Should tigers be classified into six subspecies?

By WILL EDMONDS | November 15, 2018



Genetic analysis of tigers shows there may be six distinct subspecies.

Tigers are not only the largest cat species in the world, but they are also a keystone species integral to the continued maintenance of the food chain. For decades, researchers have been warning governments around the world about the endangered status of tigers and encouraging more stringent protection on habitats and against poaching. With fewer than four thousand tigers remaining around the world, down from an estimated one hundred thousand in 1990, conservationists have been studying the best ways to keep the species alive. 

Interestingly, the actual number of tiger subspecies remaining has been a long-debated issue among the community of ecological scientists working on genomic speciation. 

A subspecies is one step below the species nomenclature. Typically, it indicates a group within a species that, despite an ability to interbreed with other subspecies, is phenotypically or genotypically different in some significant way. While some may wonder about the relevance of such an issue, the final determination will constitute a vital step in preventing a loss of genetic diversity in the tiger population. 

A paper published by leading Peking University genomic researcher Yue-Chen Liu in Current Biology supports the idea that the tiger species ought to be classified in six subspecies (or clades). 

This conclusion was reached after a thorough genome sequencing of 32 specimens. From the analysis, Liu found that it was possible to genetically differentiate the Bengal, Amur, South China, Sumatran, Indochinese and Caspian tigers from one another. Originally consisting of nine subspecies, the tiger species has lost three subspecies over the past 80 years, and this trend seems likely to continue if preventive steps are not taken. 

“In the end, we were quite amazed that, by performing a stepwise genome-wide scan, seven regions including 14 genes stood out as the potential regions subject for selection,” Shu-Jin Luo, a geneticist at Peking University and contributor to the paper, said in a press release. 

This was evidence that evolution through natural selection took place even across a relatively recent timespan, only about 10,000 years, when the common ancestor for all nine subspecies was living. 

Previously, scientists have argued for the species to be delineated into two, five or six subspecies. Such a decision will have long-term effects on how attempts at conservation unfold over the next several decades. 

In 2015, scientists from the Leibniz Institute in Germany concluded from data collected in skull measurements, fur pattern, ecology and mitochondrial DNA that there were only two distinct species remaining. 

In this study, however, the researchers separated Sumatran tigers from all other living species, which they classified into a group of “continental tigers.” Supporters of the six-subspecies hypothesis, like Luo, argued that there has only been “enough time for separate subspecies to be distinguishable genetically, but not morphologically” due to their relatively recent emergence. 

While scientists see the protection of the tiger species as their primary goal, it is also vital to “maintain the species’ evolutionary potential.” This requires identifying whether tigers belong to a particular subspecies and if so, ensuring that they only breed with other members of that subspecies under human-monitored conditions. 

As one might imagine, classifying tigers into only two subspecies would make this mission much easier, as conservationists would be able to breed far greater numbers/combinations of tigers than they could with five or six defined subspecies. 

Conservationist Volker Homes from the Worldwide Fund for Nature in Germany shared an example of how the two-subspecies study could improve conservation efforts for nearly extinct tiger species.

“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,” Homes said, according to Science.

The new classification scheme will make it more challenging to preserve any of the six subspecies, but it will protect long-term genetic diversity, which is, of course, a critical goal of any conservation effort. 

Ultimately, tigers in today’s world are endangered because of two primary factors: habitat loss and poaching. Tigers have lost an estimated 93 percent of their original habitat from deforestation and climate change, according to the World Wildlife Foundation. 

Furthermore, the World Bank also made a statement on the monetary value that is put on tiger parts.

“All parts of a tiger — the penis, paws, teeth, bones, and fur — can be traded, with a total retail value in the region of $10,000 up to $70,000 per adult male,” the statement read.

While tigers were previously poached for their fur, that market has mostly shrunk in recent years. However, according to researcher Kristin Nowell in a paper published by Traffic, a Chinese wildlife trade monitoring network, traditional Chinese medicine making use of tiger bones continues to have a network of “potential consumers [that is] widely dispersed and numbers in the hundreds of millions.”

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