Of all living tigers, only the Sumatran tiger is sufficiently different to be classed as a separate subspecies, scientists argue in a new paper.

Of all living tigers, only the Sumatran tiger is sufficiently different to be classed as a separate subspecies, scientists argue in a new paper.

Juniors Bildarchiv GmbH/Alamy

Controversial study claims there are only two types of tiger

Tiger numbers have dwindled worldwide, but tiger types may also be about to take a big hit. A controversial new study suggests that instead of nine subspecies of tiger, there are only two. The find could have a dramatic impact on tiger conservation, though not everyone agrees with the study’s conclusions.

The nearly 4000 tigers that remain in the wild are usually classed into six subspecies: the Siberian tiger, the Bengal tiger, the South China tiger, the Sumatran tiger, the Indochinese tiger, and the Malayan tiger. Three other subspecies are listed as extinct: the Bali, Caspian, and Javan tigers.

Scientists from the Leibniz Institute for Zoo and Wildlife Research in Berlin investigated the differences between these subspecies by comparing skull measurements, fur pattern, ecology, and genetics. They used data that have already been published as well as collecting new data on several museum specimens of extinct subspecies. Combining the different traits, they found little evidence to reliably differentiate the nine subspecies.

Instead, they report today in Science Advances, they propose just two subspecies: Sunda tigers, made up of Sumatran tigers plus the extinct Javan and Bali tigers, and continental tigers, encompassing all the rest. Genetically, there were differences between the subspecies to be found, says Andreas Wilting, one of the authors of the paper. “But if we looked at all the traits together, we could only reliably distinguish two subspecies of tigers.”

The paper will surely cause a stir, says Urs Breitenmoser, a zoologist at the University of Bern, who was not involved in the study. “But I find the work quite convincing and in keeping with other findings in recent years,” like a paper that suggested the Caspian tiger and the Siberian tiger were the same subspecies, he says.

Breitenmoser is co-chair of the cat specialist group of the International Union for Conservation of Nature, the organization that draws up the red list of threatened species. Two years ago, the cat specialist group asked a task force to update the taxonomy of all wild cats. Results are expected by the end of this year. “They are going to look at this new proposal as well,” Breitenmoser says.

Still, critics are pouncing. Collapsing the three Sunda subspecies into a single one may be reasonable, says Stephen O’Brien, a geneticist at the Theodosius Dobzhansky Center for Genome Bioinformatics in St. Petersburg, Russia, who generated some of the genetic data used in the paper. But the continental tiger shows enough differences genetically to be considered six separate subspecies.

Part of the problem is that tigers have had little time to evolve separate subspecies. Fossils suggest that the animals roamed across large parts of Asia 2 million years ago, but then something catastrophic happened. Genetic analysis suggests that about 70,000 years ago most of the animals were killed, probably when Toba, a supersized volcano on Sumatra, erupted. Probably just one small population survived, and all the variation seen today evolved in the last 70,000 years.

That is enough time for separate subspecies to be distinguishable genetically, but not morphologically, argues Shu-Jin Luo, a geneticist at Peking University in Beijing who works on endangered species. “Genetic data is much more reliable and objective than morphology,” she says. The nine subspecies can be distinguished genetically and that should be enough, she argues. That’s why she is skeptical of the new study, which also relied on anatomy and ecology.

If the new classification is adopted, it would spell some major changes for efforts to save the tiger. “The good thing is that it will make conservation easier”, says Volker Homes, a conservation specialist at the Worldwide Fund for Nature in Germany. For instance, Indian tigers, of which there may be up to 2000, could be used to bolster the population of South Chinese tigers, which are probably extinct in the wild, he says. Also, thousands of tigers born in zoos to parents of various subspecies would suddenly be eligible for breeding and rewilding programs.

But there may be negative consequences as well, Homes warns. Many countries are proud of hosting a unique tiger and classing several of these into one subspecies may translate into less effort to save them. “There is a danger that some countries don’t feel as responsible for protecting the tiger anymore, if it is not ‘their’ unique tiger.”