Every year migratory bats travel from Mexico to Bracken Cave near San Antonio, Tex., where they spend the summer consuming insects that would otherwise devour common food crops. But the bats have been showing up far earlier than they did two decades ago, possibly because of a warming climate, new research suggests.
This trend creates a risky situation in which bats may not find enough food for themselves and their young, as the insects they prey on may not yet have arrived or hatched. If bat colonies shrink as a result of this schedule snafu, their pest control effect could fall out of sync with crop-growing seasons—potentially causing hefty losses, scientists say.
“If the whole system becomes unreliable, then it will be a big, big problem for agriculture,” says Jennifer Krauel, a bat biologist at the University of Tennessee, Knoxville, who was not involved in the new research. “I don't think the bats will go away entirely, but even a reduced colony size will have an effect.”
Mexican (also called Brazilian) free-tailed bats, the migratory species that inhabits Bracken Cave, feast on 20 different moth species and more than 40 other agricultural pests. One favorite is the corn earworm moth, which eats plants such as corn, soybean, potato and pumpkin—costing U.S. farmers millions of dollars a year in ruined crops. A 2011 study estimated that bats indirectly contribute around $23 billion to the U.S. economy by keeping plant-eating insects in check and by hunting bugs that prey on pollinator insects.
In the new study, scientists at Rothamsted Research, an agricultural laboratory in England, used radar data from some 160 U.S. weather stations to analyze activity in the Texas bat colony (the largest in the world, with a peak population of around 40 million) from 1995 through 2017. Massive clouds of bats show up on radar images when these animals emerge for nighttime foraging. The researchers had set out to prove that radar could be used to accurately estimate the size of bat colonies. But over the course of the study, published online in February in Global Change Biology, they also discovered the creatures were leaving their winter quarters in Mexico earlier and reproducing sooner.
“This was very surprising,” says Rothamsted meteorologist Phillip Stepanian, one of the study's co-authors. The bats' behavior appears to coincide with shifting seasonal temperatures. “We weren't out looking for climate change,” he says, “but then it suddenly became very obvious.”
Stepanian and his colleagues were also startled to find increasing numbers of bats overwintering at Bracken Cave instead of heading back to their cold weather quarters in Mexico—a behavior not reported at all during the first survey in 1957. Overwintering is another sign that warmer temperatures alter the bats' annual rhythms, Stepanian says.
A separate study of migratory bats in Indiana, published last year, found that temperature variations affected arrival and departure times—likewise hinting at the potential influence of climate change. Joy O'Keefe, a biology professor at Indiana State University and co-author of that study, says early arrival at their summer roosts could expose these bats to cold snaps, and they could freeze to death.
Changing bat migration times can also clash with rainfall patterns. Many insects that bats eat breed in seasonal lakes and puddles. If the bats arrive too early to benefit from summer rainfall and the resulting abundance of bugs, they may struggle to feed their pups or skip reproduction altogether, O'Keefe says. She fears this shift could cause Midwestern bats to dwindle toward extinction, which would be bad news for humans. “Declines in bat populations could have severe implications for crop success,” she says, adding that bats also “control significant disease vectors, such as mosquitoes.”
Winifred Frick, chief scientist at the nonprofit Bat Conservation International, points to additional findings from Australia, where intense heat and resulting droughts have caused mass die-offs among fruit bats. Such events could become more likely in the U.S., Frick says.
The Rothamsted researchers are not certain that climate change alone is prompting the Bracken Cave bat colony to migrate earlier. Scientists have found a direct link between seasonal temperatures and bird migration, but bats are also influenced by factors such as changes in wind speed and direction. And there are other complications. “Bats are mysterious little animals that move mostly at night and are difficult to observe and track,” Stepanian says. “We have this conceptual picture of what might be happening, but really tying it to the cause is the next step.”