The highly folivorous diet of Francois’ langur led to the evolution of enlarged salivary glands and a sacculated stomach, which combine to encourage break down and digestion of the tough fibre in leaves, with the help of bacteria living in the upper chamber of the stomach. The lower chamber contains acid as in most mammals, whereas the upper chamber is neutral to create more favourable conditions for the bacteria. Francois’ langur is not restricted to eating leaves; it also consumes fruit, seeds, nectar, shoots and insects, varying its diet with seasonal changes in the abundance of these foods (5). Fleshy immature leaves are preferred (5), preventing the langurs from needing to drink frequently (6). The group moves as it feeds, ending each day in a new place. They settle down to sleep at dusk, preferring large trees in good weather, or limestone caves if it’s cold or raining (8).
An active and noisy species, members of troops indulge in daily mutual grooming in rest periods between feeding (5) (6). The group consists of 4 to 27 (usually around a dozen) individuals and led by the females, who operate a reasonably changeable hierarchy amongst themselves, particularly when it comes to caring for the young (2) (5) (6). There is usually just one adult male in the group who will not participate in caring for the young, except to respond to their distress calls. He will also rarely groom other members of the group, but expects to be groomed himself. Females share responsibility for the infants, who are born singly once a year. They are immediately cared for by ‘allomothers’ as well as their own mother, who will nurse the young monkey for up to two years before weaning. Once weaned, the bond between mother and infant is no different than between any other members of the group. After three or four years juveniles become sexually mature, and will commonly leave to join another group or to form an all-male bachelor group. Francois’ langurs are not fully grown until the age of six or seven (5) (6).