A subspecies is an isolated group of one organism on a slow path to becoming a new, distinct species. For instance, the world is currently home to only one species of tiger, Panthera tigris, which has existed for between 1 and 2 million years. Any tiger alive today could mate with any other tiger and produce viable offspring — that's essentially the definition of a species. However, the tigers on the Indonesian island of Sumatra have been isolated on an island for over 12,000 years, where they have become around 10 percent smaller than tigers on the mainland of Asia. They're the same, but different.
From 100,000 to 4,000 in 100 Years
This time last century, an estimated 100,000 tigers roamed their vast historical range, from the Caspian Sea, up to Siberia, all the way down to India and into the islands of Indonesia. Only 4,000 tigers remain in the wild today, covering just about 7 percent of their historical range. But a report published Oct. 25, 2018, in the journal Current Biology suggests that we're not only losing tigers, we're losing six distinct subspecies of tiger in Asia. Yes, it's depressing, but this information might also be key to conserving them.
If left to their own devices, tigers are hearty animals. Females have a litter of cubs every two to three years, and under good conditions, two to four cubs will survive. When you do that math, you find 10 tigers can pretty easily become 100 tigers over the course of a decade. The problem is, these big cats don't particularly like each other.
"They're solitary animals — the males retain huge territories of up to 150 square miles (400 square km), and they'll try to kill each other if they meet," says Dr. Matthew Luskin, a tiger researcher in the Asian School of the Environment at Nanyang Technological University in Singapore, who was not involved in the study. "The females within these territories compete for resources, too."
According to Luskin, tigers move astoundingly quickly through jungle habitat — you, frail human, could probably only walk about 6 miles (10 kilometers) a day through Sumatran rainforest, where a tiger can cover between 18 and 30 miles (30 and 50 kilometers) in the same amount of time. Point being, tigers do pretty well for themselves if they have enough space (and if they aren't killed by poachers, which is another tiger problem).
So, if tigers do well by themselves without human meddling, it follows there must be a way to convince governments that they need to give tigers more space, right? And though the world is currently home to only one species of tiger, the authors show there are six unique subspecies occupying the farflung reaches of Asia. It's one thing to lose Panthera tigris from Russia, but it's quite another when you frame it in terms of losing the Amur tiger, which is different from the Bengal, the Sumatran, the Indochinese, the Malayan, and the South China tigers. When you look at it this way, you'll notice that the Caspian, Javan and Bali tigers have already disappeared.
The Numbers Controversy
Although researchers and conservationists have talked about the distinct populations of tigers for years, there has been controversy about how many tiger subspecies there actually are.
"The tiger subspecies controversy traces back to the PLoS [Public Library of Science] Biology paper we published in 2004 that showed, based on partial genomic analyses, that tigers constitute six living subspecies," writes lead author Dr. Shu-Jin Luo, a geneticist in the School of Life Sciences at Peking University, in an email. "Other researchers challenged the findings, countering that morphological and ecological evidence pointed to fewer subspecies distinctions. Now, more affordable and robust genomic technologies allowed us to greatly strengthen the evidence affirming six living subspecies."
According to Luo and her co-authors, the proliferation of tiger subspecies in Asia has a lot to do with glaciation and good, old fashioned climate change. As the climate changes — and it's constantly going through these cycles of warming and cooling — animals are able to travel in different ways and occupy different spots on the globe. For instance, about 12,000 to 50,000 years ago during the last glacial maximum, the temperatures were so cool, there wasn't as much great tiger habitat as there had been during warmer times.
How the Species Subdivided
However, as sea levels dropped because more water was locked up in ice, the tigers walked from the mainland of Asia, to the islands of Sumatra, all the way to Java because they were all connected at the time. At the end of the glacial period, those groups of tigers that had founded separate populations got stuck on these islands, or way up in Siberia, or down in India. They started changing as a result — the populations become distinct over the course of thousands of generations.
"That's interesting, but not really surprising," says Luskin. "But what this research does is bring a lot more data to bear on this issue of tiger subspecies."
Which will hopefully help scientists and governments understand the ways in which these animals can best be protected.
"Tigers are not all alike," writes Luo. "Tigers from Russia are evolutionarily distinct from those from India — even tigers from Malaysia and Thailand are different. We should respect such uniqueness if possible and avoid admixing across different subspecies, because they represent the wonder of nature derived from long-term evolution, divergence and adaptation. However, when it comes to an extreme conservation dilemma in which reintroduction of tigers from elsewhere is required in order to restore a landscape, then we would advocate selecting candidate source individuals for cross-breeding or release, from an evolutionarily close lineage."