In a dark cave in northern Germany, an invasive brown rat stands upright, balancing with its tail. Suddenly it reaches into the night sky, plucks a bat from the air, and crunches down on it.

At first, Mirjam Knörnschild was shocked by the scene, captured with infrared surveillance video as part of her research. Knörnschild, head of the Behavioral Ecology and Bioacoustics Lab at the Museum of Natural History in Berlin, thought it could be an isolated incident—until her investigation at another German bat cave turned up more macabre evidence.

“We observed the same situation, with rats patrolling the entrances and exits of the cave, and we found caches of more than 50 dead bats that rats had stored,” she says. “It made us think that this is not a unique problem after all.”

Knörnschild and her team returned to the two caves, both important urban hibernation sites for bats, to document the extent of the rodent menace. Their findings suggest that invasive rats are an under-appreciated threat to city-living bats.

This and other recent research on the war between rats and bats has a larger implication for the spread of disease to humans, says Raina Plowright, a professor of Public and Ecosystem Health at Cornell University.

“Rodents, particularly rats, tend to do very well in areas that humans modify or inhabit,” says Plowright, who was not involved in the study. “As we move into landscapes, we’re bringing rats with us, and these rats have the potential to become a bridging host to take viruses from nature into human populations.”

Rat attack!

Brown rats (Rattus norvegicus) are adaptive, opportunistic, and one of the world’s most widespread invasive predators. At their study sites, Knörnschild and colleagues observed rats using different strategies to successfully intercept bats in flight and target hibernating bats, all in total darkness.

“Rats are smart and fascinating creatures,” says Knörnschild. “I don’t have anything particular against rats. It’s just when there are a lot of them, they can wreak havoc.”

The researchers estimate that even a small number of rats could kill thousands of bats in a single year. At a time when bat species are already under pressure from habitat loss, climate change, and disease, this added predation could push fragile populations over the edge.

In fact, it’s already happened. Invasive rats are responsible for decimating some island-dwelling bat species, or worse. The New Zealand greater short-tailed bat (Mystacina robusta) is one such casualty. It was last sighted in 1967, likely driven to extinction following an invasion of ship rats.

To prevent urban bats from suffering a similar fate, Knörnschild and colleagues recommend stronger measures to manage invasive rodents at important hibernation sites. These include rat-proof waste containers, blocking rodent access to cave entrances, and public awareness campaigns that discourage littering and the feeding of urban wildlife.

(How rats became an inescapable part of city living)

Intimidating rodents

Elsewhere, bats are showing they are capable of adapting — and possibly even coexisting with —invasive rodents.

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At Tel Aviv University in Israel, researchers maintain a semi-natural open colony of Egyptian fruit bats (Rousettus aegyptiacus). This setup allows bats to fly out to forage and return to roost as they please. Researchers make the space even more bat-friendly by setting out fruit every day before sunset. But the promise of an easy meal also attracts black rats (Rattus rattus).

Yossi Yovel, head of the university’s Bat Lab for Neuro-ecology, says that black rats do more than just steal the bats’ food. They kill and eat fruit bat pups. To understand how bats respond to this complex threat, Yovel and his team analyzed video of bat-rat interactions at the colony over seven months.

The results show that fruit bats perceive rats as dangerous. When rats occupied the food platform, bats tended to avoid it.

“When they did land and there was a rat nearby, the bats took longer to consume the fruit because they were spending more time vigilant, looking around and moving their ears,” says Yovel. “This resulted in a huge drop in foraging success.”

The fruit bats’ responses changed with the season and situation. In winter, bats acted more cautiously toward rats and favored avoidance as a strategy. In the spring, an abundance of leftover fruit on the platform led to increased rat activity. In this context, when competition was more intense, some bats actually fought back and attacked the rats.

This means that bats made strategic calculations about when to flee, when to fight, and how to feast, instead of merely avoiding rats out of fear. This behavioral flexibility may allow the bats to live alongside potentially dangerous rodent neighbors.

“Typically, you might think about the relationship between species as fitting into a single category like competition or predation,” says Yovel. “Here we see that it’s more complex than that. Not only are there different types of interactions, but they also change depending on the season and availability of resources.”

Since Egyptian fruit bats and rats both eat fruit, it’s likely that the two species naturally encounter each other on fruit trees.

“We’ve seen this anecdotally,” says Yovel. “But I think that this urban world in which they’re living increases the rate of bat-rat interactions and amplifies their complexity.”

A lethal group hug

Both Knörnschild and Yovel’s groups discovered these rat-bat interactions somewhat accidentally, while focused on how bats think and behave.

But the findings highlight concerns about increasing urbanization, Plowright says. Bats provide ecosystem services that are essential for human and environmental health. They consume billions of pest insects, pollinate economically important plant species, and serve as key seed dispersers in forests.

“As we degrade their habitats, we lose these services,” says Plowright. “These studies show we may be bringing together bat species that are sensitive to environmental change and rodent species that thrive in human-modified habitats.“ She adds, “We need to look after the bats.”

However, in less degraded ecosystems, some bats are emerging victorious in their war with rats.

With a wingspan of over a meter, the spectral bat (Vampyrum spectrum) is the largest bat in the New World tropics. They are also strict carnivores. According to Knörnschild, the diet of these “super social, very gentle apex predators” consists of birds and small mammals, including rats.

For a study published earlier this year, Knörnschild and colleagues staked out a Costa Rican tree hollow that was home to a family of spectral bats — an adult male and female along with their juvenile offspring. The researchers captured video of remarkable social behaviors, including adult bats bringing prey back to the roost for their young, as well as the occasional squabble over a meal.

What’s more, when a bat returned from foraging, it received a special greeting from the rest of its family: a kind of “group hug” where the bats use their folded wings to embrace each other.

“It’s very adorable,” says Knörnschild. “And then the bats eat the rats and crush their bones. I suppose it’s some cosmic justice, in a way.”