Elephants, giraffes and rhinos can all be caught in spider webs – or at least their DNA can.

Josh Newton at Curtin University in Western Australia and his colleagues have found that spider webs capture the DNA of creatures living nearby, providing researchers with a novel method of surveying wildlife in difficult environments.

The researchers studied 49 spider webs from two locations in Western Australia – Perth Zoo and Karakamia wildlife sanctuary, a 268-hectare area of bushland some 50 kilometres east of Perth.

Their analysis yielded nearly 2.5 million DNA sequences, which were processed in part with polymerase chain reaction (PCR) technology, the same that is used in covid-19 tests. This technique generates millions or billions of copies of DNA sequences, making it possible to study samples that contained only tiny amounts. Just over 1 million sequences were identified as human and excluded from the analysis.

The team members detected non-human vertebrate DNA in all the web samples they tested. The webs from Perth Zoo yielded a total of 61 vertebrate species, including 33 mammals, 21 birds, five reptiles and two amphibians.

For two of the zoo’s largest species – Asian elephants and northern giraffes – the team found DNA nearly 200 metres away from the animals’ enclosures.

In Karakamia wildlife sanctuary, the scientists detected 32 vertebrate species, including native animals such as western grey kangaroos and motorbike frogs, as well as three invasive species: the red fox, house mouse and black rat.

DNA from cows, sheep and pigs, which aren’t known to live within the sanctuary but are farmed throughout the region, was also found in the samples.

“The DNA is shed from the animals in the environment and becomes airborne,” says Newton. “It may be free-floating DNA or still within cells like hair and skin cells. It’s also likely to be attached to something else, like dust particles.”

He says it is also possible that the DNA of larger animals has been picked up by flies or other insects, which later become trapped in the webs.

Spider webs could be a valuable way to sample environmental DNA such as this and monitor wildlife, says Newton. “With species in decline globally, monitoring them is becoming increasingly important,” he says. “This new technology is in the early stage of application in terrestrial systems, and while it’s not a silver bullet and traditional survey methods will always be needed, it does allow us to rapidly monitor ecosystems beyond what we are able to easily see and hear.”

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