- Primatologists are finding increased evidence of mixed species associations and hybridisation between Nilgiri and Hanuman langurs in the Western Ghats.
- While hybridisation is yet to be ascertained through genetic studies, the distinct morphological characters are clear indicators, reveal a new study.
- The absence of the establishment of a new species despite long-standing hybridisation as well as the probable loss of the original genetic pool of Nilgiri langurs are concerns.
Visitors to Anamalai Tiger Reserve could be in for an unexpected treat if the most recent study by primatologists in the Western Ghats is any indicator. Not only will they get to see the recognised langur species such as the Nilgiri langur and the Hanuman langur, but they may also spot langurs with variations in colours and shapes. The study points to the likelihood of hybrid individuals resembling a combination of Nilgiri langurs (Semnopithecus johnii) and tufted grey langurs (Semnopithecus priam, also called Hanuman langurs) in certain sections of the protected area. The authors believe that this phenomenon is the outcome of the intermingling of these two langur species observed in the region for a considerable period of time.
Hybridisation between closely related species is not an unusual occurrence in the wild though it is relatively rare among mammals, says primatologist Mewa Singh, a distinguished professor at the University of Mysore, who co-authored the study. He says that there are visible signs of hybrids in mixed species groups that the Nilgiri langurs and tufted grey langurs are forming. However, the concern, according to him, arises from the absence of signs indicating the establishment of a new species, which should have occurred by now given that the hybridisation has been going on for a while. This, he elaborates, casts doubts on the viability or the reproductive capability of the hybrids. He notes that this remains largely a probability until a detailed genetic study establishes it.
Multiple shades of brown
A mixed-species group is defined as “a group of independently moving animals from more than one species found in close proximity, which interact with one another”, according to the paper. Various factors contribute to such organising including anti predation, finding a mate, increased movement efficiency and anthropogenic factors, to name a few.
In the present study conducted to explore the physical characteristics and social organisation within mixed-species assemblages of Nilgiri langurs and tufted grey langurs discovered at elevations ranging from 300 to 800 metres in the Anamalai Tiger Reserve, a significant number of mixed morphotypes were identified within the population. A morphotype refers to any of a group of different types of individuals of the same species in a population. The research paper indicates that while the dark grey langurs were observed at lower elevations, lighter brown morphs or variants were observed at mid-level elevations, and darker brown morphs were more prevalent at higher elevations.
It’s crucial to note the distinct habitat preferences of Hanuman langurs and Nilgiri langurs. Hanuman langurs are typically found at lower elevations, inhabiting the eastern slopes of hills characterised by dry deciduous and scrub forests. In contrast, Nilgiri langurs prefer the relatively wetter regions situated on ridges, valleys, and the western slopes. However, the mixed-species associations of these two species occur within the transitional zone between these disparate habitats.
The group sizes of mixed species were found to be larger than those of both Nilgiri and Hanuman langurs, comprising many adult males and females in these groups. This is another significant difference from the Nilgiri langur groups as they almost never form multi-male groups. “Nilgiri langurs generally organise into single-male, multi-female groups, with approximately 60 percent of the group being adults, and the rest consisting of sub-adults and juveniles. Multi-male groups are not observed,” highlights principal scientist at Salim Ali Centre for Ornithology and Natural History, Honnavalli N. Kumara who was a part of the study.
It is not uncommon to find mixed species organisations among monkeys. Singh points to similar observations made from parts of Bhutan and northeast India. Naturalist specialised in the wild fauna of northeast India, Anwaruddin Choudhury recalls seeing hybrid langurs during his visit to south-central Bhutan in a paper published in 2008. These “natural” hybrids between capped langurs (Trachypithecus pileatus) and golden langurs (Trachypithecus geei) were spotted in the dzongkhag (district) of Zhemgang. He writes: “The hybrid langurs observed near the town of Zhemgang resemble golden langurs from a distance, but the various shades of grey become evident on closer inspection. There is no uniform pattern. I have seen in the same group, what would seem to be an almost pure golden langur phenotype, one with a grey back, another with grey flanks, and another looking like a capped langur but with lighter grey.”
Similarities, habitat proximity as reasons
Kumara suggests that the similar genus and comparable size of the two langur species likely eased their mingling in the case of hybrids of Anamalai. Additionally, the overlapping of their habitats is cited as a contributing factor. Previous research has highlighted that minimal competition for food resources further encourages mixed species associations. Both langur species primarily consume leaves, with Nilgiri langurs exhibiting a greater preference for foliage compared to Hanuman langurs, as outlined in the paper.
Kumara emphasises that the study has crucial implications on the conservation of Nilgiri langurs whose population is on the decline. Listed as “vulnerable” on the IUCN Red List of Threatened Species, forest fragmentation, anthropogenic interventions, poaching, etc. are listed as some of the threats to this species restricted to the Western Ghats.
In a previous paper published in 2023, Singh, Kumara and team assess the distribution, social organisation and management of Nilgiri langurs to find five significant populations in five different regions of the Western Ghats — Nilgiri-Brahmagiri, Siruvani, Anamalai- Parambikulam, Cardamom Hills, and Periyar-Agasthyamalai — each with many fragmented sub-populations. The paper points out that the geographical area of distribution of these five populations is 9976 km2 out of which the suitable habitat for the species is about 7718 km2 including evergreen forests, montane forests, moist deciduous forests, dry deciduous forests, as well as forest plantations.
Kumara says that there is a misconception that Nilgiri langur is widespread and common which is far from the truth and that if hybridisation is occurring, there is an urgent need to take measures to preserve the original genetic pool of the species. Genetic research is underway to establish hybridisation and the researchers are hopeful that the original species could be conserved through concerted efforts of all stakeholders.
Read more: Golden langurs threatened by lack of suitable habitat
Banner image: Mixed morphotypes of Nilgiri and Hanuman langurs. Genetic studies are underway to establish hybridisation among the two species. Photo by Santanu Mahato.