650 grams (2) The diet consisted primarily of leaves, buds, fruit and flowers
Most water comes from foods consumed.
Observed to lick dew off leaves
In zoos, water is consumed after dry food pellets are eaten. Doucs at the San Diego Zoo dip their nutritionally-enhanced biscuits in water before eating.
Leaves Make a Meal
Douc langurs prefer to eat young leaves and unripe fruit.
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