Four subspecies were recognized by Groves (2005), although their validity was in some doubt as it was thought that seasonal variation in pelage colour may account for at least some variation. This has been reviewed recently and distinct hair patterns have been used (Choudhury 2014). Choudhury (2014) recognizes three subspecies: Trachypithecus pileatus pileatus, T. p. brahma and T. p. tenebricus.

This species is listed as Vulnerable as it is suspected to have undergone a decline of more than 30% over the last three generations (36 years) due to rampant habitat loss and is predicted to decline at the same rate over the next three generations due to increasing pressures on land for development, human settlements, plantations, monocultures, and agriculture, resulting in continuing and predicted degradation, fragmentation and loss in quality of habitat.

Trachypithecus pileatus occurs in Bangladesh, Bhutan, northeastern India (Arunachal Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland, and Tripura), and northwestern Myanmar. South Asian populations of its subspecies are known to exist in adjacent and sometimes overlapping areas of India, Bangladesh, Bhutan, and Myanmar. Various populations exist between 10 and 3,000 m in elevation (Molur et al. 2003). Records from China were disputed, but there is one record of a small population in southern Tibet (Xizang) west of the Tsangpo bend (George Schaller, pers. comm. to Anwaruddin Choudhury; photo seen) (Choudhury 2014).

Trachpithecus pileatus pileatus occurs in northeastern India (Arunachal Pradesh, Assam, Meghalaya, and Nagaland) and north-western Myanmar. Widely distributed in the highlands south and east of the Brahmaputra and west of the Chindwin; in Myanmar found in the Chin Hills south to Mount Victoria; in India, the Karbi Anglong, Khasi, Garo, Naga, and Jaintia Hills, Lakhimpur, Golaghat, Cachar Hills, Samaguting, and Sibsagar (Groves 2001) along with the Karbi Plateau and the Barail Range in Assam (Groves 2001, Das and Chetry pers. comm.), and northeastern and southeastern Bangladesh (Groves 2001).  Elevation range 0-3,000 m. Cut away by Brahmaputra from the other two subspecies (T. p. brahma and T. p. tenebricus), T. p. pileatus has a distinct hair-pattern (Choudhury 2014).

Trachypithecus pileatus brahma occurs in north-eastern India north of the Brahmaputra River (Groves 2001) and on the east bank of river Jia-Bhoreli in the state of Assam, and Arunachal Pradesh in India (Choudhury, 2014; Das and Chetry pers. comm.).

Trachypithecus pileatus tenebricus occurs in Bhutan and north­eastern India. Found in the Manas region to the north of the Brahmaputra River and between river Jia-Bhoreli in the west and Manas river in the east in the state of Assam and Arunachal Pradesh in India and Bhutan (Choudhury 2014, Das and Chetry pers. comm., Groves 2001) (from 100 m up to nearly 2,000 m) into Bhutan (Groves 2001, Molur et al. 2003).

This species is the most common langur in north-eastern India (Choudhury 2001). Although sympatric with Hoolock Gibbons in South Asia, it is not as heavily impacted by the effects of habitat loss and is more adaptive, breeds more rapidly, and can move across fragments easily (A. Kumar and J. Das pers. comm.). However, the species has probably declined by more than 30% in the last 20 years, making it very vulnerable in its extremely fragmented locations (J. Das, A. Kumar and M. Singh pers. comm.; Molur et al. 2003) and it is predicted to decline at the same rate in the next 20 years (J. Das, A. Kumar and M. Singh pers. comm.; Molur et al. 2003).

The population estimates in Assam seems to have been halved from 39,000 in 1989 (Choudhury 1989) to 18,600 in the 2008-2014 estimate due to habitat loss, fragmentation and degradation due to developmental activities, plantations and settlements (Choudhury 2014). It can not adapt like Rhesus and hence, habitat loss has impacted it severely in areas such as Sonitpur, upper assam and Barak valley districts.

The mean group size of 11 groups in Innerline RF was 11.0 (range: 8–15) and in parts of central and eastern Assam, and in Bhutan, the mean of 33 groups was 9.97 (Choudhury 2012).

These animals are diurnal, predominantly arboreal, and folivorous (Molur et al. 2003). According to Srivastava and Mohnot (2001) they are found in subtropical, broadleaf, deciduous, and bamboo forests (Choudhury 2001). The generation length is inferred from that of other langur species to be 12 years (Molur et al. 2003). Habitat is subtropical, broadleaf, evergreen, deciduous, bamboo and teak plantations, submontane. The species is sympatric with T. phayrei and S. schistaceus.

