June 10, 1998 JUNE 10, 1998--A study of mitochondrial DNA sequences of tigers, published this month in Animal Conservation, demonstrates that Sumatran tigers are a distinct species from any group of living tigers - a finding that has significant implications for tiger conservation efforts. The study was conducted by a research team supported by the Lewis B. and Dorothy Cullman Program for Molecular Systematics Studies, a joint initiative of The New York Botanical Garden and the American Museum of Natural History. The team included researchers from the American Museum of Natural History, the Wadsworth Center of the New York State Department of Health, and DePaul University.
While all tigers are under severe threat of extinction in the wild, Sumatran tigers are currently underrepresented in the captive breeding programs of the world's zoos. The discovery that they represent a unique species indicates an urgent need to increase conservation management both on the Indonesian island of Sumatra, the only location where these tigers are found, and in zoos around the world.
Traditionally, the tiger has been considered a single species with five living subspecies: the South China tiger; the Siberian tiger; the Bengal tiger; the Indochinese tiger; and the Sumatran tiger. (Three other subspecies have recently become extinct: the Bali tiger in the late 1930s; the Caspian tiger in the 1950s; and the Javan tiger in the 1970s). Prior to the current study, neither genetic analysis, nor study of tiger markings, color, or size had yielded the necessary data to distinguish whether any of the five living subspecies were sufficiently distinct to be considered a separate species.
Currently, there are approximately 400 to 500 Sumatran tigers left in the wild and 235 in captive breeding programs. Sumatrans are the smallest of the living tigers, with males ranging from seven to eight feet in length and weighing between 220 and 310 pounds. In contrast, male Siberians, the largest living tigers, range from nine to eleven feet in length and weigh between 420 and 675 pounds.
The research team was led by Joel Cracraft a curator in the American Museum of Natural History's Department of Ornithology. The team analyzed blood samples of thirty-four captive tigers, representing all tiger groups with the exception of the South China tiger, and one lion, to serve as a point of comparison. The mitochondrial DNA sequenced from the blood revealed that Sumatran tigers share three unique genetic markers found in none of the other tigers. These features support the interpretation that Sumatrans are a distinct species, and all other tiger populations on the mainland can be grouped together as a single species.
The new research supports the hypothesis that Sumatran tigers were geographically isolated and became differentiated from their counterparts on the mainland when Sumatra was cut off from the continent by a rise in sea level between 6,000 and 12,000 years ago. The genetic divergence of Sumatrans from other tigers mirrors the divergence already recognized as having occurred in the Sumatran rhinoceros, which is distinct from mainland rhinoceros populations. While the four groups of mainland tigers are currently geographically isolated from one another, this is the result of recent habitat destruction by humans. Previous to this, the mainland tigers's range was uninterrupted, allowing them to interbreed and preventing them from developing into separate species.
The ability to recognize the Sumatran tiger as genetically distinct is critical to breeding programs, which are designed to maximize the genetic diversity of the animals they seek to protect. Regulation of the illegal trade in tigers is also bolstered by the discovery, because the use of newly discovered genetic markers ensures reliable identification of products produced from Sumatran tigers. Finally, since conservation plans for wild populations depend on support from the country where the animals live, recognition of the special nature of Sumatran tigers opens possibilities for education programs in Indonesia to protect a now better understood feature of that country's natural heritage.
In addition to Dr. Cracraft, the authors of the paper are the following: Julie Feinstein, molecular laboratory supervisor, American Museum of Natural History; Jeffrey Vaughn, of the NMR/Structural Research Group, Wadsworth Center, New York State Department of Health; and Kathleen Helm-Bychowski, Department of Chemistry, DePaul University.
In addition to the support of the Lewis B. and Dorothy Cullman Program for Molecular Systematics Studies, the researchers received generous support from the Nixon Griffis Fund for Zoological Research (New York Zoological Society/Wildlife Conservation Society), and the Captive Breeding Specialist Group (Minnesota Zoo).
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