doi:10.1038/nindia.2013.90 Published online 8 July 2013
Geneticists quantifying the extent to which tiger populations have been decimated have found that only seven per cent of genetic variation of the big cats remains today as they have been completely wiped out in some regions.
Comparing the genetic makeup of museum and wild tiger samples, they also found that since tiger habitats are not connected with each other anymore, tiger populations are genetically very different — almost twice as much as they were pre-1950.
Samrat Mondol and Uma Ramakrishnan, who have been studying genetic signatures in tigers by analyzing their DNA at the National Centre for Biological Sciences in Bangalore, set out to see how tigers were faring in their final frontier.
Since genetic material is passed from parent to offspring, any unique DNA of an organism is lost forever if it dies without reproducing. Also, more variation in the DNA means the species is well equipped to deal with different hardships, like a sudden disease outbreak.
To take a look at the genetic variation from a century ago, the team used museum samples of tigers collected before 1950, from the Natural History Museums in London and Edinburgh.
Following their observations, they have formed broad groups — peninsular India (Western Ghats and Deccan), central India and the Terai region — putting tigers of similar genetic makeup together. This can help formulate better management plans to protect whatever genetic variation remains.
Even as recently as the 1930s, 40,000 tigers roamed the forests of India. By the 1970s, tiger number had plummeted to less than 2000. Historically, the tiger ranged from the Caspian Sea to the Russian Far North to the Indonesian islands of Java and Sumatra. Now, they occupy just 7% of this historical range with India home to over half of the tigers remaining in the wild.
Mondol and Ramakrishnan teamed up with Michael Bruford from Cardiff University in the UK. Bruford's lab specialises in extracting genetic signatures from old, preserved animal samples. Using tiny bits of skin and bone from tiger specimens at the Natural History Museum in London and the National Museum of Scotland in Edinburgh, they obtained a genetic profile of tigers hunted from before 1950. A lot of these museum specimens are from areas where tigers are not found anymore — Afghanistan, Rajputana and Gujarat.
The oldest sample they used was a skull from 1836. The authors compared this historical genetic profile with genetic structure in current tiger populations. They looked at two different kinds of genetic material, one inherited from only the mother (DNA from the mitochondria, found outside the nucleus of a cell) and the other, from both parents (DNA from inside the nucleus). Looking at both kinds of DNA gives an interesting, highly informative picture — one can compare how much females are moving, from the mitochondrail DNA, with how much males are moving, from the nuclear DNA.
The results show that tigers have lost about 93% of the maternally inherited mitochondrial DNA. This sharp drop is also because of the behaviour of tigers — females tend to setup territories next to their mothers, or even inherit the mother's territory. Females also hold relatively smaller territories (about 20 sq km). Males move away from where they are born and establish new territories, usually much larger than that of females (60 to 100 sq km). Wiping out tigers from one area means, a lot of related females die out, and their genetic material is lost forever.
After creating a kind of family tree using tiger samples from different areas, the authors could see that the tiger museum samples formed two populations, one with samples from northeastern India and peninsular India (Western Ghats and Deccan Plateau), and the second with samples from the Terai region and the semi-arid regions in Rajasthan (see map). This means, tigers from the peninsula were moving between the south and the northeast, but the Terai and semi-arid regions were distinct.
When analyzing the current samples, the peninsular India-northeastern population remained the same. But the semi-arid and Terai tigers form two populations; what was one population in the early twentieth century, is now two.
"This can be because of the huge amount of development, and resulting human presence in Punjab and Haryana," Ramakrishnan says. Such a sub-division of a population into two over a relatively short time period is not usually observed in mammals like the tiger that can move large distances. This means, habitat fragmentation is greatly affecting tiger movement.
The fact that populations from northeastern India and peninsular India are still connected comes as a pleasant surprise. Ramakrishnan feels that the remnant forests along eastern India have allowed tigers from peninsular India to move to the Northeast and vice versa. "However, we need more samples from eastern and central India to know more about how much these populations are connected," she added.
An earlier study by the Wildlife Institute of India, and Ralph Tiedmann from the University of Potsdam in Germany showed differences between tiger populations in the two regions. Tiedmann said, "This connectivity is shown in nuclear DNA, but not in mitochondrial DNA. This can indicate that males are moving around and connecting peninsular India with northeastern India, and females are staying close to their mothers, as usually observed in tigers". The samples that made it to the museums are the ones shot by the British.
"North India includes places where large numbers of tigers could be shot, even 120 in 3 months in the Terai as shown by some counts," says environmental historian Mahesh Rangarajan. "Also, few tigers shot by princes ended up in museums overseas," he says.