The reason ‘black tigers’ exist and why that is dangerous

Claims of Black tiger sightings have been made since at least 1773 when James Forbes, an artist, made a watercolour painting of one in Kerala. (IE)

An expert team, led by National Centre for Biological Sciences’ Uma Ramakrishnan and her student Vinay Sagar, has uncovered the genetic mystery of black tigers in Odisha’s Simlipal. The study highlighted a single genetic mutation caused the black stripes to broaden or spread.

Tigers bear a distinctive dark stripe on a light background — either golden or white. A rare variation in pattern, distinguished by broad stripes fused together, has also been observed among both captive and wild tiger populations. This pseudo-melanism, unlike true melanism, is characterised by unusually high melanin deposition.

While scientists are yet to record truly melanistic tigers, pseudo-melanistic ones have been repeatedly trapped on camera. However, all these sightings have been in Odisha’s Simlipal, a 2,750-km tiger reserve, since 2007.

The study, launched in 2017, was the first one to probe into the genetic background for this unusual appearance. The scientists used pedigree-based association analyses and whole-genome data from zoo tigers to find that pseudo-melanism is linked to only one mutation in Transmembrane Aminopeptidase Q (Taqpep). This is the same gene that is responsible for similar traits in other cat species.

Black tigers
The genetic variation that causes the mutation occurs in nature spontaneously, albeit infrequently. Claims of Black tiger sightings have been made since at least 1773 when James Forbes, an artist, made a watercolour painting of one in Kerala. There were similar claims from Myanmar in 1913 and China in the 1950s. A confiscated black tiger skin was put on display at the National Museum of Natural History in Delhi in 1993.

Unfortunately, novelty trophy hunters sought out these unusual-looking tigers until recently, with only few surviving long enough to establish bloodlines.

Pseudo-melanism is caused by a hidden or recessive gene. A cub gets from both parents two copies of each gene — the recessive gene manifests itself only if the dominant one is absent. This means two tigers with normal patterns carrying the recessive gene will have to breed for a one-in-four probability of birthing a black cub.

Recessive genes are, however, rare. As a result, it is unlikely that two unrelated tigers will carry the same gene and then pass it on to a cub together.

A black tiger may succeed in a small founding population forced for generations to inbreed in isolation, offering the recessive gene a far higher chance to show up. This is what happened at Simlipal.

The mutants of Simlipal
Much before cameras caught three black tigers in 2007, Simlipal was the source of the first confirmed mutant in 1993 when a tribal youth killed in self-defence a pseudo-melanistic tigress. Three of Simlipal’s eight tigers were black in 2018.

Three Indian zoos house pseudo-melanistic tigers — Nandankanan in Bhubaneswar, Arignar Anna Zoological Park in Chennai, and Ranchi’s Bhagwan Birsa Biological Park. All these tigers were born in captivity and have ancestral links to a Simlipal tiger.

Simlipal’s closest breeding tiger population is around 800 km away. The Bengal tiger’s average home range is 20-110 km, while their average dispersal distance is around 78-124 km. While there is documentary evidence of dispersals longer than 500 km, they are very rare, the study noted.

According to previous studies, Indian tigers have three major genetic clusters — South India, Northwest India, and Central India. The latest found that the Simlipal cluster is genetically distinct from other central Indian populations.

This means Simlipal’s isolated population in-bred. The genetic diversity loss is evident from the population’s low heterozygosity or chances of inheriting different forms of one gene from each parent. Simlipal’s heterozygosity is 28 per cent against 36 per cent in Central India. Simlipal individuals also have a mean relatedness of 38 per cent, compared to 9 per cent for Central and 13 per cent for South India.

Natural selection
Natural selection passes on the more successful traits from the gene pool, while eliminating the weakest one. The study said niche modelling showed lower melanistic leopard frequency in drier open habitats than darker tropical and subtropical forests. Similarly, darker coats may lead to selective advantage in hunting and avoiding hunters in Simlipal’s moist tropical moist and closed-canopy forest.

Takeaway
Simlipal is not the sole example of what the study described as “intense founding bottlenecks” among tiger populations endangered by human-induced habitat fragmentation. While the dangers are isolated, the depleting population manifests itself dramatically in phenotypic evolution.

Ramakrishnan told The Indian Express that while tiger populations have recovered in parts of India, there still are several small and isolated populations. She predicted that such populations would undergo genetic drift, inbreeding as well as inbreeding depression or decreased survival. Overall, she said, such populations have a higher chance of going extinct.

The introduction of fresh genes in the isolated pool can, over time, reverse the damage. While airlifting tigers is seen as a solution, in the long-term, there is still no alternative to maintaining or restoring natural connectivity between tiger forests.