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Fan of Austin's bats? Here are 6 bat facts to impress your friends

A swarm of bats in the sky above trees at dusk
Patricia Lim
Millions of Mexican free-tailed bats fly out from the Bracken Cave Preserve in the Hill Country at sunset last month.

Whether you’re new to the Austin area or you’ve been here for years, chances are you’ve heard of the bats.

The region is home to hundreds of bat colonies. At least two of them have earned a spot in the record books. The population living under Austin’s Congress Avenue Bridge is known as the largest urban bat colony in the world.

Not to be outdone, the bats at the Bracken Cave Preserve, outside of San Antonio, were recently named the largest bat colony anywhere in the world by the Guinness Book of World Records.

Both of these colonies are comprised of migratory Mexican free-tailed bats (also known as Brazilian free-tailed bats, as they occur throughout the Americas). It’s a species with so many amazing characteristics that we decided to put together this little primer of facts to share at your next dinner party or bat-viewing expedition.

1. They may be the fastest animals on earth (depending on how you look at it)

Mexican free-tailed bats have been recorded flying nearly 100 miles per hour, faster even than the speedy cheetah. That has earned them the reputation as the world’s fastest mammal.

But, Mylea Bayless with Bat Conservation International argues they could be counted as the fastest animal.

That’s because peregrine falcons, often considered the world’s fastest animals, use gravity to attain high speeds in a dive. The bats, meanwhile, attain speed by flapping their wings, what she calls “powered flight.”

“That’s not stooping like a peregrine falcon,” she says. Bats are the “fastest animal on earth, really.”

2. Their nightly emergence is visible on weather radar

During peak bat season, up to 1.5 million bats emerge from the Congress Avenue Bridge every evening looking for bugs to eat. Up to 20 million bats emerge from the Bracken Cave Preserve.

That's so many bats they're visible from Doppler weather radar and are even sometimes confused for storm clouds.

AUSTIN, TX. September 3, 2012. Crowds gather on the Congress Avenue Bridge in downtown Austin to view Mexican free-tailed bats fly out from underneath the bridge as part of their nightly feeding routine. Photo courtesy of Gabriel C. Pérez
Gabriel C. Pérez
Crowds gather on the Congress Avenue Bridge in downtown Austin to view Mexican free-tailed bats fly out from underneath as part of their nightly feeding routine.

“We partner with meteorologists to find new bat colonies that have established in highway bridges,” Richard Heibrun of the Texas Parks and Wildlife Department said. “Rather than us going out and looking at all the bridges, we can just call up the meteorologist and say, 'Hey, tell us where the bats are!'"

3. Bats eat hundreds of tons of insects every night

It is estimated that Austin’s bridge bat colony can eat up to 15 tons (30,000 pounds) of insects a night. The Bracken colony consumes up to 140 tons.

The bats aren’t too choosy about what kind of bugs they eat, but their diet depends largely on moths that are considered agricultural pests. By consuming these bugs, experts say, bats save farmers billions of dollars a yearin pesticide expenses nationally.

While the bats eat bugs, plenty of Texas wildlife eat bats. On a recent evening at the Bracken Cave, hawks, owls and snakes all enjoyed a bat dinner during the nightly emergence.

A snake eats a fallen bat on top of white rocks near Bracken Cave.
Patricia Lim
A snake eats a fallen bat near Bracken Cave.

4. Bat babies grow up fast on super-fatty milk  

The Mexican free-tailed bat colonies in Central Texas are “maternity colonies,” where pregnant female bats congregate after wintering in Mexico, to give birth to the next generation.

By August, the babies that were born in the spring need to be mature enough to join the nightly bug hunt. That’s why populations emerging from the colonies are biggest in August and September. By fall they need to be ready for the annual return south of the border.

This life cycle means the bat babies need to grow up fast. They do it by consuming milk with a 28% fat content.

“Mexican free-tailed bats have the highest fat content in their milk of any bat,” Bayless said. “Try 28% milk fat and see how fast you grow.”

5. Bat caves seem straight out of a horror movie

The Bracken bat cave maintains a temperature of over 90 degrees on the floor, and about 105 degrees on the ceiling, where the bats live. While that heat would be unbearable for humans, it helps baby bats grow quickly.

The cave floor is thick with bat excrement, called guano. In that excrement live flesh-eating beetles ready to gobble up any animal that falls.

If all that doesn’t dissuade you from sneaking into a bat cave, consider the gasses.

“The air inside the cave is highly toxic with ammonia and CO2,” Bayless said. “Bat caves are not a place you want to hang out while the bats are there.”

6. Bat populations are under threat

A close-up of a bat in the hands of a scientist. The bat has its mouth open in a squeak.
Patricia Lim
Habitat loss is providing Mexican free-tailed bats with fewer natural places to live, and insecticide use is killing off their food supplies.

Climate change seems to be altering Mexican free-tailed bats' annual migration patterns. Habitat loss is providing them with fewer natural places to live, and insecticide use is killing off their food supplies.

Because Mexican free-tailed bats migrate rather than hibernate, researchers say they are not as susceptible to white nose syndrome. The invasive fungus threatens some species with extinction by weakening and killing them during hibernation.

The fungus is present in the Bracken Cave, though.

“We don’t think it will affect our bats because the cave is pretty hot and they do migrate and stay pretty active,” volunteer Don Bergquist said.

While Mexican free-tailed bats may avoid being victims of white nose syndrome, some say they could become vectors of the disease, spreading it to other hibernating bat species they share caves with.

Mose Buchele focuses on energy and environmental reporting at KUT. Got a tip? Email him at Follow him on Twitter @mosebuchele.
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