Explained: Why black tigers in Odisha sound warning?

A team of scientists have come to a conclusion over the genetic mystery of Simlipal’s black tigers. A research led by Uma Ramakrishnan and her student Vinay Sagar from the National Centre for Biological Sciences (NCBS), Bangalore, found that a single genetic mutation in these tigers caused black stripes to broaden or spread into the tawny background.

Tigers have a distinctive dark stripe pattern on a light background of white or golden. A rare pattern variant, distinguished by stripes that are broadened and fused together, is also observed in both wild and captive populations.

According to reports, this is known as pseudo-melanism, which is different from true melanism, a condition characterised by unusually high deposition of melanin, a dark pigment.

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While truly melanistic tigers are yet to be recorded, pseudo-melanistic ones have been camera-trapped repeatedly, and only, in Simlipal, a 2,750-km tiger reserve in Odisha, since 2007.

The study was the first attempt to investigate the genetic basis for this unusual phenotype (appearance) in 2017. Through whole-genome data and pedigree-based association analyses from zoo tigers, the study found that pseudo-melanism is linked to a single mutation in Transmembrane Aminopeptidase Q (Taqpep). This is a gene responsible for similar traits in other cat species.

Why are black tigers rare in the wild?

In nature, mutants are genetic variations which may occur spontaneously, but not frequently. Black tiger sightings have been claimed sporadically at least since 1773 when artist James Forbes painted a watercolour of one shot in Kerala.

However, similar claims were made from Myanmar and China, in 193 and 1950 respectively. In 1993, a confiscated black tiger skin of unknown origin was displayed at Delhi’s National Museum of Natural History.

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Under exceptional circumstances, a black tiger may succeed as part of a very small founding population that is forced to inbreed in isolation for generations. As it turned out, that is what happened at Simlipal.

The Simlipal mutants

Simlipal furnished the first confirmed record of the mutant in 1993 when a tribal youth killed a pseudo-melanistic tigress in self-defence. In 2018, three of Simlipal’s eight tigers turned out black.

The closest breeding tiger population to Simlipal is around 800 km away, a distance much longer than the average home range of Bengal tigers and their average dispersal distance.

While the present study did not have enough data to test the hypothesis, it observed that the disappearance of black tigers from across India backed the possible deleterious effects of the mutation.

According to reports, India’s northwestern tiger population shows higher mean relatedness between individuals (46%) and lower heterozygosity (22%) than even Simlipal (38% and 28%).

Speaking to a news agency, Dr Ramakrishnan said that it was amazing that they could find the genetic basis for such a striking pattern phenotype in wild tigers, and even more interesting that this genetic variant is at high frequencies in Simlipal.