Some tropical animals are so rare, they haven’t been sighted by researchers in years. Though many are simply elusive—in dense jungle areas, it’s easy for small or skittish animals to stay hidden, even from camera-traps—others are critically endangered. Because of this, scientists who want to find evidence of rare animals, or census the jungle, can’t rely on direct observation. Instead, they’ve started employing an unorthodox strategy: leeches. 

According to The Atlantic, when leeches feed on blood, they’re also collecting animal DNA, which survives in their system for at least four months. Researchers can study DNA collected from leeches to get a sense of the range of animal life in a region. For example, researcher Thomas Gilbert, an innovator in leech DNA studies, sequenced the DNA in 25 leeches and found evidence of six mammals, including pigs, cows, and badgers. He also found DNA from the Annamite striped rabbit—a species so rare, it hadn’t been seen in more than 2000 hours of camera trap footage. 

Douglas Yu, a researcher from the University of East Anglia and China’s Kunming Institute of Zoology, is currently using leeches to search for an endangered antelope called the saola. Yu has enlisted an army of around 200 rangers from the Forestry Department of Yunnan Province in China to collect leeches—the rangers also keep track of their GPS locations as they walk, in order to provide the most information possible about the locations of different animals. They’ve collected around 20,000 leeches so far, revealing an amazing range of animal life—though they haven’t found the saola yet. 

Scientists are hopeful that as the database of animal DNA becomes more complete, leeches will make it significantly easier to keep track of rare animal species. After all, while tracking down rare animals can be extremely difficult, finding leeches is easy—as Gilbert told The Atlantic, “The leech collector simply offers his/herself up for bait.”

[h/t: The Atlantic]