Disconnected ties

Going by the size of Sunderban tigers, it was believed that they were a different subspecies, but it has now been established that they share their lineage with Central Indian tigers, writes Atula Gupta.

When mud and water is the land you tread on and the forest is nothing but a floating mass, even a mega predator like the tiger has to adapt, shrink in size and mould its behaviour, according to the habitat’s demand, in order to survive. It is for this reason that the Royal Bengal Tigers living in the Sunderban mangrove forests have become smaller in size, morphologically different from their counterparts living in mainlands.

It is also partly this that led many scientists to contemplate if Sunderban tigers were in fact a different subspecies and not just an isolated population of tigers of India. While few studies prove this theory to be wrong, a new study shows a remarkable new line of thought.

Scientists from the Wildlife Institute of India (WII) have found that despite the outer differences, at the genetic level, tigers prowling in reserves like Bandhavgarh of Madhya Pradesh and the Sunderban mangroves are exactly the same. It is like the real life version of the brothers shown on celluloid who separate at birth and adapt to their circumstances to survive.

The national animal of India might be facing dire conditions thanks to greedy human demands, urban development and habitat destruction, but it is still the regal creature that roams many varied habitats of the country. From the cool forests of Corbett in the foothills of the Himalayas to the rich and dense forests of the Western Ghats, the tiger’s home today is small fragmented pockets that each has distinct characters and climatic conditions.

The big debate

But nowhere is life more challenging than the biggest mangrove forests in the world — the Sunderbans, where land and water constantly change the dynamics of the environment. Yet, the apex predator survives. The leaner frame and lesser body mass of the Sunderban tigers makes them much more adept at moving around in the muddy terrain. It also makes them survive on lesser food, given the added difficulty to catch prey and the reduced size of the prey itself. In 2009, when US scientist Adam Barlow made a comprehensive study of the skull and body size of Sunderban tigers and compared it with other mainland tigers, he found the size difference interesting and presumed the tiger could be from a different line altogether, changing the known evolution history of tigers.

What triggered the curiosity of naturalists further was when in 2010, a Sunderban tiger that had accidently roamed out of the mangroves was captured and weighed, before being released back into the jungles. This male weighed a mere 98 kg — more than half of the average weight of 221 kg recorded of other adult tigers.

But one study by eminent scientists John Seidensticker, Sandeep Sharma and Hemendra Panwar negated this theory. It said while tigers populated Central India about 10,000 years ago, their population subdivision began only about 1,000 years ago and accelerated only 200 years ago owing to habitat fragmentation.

Sunderban tigers could not be a subspecies because for any animal population to be called a subspecies, it has to be genetically isolated from the rest of the population for at least 20,000-50,000 years, for example, the Sumatran and the Siberian tiger — two distinct subspecies of the tiger. Also, for an animal population to be declared a separate species, it has to remain isolated for a period of one million years or more.

But if the separation did take place, where did the common ancestors live? Now, through DNA analysis, scientist S P Goyal and researchers Sujeet Kumar Singh and Sudhanshu Mishra from WII have given the answer — Sunderban tigers share common ancestors with Central Indian tigers. The separation occurred between 300 and 1,000 years ago due to historical events, human pressure and land-use patterns.

“Our study has found that the gene pattern of the Sunderban tigers is identical to the big cat population of the Central India landscape, including states like Madhya Pradesh, Maharashtra, Chhattisgarh and parts of Andhra Pradesh,” said Goyal.

For the WII report titled ‘Tigers of Sunderbans Tiger Reserve: Is This Population a Separate Evolutionary Significant Unit’, the scientists used a method called DNA haplotyping and fragment analysis to study the genetic pattern of the tigers. Haplotypes are a set of closely linked genetic markers present on one chromosome which tend to be inherited together.

When the DNA Haplotypes of the Sunderban tigers were compared with that of tigers of Madhya Pradesh and Maharashtra, the pattern was found to be identical. This included tigers of MP parks, including Bandhavgarh, Kanha, Pench, and reserves like Tadoba and Nagzira in Maharashtra. Interestingly, they also found no genetic similarities between Sunderban tigers and tigers of Northern India like those roaming in Corbett reserve.

Human interference

The scientific findings open new avenues of research for biologists to further investigate the evolution and population separation of tigers. But what it also portrays is how human influence has drastically changed the habitat, homes and lives of even ferocious creatures like the tigers.

In historian Rajat Roy’s words, in 1756, when Siraj-Ud-Daulah recaptured the city of Kolkata from the British, today’s Salt Lake area used to be the main city and the Lower South Circular Road that’s now known as Chowringhee used to be the city’s southern border. “Beyond that were the forests of Sunderbans and there are beliefs that tigers were often sighted in those forests which now house busy localities like Tollygunge and Behala,” said Roy.

In the 2 million years that tigers have existed in this world, 300 to 1,000 years is a miniscule time frame. But in this short span, from a vast single homeland panning present day Maharashtra, Madhya Pradesh, Chhatisgarh, Jharkhand, Bihar and West Bengal, the tiger is now left with pockets of land in different states. And it is all man’s doing. Even as it is established that Eastern and Central Indian tigers have the same lineage, it is further proof of the way human hands have divided wildlife families and homes for their own selfish needs.