Bats carry lots of viruses – including variants of the coronavirus that sparked the pandemic. But since bats don't often attack humans, the question is: What's the risk of these viruses being passed on to humans.
A new study inadvertently discovered a possible route of transmission. Researchers were filming bats to learn how they communicate when they swarm – and during a routine watch of the live footage they saw something that shocked them: A rat grabbed a bat and bit it.
"We thought, oh well, that's an unlucky coincidence," says Mirjam Knörnschild, co-author of the paper and head of evolutionary diversity dynamics at the Museum of Natural History in Berlin. But then it happened again. And again. The rats could even snatch a bat flying in mid-air.
The study reveals as urbanization increasingly encroaches upon bat-safe caves, invasive, city-dwelling rats are likely to follow.
"This just adds to the threats that bats are already experiencing," says Knörnschild. "We need to find smarter ways to keep the wildlife that follows us around away from bats."
Knörnschild was fascinated by the attacks, which were captured in the pitch dark, using an infrared camera to film the northern Germany cave. "Bats can echolocate, so they should have been able to see the rat, but how was the rat able to see the bats? That's still a mystery, actually," says Knörnschild
The brown rats likely stemmed from a nearby open-air theater in the town of Bad Segeberg, which frequently brings in droves of trash-tossing tourists. Knörnschild and her team wondered if this rat behavior was common at other bat hibernation spots where tourists aren't hanging on, so they surveyed a different cave in the town of Lüneburg, outside of Germany's second largest city, Hamburg.
"We found the very same thing," Knörnschild says, including a cache of bitten or partially eaten dead bats near the cave. "This is something that is apparently more common whenever there are more rats, and cities breed rats."
The behavior is mildly concerning to Raina Plowright, an infectious disease ecologist at Cornell University who studies bat viruses. For bat viruses like coronaviruses to spread to people, there is often a bridging host — an animal that regularly comes in contact with both bats and people. Plowright says this study shows that rats could be a possible bridging host. "Rats are adapted to human environments, they're all around us," she says. "As we degrade habitats, we bring rats with us, and we're potentially bringing a bridging host with us to help us be exposed to the next pandemic pathogen."
Still, Plowright doesn't think this one study is cause for concern from a human health perspective. "If there was a pandemic-potential pathogen in those bats, we probably would have seen it already," Plowright says. The real issue, she thinks, is not what the bat-eating rats could do to people, but rather, what they could do to bats.
"Bats are under growing pressure worldwide," Plowright says, due to human-caused stressors like destruction of bat habitats, and noise and light pollution. Declining bat populations could have real consequences for us, given the important role bats play in insect control, seed dispersal, and pollination. "Those services are disappearing because their habitats are disappearing."
Knörnschild is equally concerned. Her research has sparked a collaboration with Umweltbundesamt, a federal agency in Germany focused on environmental research. Her team now plans to test ways to safely and humanely reduce the invasive rat population around bat hibernation spots, like the cave in Bad Segeberg. "We don't want to just report curiosity," she says. "We want this to ideally be translated into conservation action."
Bec Roldan is an independent science journalist based in Brooklyn, N.Y. They cover health and science topics and previously served as a AAAS Mass Media Fellow at NPR.
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