A tale of two tigers

Carlos Driscoll, a biologist at the University of Oxford’s Wildlife Conservation Research Unit and the US National Cancer Institute’s Laboratory of Genomic Diversity, first became interested in Caspian tigers in childhood. “I remember the tigers being declared extinct, and thought it was so wrong that this could have happened.” Decades later, he wanted to resolve questions about the taxonomy and biogeography of the tigers, long-standing unknowns. He wondered: Why not look at the mitochondrial DNA from museum specimens? (mtDNA is the tool of evolutionary studies in part because it evolves several times faster than nuclear-coding loci.)

Researchers at museums in Russia, Kazakhstan, and Azerbaijan were willing collaborators. “They were happy to see work done on animals that were painstakingly collected and often underappreciated,” says Driscoll. Except for one Caspian tiger housed in the Moscow Zoo but originally taken from the wild in Iran, the 20 specimens came directly from places like the banks of the Ili River in Kazakhstan and the Piandji River in Tajikistan.

The scientists compared the Caspian tiger samples to all other existing tiger subspecies in a phylogeny that includes leopards and clouded leopards. They found that a major mtDNA haplotype of Caspian tigers and their close relatives, the still-living Siberian, or Amur, tigers (Panthera tigris altaica) of the Russian Far East, differs by only a single nucleotide. (PLoS ONE 4(1):e4125, 2009) This finding suggests the two subspecies are too genetically similar to be considered separate subspecies, the authors say. “The tigers are too closely related to be two subspecies,” Driscoll says. “They’re in fact one.” Pronouncing the Caspian tiger extinct, he believes, may have been premature. The paper “sets forth a noncontroversial and accepted conclusion based on mtDNA evidence,” says Ron Tilson, a biologist and director of conservation at the Minnesota Zoo in Minneapolis who has conducted extensive research on tigers. “The amount of data and number of samples analyzed for this study make the conclusions well supported,” agrees George Amato, director of the Sackler Institute for Comparative Genomics and Center for Conservation Genetics at the American Museum of Natural History in New York, via email.

Some 10,000 years ago, Caspian tigers used a trail between the Himalayan Plateau and the Mongolian Gobi desert—the Gansu corridor—to migrate from eastern China to the region around the Caspian Sea. Eventually they returned across northern Asia to eastern Russia, ultimately establishing a new population that came to be called Siberian tigers. Through prehistory, Caspian and Siberian tigers intermingled, then stopped within the last 200 years, likely a result of an increasing human presence in the region. Until that point, they shared geography, natural history, and a common genetic heritage.

The findings raise the possibility of repopulating a now tigerless Central Asia with Siberian tigers, according to Tilson. “The most important consequence of these results,” he says, “is that in the right habitat, the Caspian tiger’s former range is open to reintroduction with Siberian tigers.”