Counting animals: Written in blood ONE difficulty faced by conservationists is getting hold of reliable statistics on the animals they are looking after. For good reason many species, particularly herbivores with a lot of predators, are rather shy. In tropical forests the problem is especially acute, since the dense jungle is perfect for hiding in. That makes it hard to know how well a given species is really doing. In a paper just published in Current Biology, though, a group of researchers led by Ida Baerholm Schnell, of Copenhagen Zoo, describe a clever, low-cost solution. Their idea is to take another well-known difficulty of doing fieldwork in tropical forests - the endless swarms of bloodsucking leeches - and turn it into a scientific tool. Leeches can go for months without food, surviving on blood stored from their last victim. Tipped off by a previous paper, which reported that viruses can survive inside blood sucked up by leeches for as long as half a year, Dr Schnell and her colleagues wondered if DNA from a leech's most recent victim might be recoverable as well. To find out, they fed goat blood to 26 medicinal leeches, a catch-all term for various species still used, occasionally, by doctors. These were then killed one at a time at intervals over the next 141 days and the contents of their digestive tracts examined. Using a technique called the polymerase chain reaction, which employs enzymes to produce millions of copies of a DNA sequence, the researchers were able to prove that DNA from the goats survived in the leeches for at least four months. Armed with this encouraging result, they moved on to trials in the jungles of Vietnam. They gathered 25 leeches (of a different species to the medicinal ones used in the previous study) and examined them for mammalian DNA. The results were impressive, including the first strong evidence for the presence of Nesolagus timminsi, a striped rabbit that ecologists had suspected lived in the area, but which had managed to evade more than 2,000 nights of camera surveillance. Its DNA was found inside four of the leeches tested. They also found genetic material from the Truong Son muntjac, a recently discovered small deer, and from the small-toothed ferret-badger, which is (apparently) impossible to distinguish from the related Burmese ferret-badger without getting close enough to handle it. At this stage, the study is only a proof of concept. A proper survey would require a great deal of systematic observation. It would also have to take into account the hunting patterns of the leeches, which may prefer to exsanguinate some animals more than others. But there are lots of potential benefits. The technique is cheap, and requires minimal training. The researchers note dryly that leeches are perfect for the task, because of "their diverse prey base" and "their readiness to attack humans." In other words, you don't even have to go looking for the biodiversity - it will attempt to wriggle down your socks all by itself.