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Elusive. India's tigers are hard to spot.

Pallava Bagla

Tigers Tracked by Their Scat

NEW DELHI--Hot on the heels of a study that used feces to track penguin populations from space (ScienceNOW, 6 June), researchers in India are reporting that DNA from tiger poop can help them estimate the cat's numbers in the wild. The work is a "huge breakthrough" that should help conservationists protect this endangered predator, says Melvin Sunquist, a wildlife ecologist at the University of Florida in Gainesville.

India is home to the world's largest number of wild tigers. But biologists are unsure of the population's size or distribution. Estimates from paw prints and camera traps put the number at about 1400 tigers (Science, 22 February 2008), a figure that reflects years of habitat loss and poaching. Yet even this number is unreliable, because paw prints are prone to misidentification and camera trapping is difficult to implement.

So researchers led by Ullas Karanth, a tiger biologist at the Wildlife Conservation Society in New York City, turned to poop. Earlier studies in the United States had suggested that wolf and bear numbers could be estimated via DNA in feces, so Karanth and colleagues fanned out for 6 weeks across an 880-square-kilometer area, following dirt paths frequented by tigers. They bagged 58 samples of tiger scat and hustled these to the National Centre for Biological Sciences in Bangalore. There, a team led by Uma Ramakrishnan, known for using DNA to trace the heritage of ancient South Asians, set to work identifying individual tigers from intestinal cells in the excrement. By looking for unique genetic signatures in the samples, her team was able to identify 26 individual tigers.

Camera traps set up in Bandipur National Park, which help identify tigers via their unique stripes, came to a similar figure over the time period: Twenty-nine cats. That suggests, says Karanth, that the genetic approach is accurate. The two techniques show a "great concordance," he says. Based on both data sets, Bandipur has 66 tigers, the team reports this week in Biological Conservation.

The DNA approach could be especially useful for estimating tiger numbers in locations where setting up cameras is very difficult, says Karanth, such as in the Sundarbans mangrove forest in the Ganges delta or where tiger densities are low, like in the Russian Far East. DNA samples can also help park managers get a feel for the threat that inbreeding may pose, he says. "This is how we will monitor tiger populations and how they move around the landscape in the future," predicts George Amato, director of the Sackler Institute for Comparative Genomics at the American Museum of Natural History in New York City.