The Traveling Naturalist: To the Bat Cave!

The world's largest mammal concentrations are found at Texas Hill Country bat caves, where up to 15 million bats emerge each evening. Photo: Matt Miller/TNC

The world’s largest mammal concentrations are found at Texas Hill Country bat caves, where up to 15 million bats emerge each evening. Photo: Matt Miller/TNC

By Matt Miller, senior science writer

The Traveling Naturalist, our series featuring natural wonders and biological curiosities for the science-inclined wanderer.

What’s the world’s largest concentration of mammals? Many people guess that it’s one of the great herds—the wildebeest in the Serengeti, or caribou in the Arctic.

But no: To see even more mammals, you have to look to the sky. More specifically, head to a bat cave in the Texas Hill Country, between now and the end of summer.

At caves around Texas, Mexican free-tailed bats emerge nightly by the millions. Yes, millions. The biggest? Bracken Cave, owned by the excellent conservation organization Bat Conservation International, with an estimated 15 million bats. That’s a lot of critters.

These are maternal roosts: females come here to have young. Come fall, they migrate south to Mexico.

Bats emerge en masse from caves, and within minutes they stretch out to the horizon. At a glance it resembles nothing so much as a thick cloud of smoke, swaying in the breeze.

Where there are large congregations of animals, of course, there’s also congregations of predators to take advantage of the bounty. Hovering outside the caves are often a variety of raptors, ready to snatch a wayward bat.

These bats are not just a tourist attraction, but also a valuable ecosystem service. A 2006 study estimated free-tailed bats saved south-central Texas cotton farmers more than $740,000 annually. Nationwide, some estimate that bats save farmers more than $7.3 billion annually in reduced crop damage and pesticide use.

These are tough times for bats, of course. White-nose syndrome is devastating hibernating colonies across the eastern United States. Around the world, bats are threatened by energy development, habitat loss and human ignorance. Even Bracken Cave is under threat from a large housing development.

Still, there are few places on earth where you can see 10 million of anything. If you love spectacle, put this one on your list. Sit by the front of the cave as you feel the breeze from endless wings. Take it all, and then do your part to help conserve our winged mammals. They need it.


Bats appear to stretch to the horizon at Frio Cave. Photo: Matt Miller/TNC

Bats appear to stretch to the horizon at Frio Cave. Photo: Matt Miller/TNC

The Texas Hill Country has a variety of easily accessible bat viewing caves. Bracken Cave is the biggest, and is open for tours to Bat Conservation International members.

The Nature Conservancy’s Eckert James River Bat Cave Preserve holds some four million bats from May through September, and offers regular evening tours (as well as occasional sunrise visits to see bats return to the cave).

I visited Frio Cave, a bat emergence on private property, where you can see an estimated 10 million bats each evening.  And look down: Little cave myotis, another bat species, might be darting by your feet. It’s an impressive location, and tours occur most days of the season.

There are numerous other caves and sinkholes where you can see huge numbers of free-tailed bats, as well as the famous emergence at Austin’s Congress Avenue Bridge.


The maternal colonies are there May through September or October. Now is prime time to go batty.

Citizen Science

What about white-nose syndrome? Does your visit pose a risk to bats?

White-nose syndrome afflicts hibernating bats, when their immune systems are suppressed. Mexican free-tailed bats use these caves to have pups in the summer, so they are not prone to white-nose syndrome.

All responsible tours have people sit well outside the cave: this doesn’t disturb the bats, and the views are far more impressive anyhow.

In any case, please, do not try to go inside bat caves. Give them space. Bats face enough pressures. Sit back, enjoy the show and raise a toast to the coolest pest control out there.

Matt Miller is a senior science writer for the Conservancy. He writes features and blogs about the conservation research being conducted by the Conservancy’s 550 scientists. Matt previously worked for nearly 11 years as director of communications for the Conservancy’s Idaho program. He has served on the national board of directors of the Outdoor Writers Association of America, and has published widely on conservation, nature and outdoor sports. He has held two Coda fellowships, assisting conservation programs in Colombia and Micronesia. An avid naturalist and outdoorsman, Matt has traveled the world in search of wildlife and stories.

Comments: The Traveling Naturalist: To the Bat Cave!

  •  Comment from Jayne

    Love the bats! What can we all do to stop the housing development from being approved, this would be disastrous. So tell us, what can we do???

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