Most of us aren’t super  thrilled to bump into a leech, given that they are blood-sucking parasites who  are not picky about who they snack on.

Interestingly, though, that lack  of pickiness means any given leech may contain blood from a wide variety of species. And that makes leeches an  unexpected ally for scientists trying to track endangered wildlife.

And the best part is, you don’t have  to look very hard to find leeches. If you’ve got blood in your  body, they will happily find you! So let’s get into exactly what makes these  bitey little blood-libraries just so helpful. [♪ INTRO] A key part of protecting endangered  animals is understanding which species live where and how their home  ranges might be changing.

To track wildlife distribution,  scientists can set up traps, put out cameras, and collect hair and poop. But these methods can be  difficult and time-consuming, especially for the rarest species in  hard-to-reach places like dense jungles. One handy alternative is the use  of environmental DNA or eDNA, genetic material left behind by  animals in their environment.

This DNA can be sampled from soil or water and  identified using a technique called barcoding, which means looking for short snippets of  DNA that are unique to certain species. And soil and water aren’t the only  sources of those genetic leftovers. We can also collect DNA  from the bellies of leeches!

Lots of leeches get their nutrition  by sucking blood from other animals. The digestion is pretty slow, so that  blood can linger in their tummies for months, and a DNA test can identify what  species they recently fed upon. This technique was pioneered in  a 2012 study, which sampled DNA from 25 terrestrial leeches collected  from tropical forests in Vietnam.

The researchers found DNA from  six different mammal species, some of which hadn’t been  recorded from that area before. This included Annamite striped rabbits,  which hadn’t been observed in the region even after more than 2000  nights of camera footage! That study showed that leeches can be a  great source of DNA for hard-to-find animals.

This technique has come to be called  invertebrate-derived DNA, or iDNA. On top of all that, leeches  are really easy to collect. Researchers simply wander through  the forest and before long, their clothes are covered in  leeches looking for a meal.

It’s as convenient as it is creepy! And we do mean on the clothes, by the way. Leeches found on the  scientists are generally excluded due to the contamination from Homo sapiens.

Since that 2012 study, more research  has expanded this technique, with even more promising results. A 2018 study collected over 700 leeches  across southeast Asia and identified the DNA of dozens of species of birds and  mammals of various sizes and habits. That study found that the leeches  are an especially good source of DNA for small mammals like rodents and shrews, which can be extra hard to find  using other survey methods.

One leech in that study even had bat  DNA in its stomach, which was news to the researchers, who didn’t know that  these particular leeches ever fed on bats. How exactly it got hold of a bat… we do not know. All of this is great news for scientists  hoping to help endangered species.

This leech-derived DNA can  help researchers identify which animals live where, including rare species! And that data then helps  us understand which regions are most important in order to  focus our conservation efforts. This SciShow video is supported by  Brilliant, the online learning platform with thousands of lessons in  computer science, math, and science.

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Already, scientists are putting  this novel technique into action. A 2022 study gathered DNA from over 30,000  leeches in a nature reserve in China. They had more than 150 park rangers help them out by collecting leeches during  their regular patrols of the park.

And once again, all they had to do was pick the  leeches off of the ground or off of their clothes! From those samples, not only did  researchers identify DNA from threatened and endangered species, but  they were also able to track the spatial distribution of  the species across the park. They found that native species were  most common in the interior of the park, and less common around the edges in areas with  high human activity and lots of livestock.

Another study in Borneo found a similar  pattern, where leech-derived DNA revealed lower wildlife  diversity in parts of the forest that had been disturbed by  logging or other human activities. Which means these leeches are  actually helping scientists to track how our activities  are affecting wildlife. And there is even more potential to be tapped.

For example, not all leeches  are terrestrial blood-suckers. One study in 2022 sampled a variety  of parasitic and predatory leeches and found gut DNA from fish, insects,  worms, and even other leeches! Some researchers have also  pointed out that leeches might even contain DNA from  diseases in the blood of their prey.

This could help us track the spread of diseases which can threaten endangered species, or ones that could potentially  spread from wildlife to us humans. Now, with all that said, leech-derived  DNA is not a perfect tool. For one thing, even the least picky  leeches don’t feed on literally everything.

There might be animals that  the leeches don’t like to eat or don’t regularly encounter – like the  bat I mentioned, cause you know, like...the wings. Those animals are likely to end up  mostly missing from these leech surveys. Also we don’t have a perfect understanding  of how far a leech might travel after eating, which can make it tricky to  track wildlife distribution with precision.

Also, some leeches are  protected species themselves! Certain types of medicinal leeches  are classified as species of concern, so it’s best to avoid bothering  them for these studies. That means not all leeches can be  used for this kind of research.

So, leech-DNA is not a perfect fix to  all of our wildlife-sampling woes. But it is absolutely something we  can use side-by-side with methods like cameras and other  sources of environmental DNA. And leeches aren’t the only source  of invertebrate-derived DNA.

Other studies have had similar luck  sampling DNA from other bugs that eat blood, dung, or carrion, including flies,  beetles, ticks, and mosquitoes. The world just full of little critters happily  collecting genetic samples from their neighbors! Protecting wildlife is hard work, and we need as much information  as we can get to do it right.

But it turns out there are loads of animals  out there with tummies full of data. In the future, we might owe  our conservation successes to the parasitic blood-suckers of our world. [♪ OUTRO]