Leech rangers patrol the jungle
Reviled as bloodsuckers, these worms have turned out to be a sophisticated biological monitoring tool, writes Richard Conniff.
Hard-bitten tough guy Charlie Allnut, Humphrey Bogart’s character in the 1951 film African Queen, wasn’t scared of much. Not much, except leeches. One of the film’s unforgettable moments is when he tears them from his body, muttering “filthy little creatures” and shuddering with revulsion. It is a horror many share. Few think of them the way scientists are starting to, as the most sophisticated and cost-effective biological monitoring tools ever invented.
Discovery of this useful function was a triumph of lateral thinking. Imagine your mission is to visit a remote protected area and determine the presence of the Annamite striped rabbit (Nesolagus timminsi). The animal was first discovered in the Annamite Mountains of Vietnam and Laos in the 1990s but has rarely been seen since. You have tried using camera traps in likely habitat for 2,000 nights – that’s more than five years – without success. Monitoring on foot has also failed to produce results, as rare mammals are often nocturnal and have typically managed to survive because they live in steep, wet, densely vegetated habitat. They also tend to be heavily hunted. So they naturally flee from humans. What to do?
Just go and find out what the leeches have been sucking on.
Researchers recently tested the method using the time-honoured technique of picking the leeches, 25 of them, off their own legs. “We let them hunt us,” explains researcher Tom Gilbert. “So basically we stood in likely spots, and kept our eyes wide open.”
The team had determined in a preliminary experiment that DNA from a leech’s meal could survive in its digestive tract for at least four months. So they sent off the 25 leeches from the field for DNA barcoding, a technique that sequences a small fragment of DNA as a quick way of identifying species, in this case both the leeches themselves and the species they had been feeding on. But not the researchers. The study used a sequencing technique designed to ignore human DNA.
When the results came back from the lab, 21 of the leeches contained mammal DNA and four of them had been feeding on the elusive Annamite striped rabbit. Bingo.
Researchers have already collected leeches from Vietnam in an
attempt to find an endangered species, the saola.
This kind of genetic monitoring goes by the name of “metabarcoding” or “iDNA”, for invertebrate DNA. The barcode is usually a 658 letter DNA sequence from a gene in the mitochondria, the cell’s energy organ. Mitochondrial DNA is more variable than that in the nucleus, providing a short (hence inexpensive) species-specific signature. It is is rapidly becoming a standard tool in conservation. Researchers have already collected, but not yet sequenced, leeches from the same region of Vietnam in an attempt to find another endangered species, the saola (Pseudoryx nghetinhensis). This remarkable antelope-like bovine, sometimes called the Asian unicorn, was also discovered in the 1990s. But despite weighing about 90 kg, it has proven almost impossible to find by conventional means.
Other researchers are applying the iDNA technique by sequencing an “insect soup” of mosquitoes, ticks, carrion beetles and other blood sucking or meat-eating invertebrates. In an article recently in the journal Ecology Letters, Douglas Yu and his co-authors describe a study that used this technique as well as conventional standard trapping techniques. The traditional method took highly trained experts more than 2,500 hours to assemble 55,000 arthropod and bird specimens.
For iDNA sampling, Yu says, “We collected lots of insects and other creepy-crawlies, ground them up into an ‘insect soup’, and read the DNA using sequencers that are now cheap enough to use weekly or even daily.” The soup turned out to deliver the same biodiversity information as the gold-standard datasets. It was also many times quicker to produce, less reliant on taxonomic expertise, and had the added advantage of being verifiable by third parties.
That last bit is important, Yu tells me when I track him down (without the benefit of leeches) at the Kunming Institute of Zoology in southern China. The American tropical ecologist holds a joint appointment at Kunming and at the University of East Anglia in England.
He says that if you buy natural products certified as sustainable, you are essentially taking the seller’s word that, in between visits by monitoring groups such as the Forest Stewardship Council, the landowner is following the rules. Similarly, if a mining company says it will not damage local habitat, or a country says it is designating an area for protection, you need to have a certain faith, or perhaps gullibility, to believe they mean what they say. It has been expensive and time-consuming to prove such claims. But iDNA monitoring makes it cheap and quick enough to routinely monitor the reality.
Yu gives the example of certain protected areas in China where each ranger is responsible for monitoring 500 hectares of forest. If they fail to alert authorities to illegal logging in their area, they go to jail. Now iDNA makes it possible to determine if a ranger fails to report illegal logging, hunting, or if a mining company fails to live up to its promises by answering the question, ‘are the same species present this month as last?’. The leeches also show if an area has the same diversity at the end of the mining project as at the beginning.
Researchers are already using the technique in northern Canada to monitor species in Wood Buffalo National Park, the world’s second-largest protected area, as it faces threats from mining, hydroelectric projects, climate change and other human intrusions.
For biologists, these techniques promise to revolutionise how they look at the natural world. A sister technique, eDNA, or environmental DNA, does the same sort of thing for aquatic species by sequencing DNA shed by them into a water sample. Researchers have already used it in the United States to locate the hellbender, a rare amphibian, and will soon attempt it in China to find remaining wild populations of the Chinese giant salamander.
Insect soup and other methods are also good news for the animals. Apart from the insects, leeches and other invertebrates that must die, these techniques don’t actually require killing animals, or even handling them. In one case, DNA from a leech enabled researchers to identify a mammal species in the Annamite Mountains without examining it, something previously impossible. Because DNA can identify animals not just by species, but as individuals, it is possible to release captive-bred animals into the wild and monitor their survival without recapture.
Finally, says Yu, genetic monitoring is rapidly becoming faster and less expensive. A British company, Oxford Nanopore Technologies, is developing a portable sequencing device. So researchers will soon be able to go into the field, whip up their insect soup, plug the sequencing device into their laptops, and almost instantly see a panoramic view of the species hidden all around them.
It is as if someone handed Charles Darwin a Star Trek tricorder.