New research indicates that bats could signal seasonal shifts due to climate change.

The study, published in the journal Global Change Biology, is the first to use radar to track an animal migration.

The scientists found that bats that migrate between Mexico and a cave in Texas are now arriving about two weeks earlier than they did in 1995.

Scientists know that bats boost the profits of farmers by fertilizing crops and keeping hungry insects in check. According to recent research, they also could clue farmers in to shifting weather patterns due to climate change.

“These bats spend every night hard at work for local farmers, consuming over half of their own weight in insects,” Charlotte Wainwright, a co-author of the study published online Feb. 14 by the journal Global Change Biology, said in a statement.

In the first study to employ radar to study animal migration, Wainwright and Phillip Stepanian, both meteorologists with the agricultural research institution Rothamsted Research in the U.K., found that Brazilian free-tailed bats (Tadarida brasiliensis Mexicana) now fly north to Texas from Mexico on their annual migration about two weeks earlier than they did a couple of decades ago.

The discovery was something of an accident, Stepanian said in the statement. The team was combing through radar measurements for weather surveillance around Bracken Cave near San Antonio, Texas, where millions of Brazilian free-tailed bats roost in the spring, summer and fall. This data, it turns out, is a handy tool for estimating the size of bat populations.

“Our initial goal was just to show that the populations could be monitored remotely without disturbing the colony,” Stepanian said. “We weren’t expecting to see anything particularly noteworthy.”

But when they looked at the measurements between 1995 and 2017, which tracked the massive exodus of bats from the cave each evening heading out to hunt, a pattern emerged. It showed that the bats now arrive at the cave earlier on average in the spring than they did in 1995 — currently around mid-March each year.

They were also able to document a growing proportion of the population sticking around through the winter. Research on Bracken Cave in the 1950s reported that the bats typically cleared out by mid-November each year. But the earliest data in the current study indicated that about 1 percent of the bats had taken to wintering in the southern Texas cave. By 2017, that figure rose to 3.5 percent.

“We can’t tell if the overwintering bats are bats that arrived in March and have not returned south, or if they migrated to Bracken Cave from farther north,” Stepanian said.

The research validates the use of radar to help keep tabs on bat numbers. And while the study has demonstrated that bats are capable of adapting to changes to their environment, other questions have arisen. It’s not clear how they’ll respond to more extreme climatic changes or whether bats’ malleable behavior will allow them to cope. If they can’t, of course, that’s likely to have an impact on farmers’ yields.

What is clear, Stepanian said, is that Bracken Cave’s seasonal bat colony is responding “to some environmental change, and to the presence of insect prey earlier in the year.”

Banner image of free-tailed bats leaving Bracken Cave by Phillip Stepanian/Rothamsted Research.

CITATIONS

Eads, R. B., Wiseman, J. S., & Menzies, G. C. (1957). Observations concerning the Mexican free-tailed bat, Tadarida mexicana. Texas. Texas Journal of Science, 9(2), 227-242.

Federico, P., Hallam, T. G., McCracken, G. F., Purucker, S. T., Grant, W. E., Correa-Sandoval, A. N., … & López, J. D. (2008). Brazilian free‐tailed bats as insect pest regulators in transgenic and conventional cotton crops. Ecological Applications, 18(4), 826-837.

Stepanian, P. M., & Wainwright, C. E. (2018). Ongoing changes in migration phenology and winter residency at Bracken Bat Cave. Global Change Biology.

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