For many, Halloween and the coinciding Bat Week are open invitations to break out the bat-shaped cookie cutters, costumes, and seasonal decorations. But for a team of University of Oklahoma researchers, the batty holidays mark the end of a months-long field season. By the end of October, their primary research subject, the Mexican free-tailed bat, has largely migrated from the state. 

 

The Mexican free-tailed bat, Oklahoma’s state flying mammal, is known for its remarkable flights from maternity caves while in the state. Less is known about its migration habits. A research team from the University of Oklahoma has been capturing the bats to learn about their movements and their overall health leading up to migration. 

The timing of this migration, from Oklahoma to Texas or into Mexico and back, is one focus of the team’s ongoing study. 

“We hope to learn if the bats are leaving and arriving in one big wave, or if they come in pulses,” said Daniel Becker, assistant professor at the University of Oklahoma and principal investigator of the multi-year study. “We’re also conducting general health assessments – is the bat stressed, are there blood parasites or viral infections, how does their immune system look  – and then asking how these factors shape the bat migration.” 

To answer these questions, graduate students Kristin Dyer and Meagan Allira have been making regular treks to the Wildlife Department’s Selman Bat Cave Wildlife Management Area, home to one of the state’s four known Mexican free-tailed bat maternity caves. They’ve been capturing bats throughout the spring and summer, collecting blood and other biological samples, and tagging the animals with tracking devices before releasing them back into the prairie sky. 

Dyer, a Texas native, is studying the bat’s movements and ectoparasites like bat flies, but never intended to work with mammals. “I’ve always been a snake person.” In a welcome twist, Dyer instead landed a job along the Gulf Coast studying the roosting ecology of evening bats just days after graduating from Texas State University. “It was really exciting from a new scientist’s standpoint – there’s just so much we don’t know about bats.” 

Allira, originally from Washington state, is focusing on the role migration plays in immunology and pathogen transmission. She, too, had an indirect journey to the field of bat research. “I always thought I would go the amphibian route, focusing on chytrid research. Then I started working with a researcher while at Colorado State University. We netted bats all over Colorado and I was sold. It was exactly what I want to do for ever and ever.” 

A Weekend at Selman Bat Cave

In mid-September, Dyer and Allira, along with University of Oklahoma research technician Bret Demory, undergraduate researcher Mackenzie Hightower, and Alicia Roistacher, a PhD candidate working on a separate bat project, returned to their temporary field headquarters at the University of Central Oklahoma’s Selman Living Laboratory before splitting into teams for a weekend of bat netting. 

While they shared common goals of capturing bats and collecting their final samples for the year, the teams were targeting different species at nearby locations. Roistacher and Demory traveled to Alabaster Caverns State Park to capture cave myotis, a nonmigratory bat that hibernates in the gypsum cave system in the winter. Dyer, Allira, and Hightower hiked to Selman Bat Cave WMA’s namesake feature to capture Mexican free-tailed bats before the animals left the state during their annual migration. 

On their arrival, the Selman trio stretched a fine mesh net just under the cave’s mouth, raising the three tiers, or connected shelves, as high as possible at sunset. Within minutes, the first bats were emerging from the cave, some zipping around the net while others became entangled in the fine mesh. The net was lowered each time a bat was captured, and the team worked quickly to untangle and stow the bats in individual cloth holding bags. 

Jena Donnell

Meagan Allira, a graduate student at the University of Oklahoma, prepares to stretch a mist net at the mouth of Selman Bat Cave. The black triangular-shaped tubing in the upper left quarter of the opening houses a Passive Integrated Transponder detection loop that can help researchers time the bat’s migration. 

“A lot of the bats have left since the last time we were here,” Dyer said. “That was also before the first cold front of the season moved through.” 

Allira agreed, “Normally, when the bats are fully here, we don’t even need to raise a net. We can just use a butterfly net.” 

Despite the slower capture rate at the end of the season, the trio had netted their nightly goal within an hour and a half. By 9:15 p.m., the team was disassembling the mist net; carefully hanging the bat holding bags in a five-gallon bucket; and hiking away from the cave. 

Field Work Meets Lab Work

Back at the Selman Living Laboratory, the teams revealed their catch; the Selman trio ended the night with 20 Mexican free-tailed bats and the Alabaster Caverns duo had captured nine cave myotis and three bonus Mexican free-tailed bats. 

The researchers then settled into the night’s second phase: data and sample collection. 

