There are six living subspecies of tigers: Bengal, Amur, South China, Sumatran, Indochinese, and Malayan, according a study published today (October 25) in Current Biology. The distinctions could help efforts to conserve the world’s 4,000 remaining tigers, the authors say. 

For years, researchers have debated the number of tiger subspecies. In 2004, Shu-Jin Luo, now at the University of Peking, and colleagues proposed that there were six, based on an analysis of several molecular markers. Yet in 2015, other researchers claimed there were only two subspecies, using molecular, ecological, and morphological data. In the present study, which relied on whole-genome sequencing, Luo and colleagues confirm their original finding.

The number of recognized subspecies has implications for conservation efforts, and acknowledging only two would harm tigers, Luo tells The New York Times. “If you think that all tigers are genetically homogeneous, you might say...

To determine the number of extant tiger subspecies, the researchers performed whole-genome sequencing on 32 tigers. They found that the genomes fell into six distinct clusters, each of which showed evidence of a unique evolutionary history. For example, the genomes of the Sumatran tiger bear traces of natural selection in the ADH7 gene, which affects body size. The researchers suggest that this tiger responded to evolutionary pressure to become or stay small, in order to survive by eating the small prey animals available on the island. 

The authors recommend that each subspecies be protected as a separate entity at risk of extinction, Newsweek reports. The result would be more conservation efforts for tigers. Of the six subspecies, the South China tiger is already extinct in the wild and there are about 150 individuals in captivity. The Indochinese tiger is also thought to be on the brink of extinction in the wild.

“To preserve such genomic signatures is to preserve evolutionary uniqueness that tigers have accumulated over thousands of years,” Luo tells the Times. “We need to respect this uniqueness by maximizing our efforts for all tiger subspecies.”

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