The skin and bones of long-dead tigers from the days of the British Raj have helped reveal how the latest threat to the endangered species is their own DNA.
Taking DNA samples from game hunters' trophies in the Natural History Museum and National Museum of Scotland’s collections, researchers compared these with samples from modern tigers. They found that genetic diversity among the remaining Indian tigers may be low enough to make the population unviable.
The tiger population has dropped precipitously from around 40,000 in 1850 to fewer than 2,000 today, and as their numbers and habitat have shrunk, so has their choice of mating partners. The findings will come as a blow to the Indian government, whose conservationists believe that several years of rising numbers suggest the species has been saved from extinction.
The study, conducted by Samrat Mondol and Uma Ramakrishnan from the National Centre for Biological Sciences in Bangalore and Professor Michael W. Bruford from Cardiff University, was published in the Proceedings of the Royal Society B.
Bruford said, “The results were staggering. We found 93% of the DNA genetic variants we measured in the historical tigers are not found in modern tigers. This is a much bigger fall than we expected.”
For most mammal species, a population of more than 1,000 is a viable one. But Bruford explained it was more appropriate to think of them as pockets of much fewer numbers, as the tigers were spread thinly across the sub-continent.
The researchers divided the tigers they studied into two broad genotypes – those that live in the uplands of Nepal, Tibet and northern India, and those that live in the lush lowands of the south.
“If you look at modern tigers now, you’d assume they fell into two main genotypes. But when we overlaid them with historical populations, we realised that wasn’t the case at all – it’s just that all the intermediate populations have been exterminated,” Bruford explained.
“Not only have tigers lost a huge amount of genetic variation since the days of the Raj, they have also been partitioned. And because they cannot disperse around their habitat as they would normally, they are losing their genetic viability as a population even faster.”
Hunting by the British and by Indian and Mughal princes alongside rapid loss of habitat from agriculture and urbanisation is to blame for the tiger’s decline. Adding to their woes was that the biggest animals were taken by game hunters as trophies – removing the DNA of evolutionary winners from the genepool.
In the short term, Bruford said, the threat posed by poachers is still more concerning, but the declining quality of tiger genes was a long-term problem. “This won’t go away until wild spaces are managed coherently, with corridors to link up the habitats,“ he said. "When the Indian government measures its conversation success it needs to rely on more than just numbers.”