What are melanistic tigers? | Explained

Odisha has moved a tigress from Maharashtra to its Similipal to address the increasing number of pseudo-melanistic tigers

The story so far: A tigress from the Tadoba-Andhari Tiger Reserve, Maharashtra, was sent 800 km away to the Similipal Tiger Reserve on Sunday (October 28, 2024) so as to diversify the genetic pool of the tiger population in Odisha. She is part of a plan by the Odisha government to tackle inbreeding in the tiger population that has resulted in increasing numbers of the once elusive black tigers or pseudo-melanistic tigers. 

Aged two years and seven months, the tigress was released into an enclosure where she is kept for observation. She will be later released in the South Division of Similipal. 

According to Susant Nanda, Principal Chief Conservator of Forest (Wildlife), the presence of so many melanistic tigers in Similipal is not a cause of concern. However, he stressed that the tigress will bring in different genetic matter which would remedy the inbreeding situation.

What are pseudo-melanistic tigers?

As far as appearances go, pseudo-melanistic (pseudo: false; melanistic: dark coloured) tigers appear to have a black coat with white and orange stripes occasionally peaking through. Their pelt looks almost the inverse of a normal tiger. 

Till the 1700s, these tigers were considered mythical. One of the first pieces of evidence of a black tiger was a watercolour painting by a British artist and writer named James Forbes.

Even two centuries later, there were only sightings recorded by locals and British hunters. This is until 1970, when a tigress at the Oklahoma City Zoo gave birth to a cub with bizarre patterns. The cub was killed soon after by the mother. In the 1990s, an unusual tiger pelt was seized from poachers in New Delhi which finally confirmed the existence of this rare tiger.

The most recent sighting of the black tiger was in 2017-18 in the Similipal Tiger Reserve. 

How do the tigers get their colours?

The pseudo-melanistic tiger is a colour variant of the Bengal tiger. Its strange coat is a result of a mutation in a particular gene. This genetic quirk cause their stripes to widen and spread across the orange or tawny coat and at times give the appearance of being completely black.

A comparison between tiger populations showed that apart from the Similipal population, the mutation of the gene is extremely rare or even non-existent. The only other black tigers that have been found beyond the Similipal population are in captivity—Nandankanan Zoological Park at Bhubaneswar and Arignar Anna Zoological Park at Chennai—whose ancestry could be traced back to the Similipal population. 

The mutated gene

A 2021 study by the National Centre for Biological Sciences (NCBS), Bengaluru, published in the Proceedings of the National Academy of Sciences, looked at the gene which gave the black tigers their colour. 

The team, led by Uma Ramakrishnan, found that the mutation of the Transmembrane Aminopeptidase Q or Taqpep gene was responsible change in colour. This gene gives other cat species their markings too. For instance, tabbies with darker or more blotchy stripes have this mutated gene. A King cheetah’s wider and darker spots are also because of the mutated gene. 

The anomalies in colour and patterns are caused due to a missense mutation in the Taqpep gene. This mutation changes the amino acid Cytosine (C) to Thymine (T) in position 1360 of the gene sequence.

The prevalence of this particular mutation is abnormally high in the Similipal Tiger Reserve population. According to the 2021 study, the frequency of the mutated gene occurred at 0.58 or a tiger born in this population had a 60% chance of carrying this mutated gene. 

Why are the mutations more common in Similipal?

A majority of the tigers found in Odisha are in the Similipal Tiger Reserve. The All Odisha Tiger Estimation (AOTE-2023-24) says a total of 30 tigers were found in Odisha’s forests of which 27 of them are in Similipal.

Of these 27, atleast 13 adult tigers (seven females and six males) were found to be pseudo-melanistic. No other wild habitat in the world has pseudo-melanistic tigers.

To find out why the concentration of pseudo-melanistic tigers is so high, researchers dug further. The genetic analysis of the tigers along with computer simulations showed that the reason for such high mutation was genetics and not adaptation as previously theorised. The Similipal tiger population is cut off from the other populations by a great distance and has been isolated for an extended period of time.

This has led to inbreeding in an already small founding population leading to higher chances of the mutated gene being passed down. The study also attributed the phenomenon of genetic drift which suggests that a mutation may appear in high frequency or die out all together depending on pure chance.

Published - November 01, 2024 06:00 am IST