Capped langurs live in groups that consist of a dominant male, several females, and a handful of juveniles and infants. The average troop size is around 9 to 11 individuals, and the ratio of males to females in a troop is typically 1 male for every 7 females. Once mature, if two or more males live together in a troop, they will eventually compete for dominance through aggressive displays. Comparatively, females care for their troop’s infants and juveniles and instead develop close bonds with one another.

In the morning, troop members wake early but remain at their sleeping sites until the sun has fully risen. Once risen, they move to higher branches to bask in the early-morning sun and heat, before eventually venturing off to forage and feed. After splitting up into smaller sub-groups, when foraging, langurs may spread out over wide distances to maximize their feeding potential. To avoid getting lost, they use contact calls to communicate to one another and share locations. 

Because the home ranges of capped langurs overlap with those of other primate species, it is not uncommon to find capped langurs interacting with other primates whilst foraging. Examples of such primates include Hoolock gibbons (Hoolock), Nepal gray langurs (Semnopithecus schistaceus), or Phayre’s langurs (Trachypithecus phayrei). 

Although many of these interactions result in a sympatric relationship, in which different species may occupy the same geographic area without interbreeding, some interactions do not have positive outcomes. Male langurs are generally considered territorial and may produce aggressive displays when threatened, especially when they feel that their home range is in danger. For example, when two or more males encounter each other, they may leap towards intruders or rapidly jump from branch to branch to cause disruption and disturbance. 

Unfortunately, there are other problems between langur species, particularly when it comes to species interbreeding. For example, in Bangladesh, a recent study revealed instances in which capped langurs and Phayre’s langurs bred and hybridized. 

In biology, hybridization refers to the interbreeding of two genetically distinct individuals that results in an offspring containing traits from both parents. Interspecific hybridization refers to the interbreeding of two different species. For many wild species, hybridization is dangerous because it may lead to a loss of an individual species’ unique genetic traits. When small, isolated populations breed and hybridize, gene flow, or the movement of genes from one population to another, becomes limited. When gene flow is impeded in this way, individuals are likely to interbreed and form a new species through a process known as speciation. Increased speciation can lead to the extinction of one or more parent species, causing all involved to be vulnerable. 

Unfortunately, there is no specific, universally recognized name for the offspring of Phayre’s and capped langurs. Although mixed species groups of capped langurs and Phayre’s langurs have been observed successfully interacting with their hybrid, the interaction will not be observable for years to come. Hybridized individuals are usually born infertile, which means they cannot produce viable offspring, so each generation of hybrids will probably only exist for around 12 years. Unfortunately, in Bangladesh specifically, national conservation laws only protect pure langurs, not hybrids, which puts the hybrid species at an even greater disadvantage.