The Surprising Link Between Bats Dying and Human Infant Mortality

Bats get a bad rap. Often, the flying mammals are associated with vampiric monsters or the threat of rabies. But ecologists have long known that bats play an important role in maintaining balance in ecosystems by eating insects that would otherwise get out of control.

This week, a study published in the journal Science finds that the benefits of bats apply to humans, too. We shouldn’t fear the presence of bats, the research suggests, but their absence.

“Bats are a fantastic example of a species that we like to keep a distance from, but are really impactful in terms of the role they play in ecosystems,” Eyal Frank, an environmental economist at the University of Chicago and author of the new study, tells the Washington Post’s Dino Grandoni.

Frank found that in U.S. counties where bat populations have been decimated by white-nose syndrome, human infant mortality rates rose by about 8 percent. That equates to 1,334 infant deaths between 2006 and 2017 that Frank says are attributable to a loss of bats.

The research began when Frank came across information about white-nose syndrome, a disease caused by the fungus Pseudogymnoascus destructans. Introduced from Europe to the New York area around 2006, perhaps through contaminated hiking or caving gear, the fungus has since spread to at least 40 states. When it infects a colony of bats, their population can plummet or even get wiped out entirely.

Frank realized that the well-documented spread of the fungus, and the resulting decline in bat populations, was an opportunity to quantify the impact of bats on their ecosystems and human lives.

“Reading how this disease is spreading from county to county, decimating bat populations, made my economist senses go, ‘Oh, this is probably the best natural experiment you can have,’” Frank recalls to the New York Times’ Catrin Einhorn. “It’s the closest we’re going to get to just going out there into the wild and randomly manipulating bat population levels to see what happens at a large, meaningful spatial scale.”

He compared the spread of white-nose syndrome to county-by-county data of infant mortality. The connection was stunning. Winifred Frick, chief scientist at Bat Conservation International who wasn’t involved in the research, tells Science’s Erik Stokstad that when she saw the study’s findings, her “jaw dropped.”

Frank suggests the link is due to the positive impacts of bats’ diets. A single bat consumes up to 40 percent of its body weight in insects every night. In agricultural areas, this means that when bats disappear, farmers might use more insecticides on their fields. In counties with outbreaks of white-nose syndrome, farmers used 31 percent more of these toxic chemicals, on average, per the study. Putting more insecticides into the environment seems to be the cause of the increased infant deaths, Frank writes.

To make sure his ideas stood up to scrutiny, Frank tells the Guardian’s Rebekah White that he spent a year “kicking the tires” and ruling out other possible causes of infant mortality, like the opioid epidemic, parental unemployment, genetically modified crops and even the weather. Nothing else fit, showing “compelling evidence … that farmers did respond to the decline in insect-eating bats, and that response had an adverse health impact on human infants,” he adds.

Paul Ferraro, a sustainability scientist at Johns Hopkins University who was not involved in the research, tells Science that the study proposes a “pretty dramatic claim that’s going to get a lot of attention.” But, he adds, it’s the “most convincing evidence to date” that losses of a wild species can have huge impacts on the economy and human health.

Bats, by eating potentially harmful insects, aren’t the only species with contributions to the environment that benefit humans—a phenomenon some scientists call “ecosystem services.” Frank found earlier this year that the loss of vultures in India led to the death of an extra 500,000 people on the subcontinent between 2000 and 2005. In a more positive example, researchers found that the reintroduction of wolves to Wisconsin reduced car crashes involving deer by 24 percent.

The loss of such ecosystem services highlights the potential devastating impact of the extinction of species, which has been accelerating in recent years. But, Frank says, unexpected harms can appear even without a total extinction event, when only local ecosystems are diminished.

“We often pay a lot of attention to global extinctions, where species completely disappear,” he tells the New York Times. “But we start experiencing loss and damages well before that.”

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Rudy Molinek is Smithsonian magazine's 2024 AAAS Mass Media Fellow.