One Tiger, Two Tigers, 1,500 Tigers …
How they count tigers in the wild.
Wild tigers in India are under increasing threats from poachers, and seven of the country's 28 reserves can barely sustain a breeding population, according to a Sunday article in the Miami Herald. The article went on to note that "no one is sure how many wild tigers remain in India, but most estimates put the total at only 3,000 to 3,500. Some believe it's closer to 1,500." How do wildlife researchers count tigers?
With camera traps: faux-bois gadgets consisting of a camera, an infrared motion sensor, and a battery pack. Whenever an animal triggers the sensor, the camera goes off, delivering photographic evidence of a wild beastie. The trick is figuring out where to place the traps—there aren't many tigers left, but the natural reserves they inhabit are vast. (Kanha National Park in Madhya Pradesh, for example, is 940 square kilometers.) Luckily, big cats are predictable: They travel along natural trails, communicating with their brethren through scent markings. Researchers identify these trails by looking for tracks or scat and then set up 12 to 15 cameras along each one.
Typically, the researchers check the traps daily to make sure they haven't been knocked over by animals. After a sampling period of perhaps a week, they take the cameras back to their base camp. Now comes the tough part. To arrive at a population estimate, the researchers can't just count the number of photographs, because the cameras are triggered by all sorts of things, like monkeys or leopards. And they can't just count the total number of tiger photographs, because the cameras might capture the same fame-hungry cat several times over. Instead, the researchers examine each snapshot manually and then analyze the tigers' stripe patterns, which, like fingerprints, are unique.
After a second sampling period, the researchers have enough data to conduct a capture-recapture study. They multiply the first tiger count by the second and then divide by the number of repeats— that is, the individuals who turned up in both samples. The quotientis an estimate of the total population in the study area. Of course, the math gets more complicated when researchers input multiple sampling periods. In such cases, they'll rely on specialized computer programs.
Paw print, or pugmark, tracing is another common technique for counting tigers. Based on the premise that each tiger leaves a distinctive set of prints, researchers survey wildlife reserves for tracks and collect digital photographs. The researchers then use the pictures to measure the length and width of each pugmark, the angle between toes, and 10 other variables to establish an ID. Unlike the capture-recapture method, which estimates a total population, pugmark tracing yields a minimum census—if there are 25 unique sets of tracks, then there must be at least 25 tigers in a given area.
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Explainer thanks Brian Gratwicke of the Save the Tiger Fund.
Juliet Lapidos is a former Slate associate editor.
Photograph of tigers courtesy Save the Tiger Fund.