redefining tiger subspecies
by Stephen J. O'Brien
Of all the big cats, or perhaps of all endangered species, the tiger may be both the most charismatic and the most feared. What has been the evolutionary history framing the tiger into the exquisite predator we admire today? The rapidly changing field of molecular genetics, particularly advances in genome-sequence analyses, has provided new tools to reconstruct what defines a tiger and its origins.
DNA evidence shows that all the thirty-seven living cat species trace back to a pantherlike predator that lived in Southeast Asia in the late Miocene Period more than eleven million years ago. The earliest tiger fossils, found in northern China and Java (Indonesia), date back around two million years. By the end of the Pliocene and the beginning of the Pleistocene, tigers were widely distributed in eastern Asia. The coalescence time of modern tiger DNA (that is, the time elapsed since the merging of lineages backward in time) occurred around 73,500 years ago, during the late Quaternary, and coincides with a catastrophic volcanic eruption of Toba in Sumatra, the largest known explosive volcanic event on Earth. The associated hemispheric volcanic winter of the Toba supereruption likely persisted for several years and was followed by a millennium featuring the coldest, driest period in Earth's history.
These events would lead to a near-extinction event for tigers, which, like many other species of the region, dropped to precariously low numbers. But from these survivors of the Sunda Shelf cataclysm would arise modern tiger subspecies, the geographically isolated races of tiger (Amur, Bengal, Indochinese, Malayan, and Sumatran tigers) that occur today from eastern Russia to Indonesia and India. The subspecies concept provokes both scientific and political controversy because living subspecies are considered to be specific units of conservation, which are protected by international treaties and organizations concerned with the stewardship of wildlife on the species level. The recognition of subspecies has particular relevance here because tiger-conservation strategies are inextricably tied to how subspecies are defined and protected.
Captive populations of living subspecies of tigers have been established based on the principle that they are genetic representations of their natural counterparts and thus insurance against extinction in the wild. However, debates persist over (1) the role of captive tigers in conservation efforts, (2) whether managed captive populations serve as adequate genetic reservoirs for the natural populations, and (3) whether the presumptive generic tigers thought to be decedents of intercross matings between different subspecies have any conservation value. The most direct way to address the dilemma is by understanding the genetic ancestry, the extent of genetic admixture, and the level of genetic diversity of captive tigers in relation to the wild populations. Traditionally, subspecies were defined by their geographic distribution combined with morphological traits such as body size, skull traits, coat color, and striping patterns. Later, several lines of evidence suggested that the classical subspecies designations were not so reliable. In 2004, my group and our collaborators published the conclusions of a twenty-year study to characterize differences among living tiger populations and subspecies using molecular genetic approaches. We genotyped biological samples from 134 tigers verified as wild-born from a specific geographic locale or descended in captivity directly from parents of known geographic origins. The results were striking in providing a variety of subspecies-specific DNA fingerprint markers unique to different subspecies. Not only were the subspecies shown to be remarkably distinctive, at least in DNA terms, but the genetic profiles also indicated that there had been little gene exchange between living subspecies despite few physical barriers that would keep the tiger populations isolated. We suspect that behavioral reinforcement--that is, territorial defense as part of the rigid tiger home-range system--could explain this isolation as it does for other large cat species.
First recognized as endangered back in 1975, the tiger is vanishing rapidly from its natural habitat; only an estimated 3,200 remain in the wild as compared with 100,000 a century ago. In contrast to the declining wild tigers, worldwide captive tiger populations are booming: Between 15,000 and 20,000 tigers live in captivity--five to seven times more than their wild relatives. Only a relatively small portion (less than 1,000 individuals) of the captive-tiger population is managed through coordinated breeding programs among zoos with the goal of preserving genetic variability representative of geographic and subspecies groupings found in the wild. The vast majority of captive tigers are not part of these managed breeding programs. Most reside in roadside zoos, breeding farms, makeshift breeding facilities, and circuses and as pets. Conservation managers generally consider these tigers generic--members of hybrid subspecies or of unknown origins--and therefore of little value for conservation matters. This perception may change a bit, however, because our genetic analyses of captive animals of unknown provenance revealed that though most were indeed hybrids, between 14 percent and 23 percent were pure subspecies, meaning there are as many pure validated subspecies in captive settings as there are in wild habitat. The potential genetic value of tigers housed in these private facilities means that they may certainly be considered as a source population for future breeding or even reintroduction programs.
Modern tiger-genome diversity is traced to a founder event that occurred 72,000 to 108,000 years ago, coinciding with the Toba volcano supereruption in Sumatra, Indonesia, which may have reduced the historical tiger population to a small demographic bottleneck. Since then, ecological and biogeographical factors have led to the distinctive population differentiation of at least five surviving subspecies. Yet a tiger from Sumatra and a tiger from south China have less genetic distinctiveness between them than a person from Ireland and a person from India.
Assessment of subspecies ancestry based on DNA, if applied to captive tigers of uncertain background, would increase by thousands the number of tigers suitable for conservation management. Considering how dramatically the tiger population has diminished in wild settings, these captive populations should probably not be dismissed so quickly.