Trachypithecus francoisi (de Pousargues, 1898)

Species Identification

François' langur (Trachypithecus francoisi) is a medium-sized primate covered in black, silky hair, with very distinct white sideburns that extend from its ears down to the corners of its cheeks.

Sexual Dimorphism

This species displays sexual size dimorphism. Males have a head-body length of 55–64 cm (22–25 in), while females measure 47–59 cm (19–23 in) in head-body length.

Tail Length

Males also have longer tails, ranging from 82–96 cm (32–38 in), compared to female tails of 74–89 cm (29–35 in).

Adult Weight

Males are significantly heavier than females, weighing 6.5–7.2 kg (14–16 lb) while females weigh 5.5–5.9 kg (12–13 lb).

Infant Traits

Infants weigh 0.45–0.50 kg (16–18 oz) at birth, and are born with bright orange fur that fades to black over the course of their several-month-long infancy.

Infant Coat Hypothesis

It is unknown why infant coats are so conspicuous, but current hypotheses suggest the bright color may elicit attention, protection, and caregiving from adult group members.

Dietary Adaptations Overview

As a folivore, François' langur has multiple morphological adaptations for digesting a leaf-based diet. It has large salivary glands that start breaking down tough leaf fibers.

Stomach Specialization

Its most notable evolutionary adaptation is a specialized sacculated stomach; the original description notes both a four-chambered complex stomach and a two-chambered sacculated structure, where bacteria in the upper chamber continue breaking down fibers started by saliva.

Stomach pH Function

The upper chamber maintains a relatively neutral pH to support favorable conditions for bacterial growth, while the lower chamber resembles the stomach of other mammals and contains acid to finish breaking down food components.

Preferred Habitat

The preferred habitat of François' langur is karst topography, specifically limestone cliffs and caves in tropical and subtropical zones.

Sleeping Site Preference

Living on these limestone cliffs gives the langur an advantage for sleeping: it sleeps on ledges or in caves, and prefers caves as sleeping sites. The species has also been recorded using sleeping sites in evergreen forests in areas with terrain temperatures above 16 °C (60 °F).

Predation Risk Reduction

By living and sleeping in these limestone caves and cliffs, far from flat lowland, François' langur has greatly reduced its risk of predation. It uses cryptic behavior and remains very vigilant when entering its cave to rest for the night, as a tactic to avoid predators.

Territorial Behavior

It also produces loud calls to mark its territorial boundaries.

Sleeping Site Selection

François' langur selects its sleeping habitat based on proximity to foraging areas, choosing sleeping sites close to potential foraging grounds to conserve energy and cut down on travel costs. Sleeping sites are not located within the core of foraging areas, but lie within reasonable proximity, as preferred nesting and foraging sites do not align completely.

Travel Routine

When traveling to forage, the species tends to follow the same route and returns to the same sleeping site on consecutive nights, another behavior that reduces predation risk.

Sleeping Site Rotation

François' langur typically has around 6 to 10 regularly used sleeping sites that it uses at different times throughout the year as water and food resources change.

Geographic Range Overview

François' langur has a restricted geographic range. It is primarily found in Southwest China and northern Vietnam.

Research Locations

Most wild scientific research on this species is conducted in China's Guangxi Province, at the Nonggang Nature Reserve and Fusui Nature Reserve.

Home Range Size

The average home range size for the species is 19 hectares (230,000 square yards), and average daily range size is 341–577 square metres (3,670–6,210 square feet).

Diet Impacts on Movement

In general, the low nutritional quality of its folivorous diet leads to nutritional stress, smaller home ranges, and reduced daily travel time.

Group Size Extremes

The largest recorded group of François' langurs numbered 500–600 individuals, found in Mayanghe National Nature Reserve. Average group size ranges from 4 to 27 individuals.

Population Decline

In 2009, Fusui Nature Reserve reported that the local François' langur population had declined by 73% over the preceding five years, which further reduced the species' already limited distribution.

Current Distribution

Recent population censuses have found the species is now restricted to 14 localities across 10 different counties.