At the lab’s first station, each bat was weighed and measured, and swabs and blood samples were taken.  

“These samples will show the immune status days before the migration,” Allira said. “I’m especially interested in comparing the blood samples and swabs throughout the season to see if there are any changes in pathogen shedding or specific immunity levels, or if any patterns emerge.”

“Migration is a stressful and hard time,” Dyer added. “When there’s more stress, they’re more likely to get sick and more likely to spread it.” 

“Bats have evolved with viruses over millennia – some won’t even show symptoms,” Allira said. “One virus we’re studying, herpes, relies on latency. It has a similar pattern in people. It could be a thing you have all the time, but it’s not until an individual gets stressed that they get a cold sore.” 

While blood samples were being deposited in clear test tubes and added to glass slides at the first station, Dyer and Allira readied the Mexican free-tailed bats already sampled for the tracking portion of the study.

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Jena Donnell

Kristin Dyer, a graduate student at the University of Oklahoma examines a Mexican free-tailed bat’s wings before collecting samples and attaching a tracking device.   

“We can use weather radars to detect movements, and every bat we process gets a PIT tag to help us understand when they get here and when they leave,” Dyer said. “We can then compare the arrival and departure dates with climate conditions – if there was rainfall or drought, if it was hot or cold – to find out if any weather patterns trigger the migration. They might come for a reason we don’t yet understand.” 

Since the project launched in 2022, the team has implanted small Passive Integrated Transponders, or PIT tags, under the skin of about 400 Mexican free-tailed bats. A detection loop, about the size of an inflatable swimming pool and installed at the upper mouth of the cave, logs any PIT-tagged bat that flies through the loop when entering or exiting the cave. Even though the loop doesn’t span the full mouth of the cave, this method will likely provide the most accurate arrival and departure dates of the tagged bats. 

The radars and PIT tag detections can reveal the presence of bats at the cave, but a third approach, automated telemetry, may also reveal the bat’s migration path to and from the state. 

Jena Donnell

This Mexican free-tailed bat has been equipped with a Motus tag that could help researchers better understand the migration path from Oklahoma to Texas or into Mexico and back. 

The Motus Wildlife Tracking System uses a network of receiving stations, including a newly installed station at the Selman Living Laboratory, to detect the signal of any Motus-tagged animals that pass within a station’s detection zone. 

“If the bats are large enough – if they weigh enough – we plan to deploy the rest of our 40 Motus tags this weekend,” Dyer said. 

The tags can only weigh a fraction of the bat’s weight so as not to impact movement, and detections are only possible when the animal passes near one of the system’s active receivers. 

More about Motus

 

Learn more about the Motus Wildlife Tracking System, and the installation of the receiving station at the University of Central Oklahoma’s Selman Living Laboratory in a recent Outdoor Oklahoma Journal post. 

By the close of the weekend, the research team’s goals had been met. Biological samples had been collected prior to migration to learn more about the population’s overall health; PIT tags had been implanted to best detect the bat’s arrival and departure dates; and the study’s 40 Motus tags had been deployed to hopefully show the bat’s migration path. The team will spend the off-season analyzing samples and monitoring the trackers for detections. 

? As of late October, at least two of the bats tagged in mid-September were still detected by the Selman Living Laboratory’s receiver. Though none of the Motus-tagged bats have yet been detected by other receiving stations, the Selman station has so far contributed to three other research projects. A Sprague’s pipit tagged in Montana by the Smithsonian Conservation Biology Institute in late July had migrated through the Great Plains and was tracked to northwestern Oklahoma on Oct. 9; a second Sprague’s pipit tagged in South Dakota by Environment and Climate Change Canada in early October was detected in northwestern Oklahoma on Oct. 13; and a third Sprague's pipit tagged in Montana by the University of Oklahoma was detected near the station on Oct. 16.   

Funding for this project is provided by the University of Oklahoma’s Health Sciences Center and Center for Biomedical Research Excellence, COBRE, an award from the National Institute of Health, as well as by the National Science Foundation (BII 2213854), the University of Oklahoma’s Data Institute for Societal Challenges, the Research Corporation for Science Advancement, and the Edward Mallinckrodt, Jr. Foundation. The research team is able to capture bats at the Selman Bat Cave Wildlife Management Area and Alabaster Caverns State Park with permission from the respective agencies and with Scientific Collectors Permits issued by the Oklahoma Department of Wildlife Conservation. 

The Selman Bat Cave WMA is closed to the public except for scheduled events.