Traipsing around a field site in Malaysia, Mads Bertelsen, a biologist with the Copenhagen Zoo, noticed leeches glued to his colleague’s body. Rather than grossing him out, it got him wondering: Could he collect DNA from blood consumed by leeches to find out which species they fed on? If so, he thought, it could provide a simple, inexpensive way to track species and help pinpoint critical habitat. Turns out, he was right. Bertelsen and one of his colleagues, Thomas Gilbert, showed that the genetic material stayed intact in the bloodsuckers for weeks. When they analyzed the blood in 25 leeches from a field site in Vietnam, Bertelsen and colleagues were able to identify three poorly understood species and two recently new to science: the Truong Son muntjac, a deer discovered in 1997, and the Annamite striped rabbit. They also found an animal that is difficult to monitor without physically trapping it, the small-toothed ferret-badger, the smallest badger of its kind, as well as the...