It is largely folivorous but also eats young mature leaves, fruits, bark, seeds, flowers, caterpillars. Studies in Madhupur, north-central Bangladesh report 53% of diet made of leaves and leaf parts, seeds and fruit make 34% of the diet, flowers make 7%, buds and bark and caterpillar.  While mature leaves made 42% of the diet, young leaves formed 11% of the diet. Observations in different parts of central and eastern Assam showed that its diet consisted of 63.2% leaves (including petioles), 12.3% flowers, 11.1% bamboo shoots, 8.6% seeds, and 4.3% fruits (Choudhury 2012). Important fruit trees include Ficus benghalensis, Ficus religiosa, Spondias mangifera and Terminalia bellerica. Two species preferred for seeds are Mallotus philippensis and Litsea polyantha. According to Stanford (1991) and Choudhury (2012), the species prefers many fruiting trees.

Trachypithecus pileatus tenebricus spent 67% of its time eating mature leaves, 57% young leaves, 6% mature leaves, and 4% petioles, fruits/seeds were 16% and flowers and buds 16%. Important plant species in their diet are Gmelina arborea for leaves, Albizia lucida for leaves and fruits, Ficus glomerata for leaves, Mikania micrantha for leaves, Morus laevigata for leaves. In Pakke Wildlife Sanctuary and Hollongapar Gibbon Sanctuary they were seen occasionally feeding on water lilies.

Trachypithecus pileatus pileatus was found mostly in tea gardens close to forests and less often in areas that lack forest proximity or here there is abundance of other primate species such as gibbons and Phayre’s Langurs. Preferred food trees include, Gmelina arborea, Dalbergia sisso and Albizia lebbeck (Mazumdar et al. 2014).

Breeding occurs during dry season; September-January, April-May in case of T. p. tenebricus. Gestation of 200 days (195-205 days). Birthing from late December. Interbirth interval of ca 2 years. Allomothering restricted to parous females-a feat of ‘altruism’ that offers mothers time to feed while the aunt takes care of the infant.

The species is diurnal and arboreal. Occasionally, they use the ground for social activities such as playing. Feeding constitutes 35% of the total time budget while resting 40% but there is seasonal variation in this proportion. Sometimes feeding can be as low as 26%. Grooming was observed as a predominant social activity in Pakke Wildlife Sanctuary, Arunachal Pradesh where 56% of their grooming time was spent in social grooming and 46% for auto grooming (Kumar and Solanki 2014).

They live in unimale-multifemale groups, with home ranges of 21-64 ha. Daily movements of 320–800 m have been recorded. Golden Jackal (Canis aureus), Clouded leopard (Neofelis nebulosa) and Leopard (Panthera pardus) are natural predators.

Populations of this species face a range of threats particular to their geography, with most stemming from human alteration of habitat. Some major causes are jhum cultivation, monoculture plantations, timber and firewood harvests, and other development, resulting in the loss of fruiting and lodging trees. Also, these animals are subject to trade for their meat and other body parts, and as pets (Molur et al. 2003).

Encroachment, slash and burn shifting cultivation and fragmentation are the other threats. Hunting for meat is of serious concern in some parts of Assam, central and eastern Arunachal Pradesh, hill districts of Manipur, Nagaland and Mizoram. They are also hunted their pelts which is used to make dao sheaths. Unsustainable bamboo harvest in Jagiroad, Panchgram and Jogighopa in Assam and Tuli in Nagaland. Petroleum, open-cast coal mining and limestone mining, poor law enforcements, fragmented population susceptible to local extinction. Mijikajan tea estate (Sonitpur district), Hatipoti village (Tinsukia district), Diroi and Sola reserved forests (both in Sivasagar district) are such isolated fragments where the capped langur could go extinct soon. Border skirmishes with Nagaland have already extirpated the langur from Nambhur, Dhipu, and Rengma reserve forests in Golaghat (Choudhury 2014).

Large scale habitat destruction and  encroachment for human settlement and agricultural activities have been recorded and going on in the north bank of Brahmaputra (Das and Chetry pers. comm.). The population estimates in Assam seems to have been halved from 39,000 in 1989 (Choudhury 1989) to 18,600 in the 2008-2014 estimate due to habitat loss, fragmentation and degradation due to developmental activities, plantations and settlements (Choudhury 2014). It can not adapt like Rhesus and hence, habitat loss has impacted it severely in areas such as Sonitpur, upper Assam and Barak valley districts.

Subject to trade for their meat and other body parts, and as pets.

All South Asian populations are listed under Schedule I, Part I of the Indian Wildlife (Protection) Act (Srivastava and Mohnot 2001), amended up to 2002, and are also listed under Appendix I in CITES. Populations in Bangladesh are protected under Schedule III of the Bangladesh Wildlife (Preservation) (Amendment) Act, 1974. It is considered First Class protected under the Wildlife Protection Act in China. In Myanmar it is nominally protected under the Wildlife Protection Act. This species is found in a large number of protected areas across its range.