Soil DNA analysis of pawprints could help Sumatran tigers

Editor’s note: Netflix’s Tiger King exposed how tigers were abused and exploited. This Sunday at 10 p.m. ET, “This Is Life with Lisa Ling” is hitting up some of the people trying to shut it down.


dr Mrinalini Watsa, a researcher at the San Diego Zoo in California, scoops soil from a fresh paw print from Rakan, a 4-year-old male Sumatran tiger who lives at the zoo, and secures it in a specimen jar.

Back in her lab, Watsa analyzes the sample using a small electrophoresis device connected to a smartphone. Jackpot. She can detect Rakan’s DNA in the ground.

The proof-of-concept experiment is part of their work, which will adapt existing genome sequencing technology so that it can be easily used to detect individual tigers in the wild from their DNA. Watsa hopes the application will make it easier to track Rakan’s wild counterparts on Sumatra, Indonesia’s largest island, and tiger populations in the rest of Asia.

“Rather than saying that we saw about 40 footprints in this 3 square kilometer area, you can actually see that those 40 footprints add up to four tigers and that gives us so much more power as we count them,” she said in the latest episode of the CNN original series, This Is Life with Lisa Ling.

All living organisms, including humans, release genetic material into the environment when they defecate, bleed, or shed skin or fur.

Conservation scientists are increasingly using this environmental DNA—be it in the soil, in the water, in the snow, or even in the air—to gather information about specific species or ecosystems. It can alert scientists to the impact of the climate crisis or the existence of harmful pathogens and help them track animal populations.

In her previous experiments, Watsa was able to detect DNA from the Sumatran tiger in the soil and determine the sex of the animal. Watsa wants to refine her approach so she can identify individual tigers before testing them in the field.

Tiger numbers have increased 40% in seven years, from 3,200 in 2015 to 4,500 in 2022, according to the latest estimates released in July by the International Union for Conservation of Nature.

This promising population growth has been hailed as a conservation success story, but Watsa and other tiger experts say the mission is not accomplished. Tigers are still listed as Vulnerable on the IUCN Red List of Threatened Species and are only a fraction of the 100,000-strong population that roamed Asia in the early 20th century.

Also, the headline numbers hide a more nuanced picture.

Tiger populations are growing in some places in India and Nepal, but the big cats lead a much more fragile existence in Southeast Asia. Tigers have been extinct in Vietnam, Cambodia and Laos since 2000 and are on the brink in Malaysia. In Sumatra, where Watsa’s work is focused, there are believed to be fewer than 800 tigers and only two sanctuaries are home to more than 25 breeding female tigers.

Also, it is not clear to what extent the observed increase in numbers is due to intensified and improved tracking techniques or actual population increases. Tiger counts are rarely based on direct sightings; Instead, population numbers are derived from tracks or pugmarks, or how often tigers are spotted by hidden cameras.

“It’s cautious optimism. Tiger numbers are better known than ever. More than a recovery, I’d say it’s a much more accurate estimate,” said Abishek Harihar, deputy director of the tiger program at big cat conservation group Panthera.

“A lot of so-called increases have more to do with better estimation methods,” he added.

Watsa hopes the DNA detection methods she is developing with Rakan's help will improve tracking of tiger populations in the wild.

For example, Harihar said India, which accounts for about 64% of the world’s wild tiger population, conducts a survey every four years – but the area surveyed has increased over the past 12 years, making it difficult to really understand population trends.

Population surveillance in India is usually done with camera traps, Harihar added. He believes DNA techniques could help scientists better understand how some tigers spread between different areas, which is difficult to capture with cameras.

“It’s good to understand where the different tigers are coming from and then we can secure those dispersal routes,” he added. “DNA techniques will also be useful where cameras are difficult to capture,” he added, such as in remote mountainous regions in Southeast Asia.

Watsa believes the techniques she is developing will overcome some of the weaknesses of camera-based surveillance.

“The camera only sees a very small radius around them, so an animal could walk just outside of it and miss it completely. That means they have a massive margin of error,” she said.

By developing techniques that are less expensive and easier to use, Watsa is aiming for more accurate tiger population numbers.

Watsa also hopes her portable DNA analysis techniques could be used for forensic investigations. The biggest threat to tigers today is poaching and the trade in their body parts, which are prized for traditional medicine in countries like China.

Analysis of tiger DNA in soil samples could help forensic investigations in the fight against illegal wildlife trade.

Tigers occupy just 45% of the 2.1 million square kilometers (1.3 million square miles) of remaining tiger habitat that still exists in South Asia and East Asia, an indication of the extent of poaching, Harihar said.

Analysis of DNA samples from confiscated skins, bones and animals could help identify tiger populations most at risk from poaching and track down individuals and organizations involved in the illegal tiger trade, Watsa said.

According to Traffic, a group that monitors the illegal wildlife trade, 2,205 tigers and their body parts were confiscated in 50 different countries between 2000 and June 2022. Of these, a third involved whole tigers, with 665 found alive and 654 dead.

In the United States, the popular 2020 Netflix documentary Tiger King publicized the exploitation of tigers for entertainment. In late July, the House of Representatives passed the Big Cat Public Safety Act, a bill designed to restrict private ownership of tigers and prevent big cats from entering the illegal pet trade.

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