Animal Hybridization, Signs of Dying Forests

Visitors to the Anamalai Tiger Reserve in Tamil Nadu might notice something unusual in the forest canopy. Among the glossy black Nilgiri langurs and the more familiar grey Hanuman langurs, there are animals shaded in brown, gold, and smoky grey — langurs that don’t match the field guides. Researchers say these unusual morphs are likely hybrid individuals, the physical outcome of two species increasingly pushed into a shared and shrinking habitat.

Hybridization among mammals is uncommon. The Western Ghats, one of the world’s biodiversity hotspots, is experiencing a level of ecological stress that is erasing natural boundaries, and langurs appear to be among the first primates showing visible signs of this unravelling.

Forest Glum

According to B.K. Singh, retired Principal Chief Conservator of Forests for Karnataka, the last decade has fundamentally altered the landscape langurs depend on. “Langur habitat has undergone a big change in the past decade,” he says.

“Nearly 2,500 sq.km. forests of Tamil Nadu, Karnataka, and Kerala, known as the Nilgiri Biosphere Re-serve, provide habitats for tigers, elephants and other wildlife species, including langurs.

These forests have been subjected to heavy anthropogenic pressures like human

and livestock movements, illegal harvest of wood, encroachments, NTFP, and wildfires.”

Struggle For Food

The collapse is also botanical. “Fruit-bearing trees important for langurs are either damaged or degraded or have been suppressed by the invasion of invasive species like euphonium or lantana or Cassia spectabilis or wattles,” Singh says. “One of the common sites of langurs on Amla trees and spotted deer and sambhars under the same trees, all enjoying the fruits, is now missing.”

Without food trees anchoring territories, langurs move more widely and into one another. In other words, two species have been pushed into one space. For decades, Nilgiri langurs have inhabited the wetter western slopes and valleys of the Ghats, while Hanuman langurs occupy lower, drier deciduous forests. Between them lies a transitional band, where forest types shift with rainfall and altitude. That band is now the centre of scientific attention.

“Habitat overlap between the Nilgiri and Hanuman langurs is definitely accelerating owing to large-scale habitat loss and fragmentation, leading to some cases of hybridization too,” Singh says.

This aligns with what Honnavalli N. Kumara, Principal Scientist at the Salim Ali Centre for Ornithology and Natural History, observes in the field. “Grey langurs have been divided into many species and subspecies,” he explains. “Most of the species in the forest area are doing well. However, in the plains or outside the protected area, their habitat has been affected.”

The consequences ripple outward. Tree lines around agricultural fields that were once crucial corridors have been cut. “In most of the places they have cut, thinking that langurs damaged crops,” Kumara says. “So that idea that they should not be there has affected their habitat.”

Hunting has compounded the loss. “In some parts of Kerala and outside protected areas, you don’t see anything because they’re finished, everything is consumed,” he says.

With fewer intact forests remaining, especially in the Deccan Plateau and central India, langurs are pushed into villages, plantations and roadside fringes. “Once there is less habitat and more conflict, that’s what is happening,” Kumara says. “Earlier, only macaques used to be in conflict, but now even langurs are in conflict in many parts of the country.”

Signs Of Hybridisation

When langurs converge in the same forest fragments, hybridisation becomes possible. Kumara’s field observations suggest this is now widespread not only in Anamalai, but in parts of the Northeast and even Sri Lanka.

“Hybridisation, we presume, is a natural phenomenon,” he says. “Wherever different species are there, since they belong to the same genus level, obviously different types of morphos are coming out.”

In Anamalai’s transition slopes, researchers have documented a clear elevational pattern: at the highest reaches, Nilgiri langurs dominate but show slight traces of Hanuman features; mid-elevations host the largest number of mixed morphs; lower areas show the opposite, with grey langur traits prevailing. “That way the beautiful pattern has emerged,” Kumara says. “It is the obvious reason that it is basically played by the forest type.”

He notes that similar hybridisation zones exist between capped langurs and golden langurs on the Bhutan–Assam border, and between grey and purple-faced langurs in Sri Lanka.

Points To Ponder

Despite the visible signs, Kumara cautions against panic. Hybridisation itself is not necessarily a conservation emergency. “If there are 40% or more than 40% hybrids, this is not a recent phenomenon. It may have been happening for over a century,” he says. “Right now, we don't need to really bother about it. I consider it a natural phenomenon.”

Nevertheless, he stresses the need for ongoing genetic work. “Proper analysis has to be done. Understanding the entire process and documenting it will help in the future to decide if the hybridisation goes very fast and if something emerges.”

Changing Landscape

In the Nilgiris and elsewhere, habitat change is driven not only by fragmentation but by the historical spread of monoculture plantations. R.P. Singh, a senior forest officer in Satpura and the Ghats, explains the ecological consequences.

He describes langurs as “very versatile in ecology,” pointing out their roles in seed dispersal, feeding ungulates and bears by dropping fruit, and warning other animals of predators. But these roles depend on diverse forests, not stands of alien trees. The rapid spread of eucalyptus, wattle, silver oak, and other exotics has uprooted native ecosystems. “Spreading of urbanisation and cutting of trees, plantation of monoculture, exotics… are spreading fast in the lower side of hills,” he says. “This is all done at the cost of the destruction of natural flora.”

Even where forests remain, roads are taking a toll. “Langurs getting killed in road accidents are quite common,” B.K. Singh notes. “Although night traffic is regulated, the increasing use of road infrastructure has been killing the animals.”

The Future

Legal protections exist, but Singh says they’re not enough: “The management must connect with the communities for education and awareness.” Hybrid langurs may be the most visible sign of a forest under pressure, but the deeper crisis is ecological fragmentation: vanishing food trees, invasive plantations, loss of corridors, and human-dominated edges forcing species together.

As Kumara puts it, the hybrid zone of Anamalai is simply where the landscape now gives wildlife no other choice.

At A Glance

Category Hanuman Langur Nilgiri Langur

Range North, Central & West India; Western Ghats

Nepal, Bhutan, Bangladesh, (Kerala, Tamil Nadu, Pakistan Karnataka)

Habitat Dry/moist deciduous forests, Evergreen & forests;

scrub, grassland, semi-evergreen

urban–temple areas shola–grassland mosaics

Appearance Grey/light brown coat; Black body; golden/

black face; long tail reddish mane; long tail

Group Size 10–60 10–20

Diet Leaves, fruits, Young leaves, shoots,

flowers, seeds fruits, flowers

Threats Fragmentation, electrocution, Habitat loss, fragmentation,

road kills, urban conflict historical poaching