Apr. 22, 2008 — Tigers held in captivity around the world--including those in zoos, circuses, and private homes--may hold considerable conservation value for the rapidly dwindling wild populations around the world, according to a new report published online on April 17th in Current Biology. Using a new method for assessing the genetic ancestry of tigers, researchers discovered that many apparently "generic" tigers actually represent purebred subspecies and harbor genomic diversity no longer found in nature.
"Assessment of 'verified subspecies ancestry' (VSA) offers a powerful tool that, if applied to tigers of uncertain background, may considerably increase the number of purebred tigers suitable for conservation management," said Shu-Jin Luo, of the National Cancer Institute, Frederick. "This approach would be of particular importance to tiger subspecies that have suffered severe population decline in the wild and/or lack of efficient captive breeding."
For instance, he said, the Indochinese tiger has been classified as a different subspecies from the Malayan tiger, leaving just 14 recognized Indochinese individuals in captivity. "Thus," Luo added, "verification of VSA Indochinese tigers, establishment of captive breeding programs, and preservation of remaining Indochinese tiger populations in the wild should be set as one of the top priorities in the global tiger conservation strategy."
Tigers in general are disappearing rapidly from the wild, from over 100,000 in the 1900s to as few as 3,000 last year, said the researchers, led by Stephen O'Brien also of the National Cancer Institute, Frederick. By contrast, captive tigers are flourishing, with 15,000--20,000 individuals worldwide, outnumbering their wild relatives between five and seven to one.
A relatively small portion of the world's captive tigers--some 1,000 individuals in all--are managed through coordinated breeding programs that aim to preserve genetic variability representative of geographic and subspecies groupings found in the wild, the researchers said. The rest are of hybrid or unknown origin and are kept in zoos, farms, breeding facilities, circuses, and private homes. Scientists have long debated the role that captive tigers might play in conservation efforts.
To address the issue in the new study, the researchers developed a strategy for assessing the subspecies affiliation of tigers on the basis of diagnostic genetic markers obtained from 134 "voucher" tigers. They applied the method to samples from 105 captive tigers from 14 countries collected over 20 years. Of those, 49 individuals were found to represent one of five purebred subspecies, or VSA. The rest of the cats had mixed backgrounds.
They suspect that the proportion of purebreds observed in their study will be an overestimate for captive tigers worldwide. Nevertheless, they said, "If 14--23 percent of the over 15,000 existing captive tigers would prove to be VSA, the number of tigers with pure subspecies heritage available for conservation consideration would considerably increase.
" Also, an important fraction of captive tigers retain genetic diversity unreported, and perhaps absent, in the wild populations. A wide-ranging identification of captive VSA tigers to assess their potential for inclusion into comprehensive, integrated in situ and ex situ management plans could significantly increase population sizes and help maintain genetic variability and population viability of this iconoclastic species."
The researchers include Shu-Jin Luo, National Cancer Institute, Frederick, MD, Department of Fisheries, Wildlife, and Conservation Biology, University of Minnesota, St. Paul, MN; Warren E. Johnson, National Cancer Institute, Frederick, MD; Janice Martenson, National Cancer Institute, Frederick, MD; Agostinho Antunes, National Cancer Institute, Frederick, MD, CIMAR, Universidade do Porto, Porto, Portugal; Paolo Martelli, Veterinary Department, Ocean Park, Aberdeen, Hong Kong; Olga Uphyrkina, Institute of Biology and Soil Sciences, Vladivostok, Russia; Kathy Traylor-Holzer, Conservation Breeding Specialist Group/SSC/IUCN, Apple Valley, MN; James L.D. Smith, Department of Fisheries, Wildlife, and Conservation Biology, University of Minnesota, St. Paul, MN; and Stephen J. O'Brien, National Cancer Institute, Frederick, MD.