Along the Tiger’s Trail: It’s All in the Stripes
Editor’s Note: “Along the Tiger’s Trail” is a series about the efforts to monitor tigers and their prey in the Malenad landscape in southwestern India that harbors one of the world’s largest population of wild tigers. The series tracks on-going annual activities of the world’s longest running research project on tiger and their prey, implemented under the leadership of Dr. K. Ullas Karanth of the Wildlife Conservation Society. These tales from the Indian jungles will take you through a virtual journey into the lives and work of people dedicated to the cause of India’s wild.
Counting secretive, elusive animals, such as the tiger, is a challenging task. Unlike other big cats that are easily seen in open African savannahs, tigers largely inhabit dense tropical forests. This makes tigers hard to find, and aerial surveys or vehicle-based counts become logistically infeasible and do not yield sufficient information for reliably counting them
In the 1990s, frustrated with the inaccurate ‘pugmark’ based counting approach widely used in India, which relies on making and comparing plasters casts of animals’ footprints, Dr. Ullas Karanth devised an innovative approach to solve this. He set up motion-sensitive cameras inside forests that allowed tigers to trigger and click their own photographs. Camera traps were not new. In India during the 1920s, British forester F.W. Champion used cameras with tripwires to photograph tigers and leopards in the Western Himalayan foothills—a primitive wildlife selfie of sorts. Dr. Karanth adopted this method to identify and count tigers scientifically.
Camera trapping is a strenuous and sometimes difficult task. It requires driving and walking through rough terrain to set up cameras, discreetly placed along forest roads and trails frequented by tigers. Once set up, cameras have to be routinely checked. Occasionally they are stolen by people or damaged by elephants. Our field season lasts between six to eight months. We set up over 700 camera trap stations covering 6,000 square-kilometers of prime tiger habitat every year in India. The outcome is over 15,000 images of tigers, leopards, dholes, elephants, deer and a suite of other secretive animals.
So how are tigers counted using photographs? Every tiger has a unique stripe pattern that allows us to identify each individual. As with human fingerprints, no two tigers have the same stripes. This enables us to count the number of tigers ‘captured’ in the camera traps. Innovative statistical modeling also allows us to estimate the proportion of tigers missed.
Sifting through thousands of tiger photographs to match individuals based on their patterns is a monumental task! Our collection of tiger images spans two decades and we have now identified almost 800 individual tigers. As new tigers are photographed every year, it becomes increasingly difficult to match individual tigers. To make this easier we use a pattern matching software called ExtractCompare (developed by British statistician Lex Hiby). The pattern-matching algorithm extracts stripe patterns to shortlist the most closely matched tigers. Trained technicians help identify the tiger from this shortlist, if it exists in the database.
Over the last two decades, we have expanded from 15 cameras in a single reserve to 700 cameras in seven tiger reserves across India. We estimate tiger numbers accurately and have gained fascinating insights into tiger ecology, behavior, survival, recruitment and dispersal in one of the most important tiger populations in the world. As a bonus, we now have information on a host of other rare and elusive animals such leopards, dholes, leopard cats, civets and mongooses, making tigers the quintessential “umbrella species” in the Western Ghats of India.
Scientific American By Arjun Srivathsa and Krithi K. Karanth