The time I saw ten million flying mammals

When I walked into lab last Monday, I waited with bated breath for the guy who sits across the bench from me to inquire about my weekend. I don’t usually have much to offer in terms of excitement, but this weekend was different.

“Hey Eva. How was your weekend?” he asked.

“Hi Sean. It was nice and relaxing—oh, and I saw the world’s largest colony of bats emerge from their cave to eat twice their weight in insects.”

He was duly impressed.

And it was not just the world’s largest bat colony that I saw, but also the world’s largest aggregation of biomass—there are more living, breathing bodies packed into this cave than anywhere else on earth. This incredible multitude of mammals is the Mexican Free-tailed Bat colony at Bracken Cave. Ten million strong, the colony leaves the cave every night to fly up to 50 miles in search of a meal.


The bats flying out of their cave

To book a bat “appointment,” you have to be a member of the non-profit organization Bat Conservation International. As it turns out, my uncle is an active member, although I’m not sure why. Once you have scheduled your bat trip, the waiting list is usually 6 months to a year.

We drove up to the site at around 6 p.m., and the sun was still a couple of hours away from setting. A guide led us down a hill to the mouth of Bracken Cave, which is a yawning semicircle set in a perfect bowl, like an amphitheater. Since the bats didn’t come out until dusk, we had some time to just sit and enjoy the outdoors. It was pretty peaceful…until the non-bat (but still batty) entertainment started.

I had not realized the bats came accompanied by live music by the one and only Lucas Miller, “The Singing Zoologist.” Lucas was a tall, skinny guy holding a guitar and wearing a shirt with a bat on the front. The strap of his guitar was embroidered with tiny colorful animals, and he was wearing those khaki field pants that are the unspoken dress code of field biologists.

“I promise I’ll shut up when the bats come out,” he said as he strummed a preliminary chord, and then launched into a passionate rendition of his song “Wah Wah-Wah Wildlife.”

About an hour later, we had been serenaded with several of his other original compositions, including “Out on the Prairie (Whoop-de-doo, Poo-poo!)” (poop jokes galore), “The Monarch Butterfly Song” (funny and cute), and “The Fire Ant Song” (take out the words about fire ants, and you have yourself a not-quite-tasteful-or-appropriate song about illegal immigrants).

While Lucas was singing, the bats had been flying round and round in increasingly frenzied loops, the swarm pushing closer and closer to the edge of the cave like a single entity. The flapping and whirring of wings was growing louder in a steady crescendo until finally, the first few swooped out and flew in a stream of wings and fur out over the back of the cave.


The bats flew in circles inside the mouth of the cave until they were ready to leave in a long stream


They reminded me a little bit of girls getting ready to go downtown. “Hold on, I forgot my lipstick.” “Wait, don’t leave yet, I’m not ready!” “Y’all go on, I’ll catch up later with Sally.”

While the bats were not gearing up for a night on the town, I was right about one thing—they were all girls. The guide explained that some bat colonies are a mix of both female and male bats, while some are only female, and others are BATchelor pads (ha!). The bats at Bracken Cave are a female colony, who feed every night and come back with full stomachs. Each bat weighs approximately as much as two quarters, and each consumes twice its weight in insects each night. Some of the food goes to the bats, while the rest is used to make milk that they feed to their babies, which are huddled upside down on the roof of the cave at a density of 500 baby bats per square foot.

Did you hear that? Five hundred. Baby bats. Per square foot. And the most amazing part is, the mother bat will come back to the cave after feeding, and go right up her baby.

After the first wave of bats, we began to see predators sidling out from their hiding places in the rocks around the cave. Several snakes twisted sinuously down along the cave mouth, ready to pounce on some unlucky bat lady. A confused-looking skunk (honestly I have never seen a skunk that did not look confused) ambled across the rocks eyeing the opening of the cave hungrily.

For the second wave, we ran around to the other side of the cave to stand directly under the stream of bats, watching their little brown bellies bob and dip above us. The most amazing part to me was the sound they made. It was a quiet whirring, the kind of noise you might hear from a sewing machine. Thousands of tiny parts, working in unison. Millions of leathery winds, flapping to support small furry bodies.

Watching the bats emerge was an amazing experience—and like most natural wonders, incredibly humbling. It’s easy to spend a sunset hour sitting in your apartment, wrapped up in your own little world with a book and a drink. In this setting, it’s easy to feel big and important. But while you are sitting there, ten million bats are flying out of Bracken Cave. Ten million bats are eating insects, ten million bats are flying back with full stomachs and nutrient-rich milk for the ten million babies. We’re not that big or important—we are just tiny parts of a huge world, just like one bat in a colony of millions. Or at least, that’s how the bats made me feel. Small, but integral. Small, but not insignificant.


Off to eat twice their weight in insects!


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