The Echo
Bracken and its Bats: A Natural Wonder of the World

The Echo

Bracken and its Bats: A Natural Wonder of the World

Published on July 5, 2017
Written by Michelle Donahue


 

Every night, at a 100-foot-wide crescent crack in the earth in south-central Texas, a phenomenon occurs that’s as dependable as the eruptions of Old Faithful. Tens of millions of Mexican free-tailed bats, seasonal residents of Bracken Cave, stream out in a nightly ballet, swirling far and wide over Texas hill country. Over four hours, these hungry hunters emerge in search of insects to feed themselves and the millions of baby bats they birth in the warmth and safety of the cave.

To be sure, the spectacle of the bats leaving the cave is astonishing all on its own. But the bats’ presence in Bracken also means that the cave itself is peculiar, a scientific, historic and ecological treasure whose mysteries are still being unraveled.

Read on to find out how there’s more to Bracken than just the bats that roost there.

 

1. It’s the largest known bat maternity colony in the world.


Bracken Cave hosts the largest known bat maternity colony in the world, playing nursery to millions of Mexican free-tailed bat babies each year. At a toasty 102 to 104 degrees Fahrenheit, the cave is a perfect incubator.

In March and April, pregnant females return to Bracken after overwintering in Mexico. In June, females pack the cave walls with naked, newborn pups—as many as 500 per square foot.

All those hungry bat moms put a big dent in the bug population, too, collectively chowing down on 140 tons of flying beetles, winged ants and moths every night. One recent study also found that the Bracken bats save the region’s cotton growers $741,000 every year in pesticide and crop damage costs.

 

2. The bats are food, too. 

The bats aren’t the only animals feasting in the vicinity of the cave every night. From March to October, as the bats prepare to exit Bracken’s mouth, they nervously circle around inside before emerging.

And for good reason: outside, a host of predators including owls, Cooper’s and red-tailed hawks, raccoons and ring-tailed cats await the chance to nab the bats as they make their evening dash. Western and Eastern coachwhip snakes gather in the cave’s sinkhole, ready to pounce on any unlucky crash-landers. 

 

 3. Lots of bats = lots of poop.

As the story goes: everybody poops. And in Bracken Cave, with more than 15 million Mexican free-tailed bats living there, you know that’s got to be a lot!

Each year, bats deposit an estimated 50 tons of guano on the floor of the cave. Historic accounts from guano miners suggested they’d dug down 30 to 60 feet in places without hitting bottom. But its full extent is still unknown.

Extracting a “core” of guano with a hollow drill could answer this question. Scientists could use the core for radiocarbon dating to establish the age of the oldest layers, which could reach back centuries or millennia. It could also reveal what bats have been eating over time, and whether other species of bats have ever lived in Bracken.

But getting to the bottom of the deepest parts has proven to be tricky. A method to estimate guano thickness with electrical pulses can see that some areas of the cave are at least 59 feet deep, but it may extend another 33 feet. With such a depth, finding a drill long enough that can be brought into the cave is a problem that BCI is still trying to solve.

 

 4. The cave floor is a dangerous living organism.

If you were to enter Bracken Cave (wearing protective and breathing gear!), the floor would appear to be alive and moving.

“Guano is a food source for a lot of cave critters, including crickets, fleas, mites, and many other invertebrates,” says Fran Hutchins, BCI’s Director of Bracken Cave. In Bracken alone, there are six different species of flesh-eating, or dermastid, beetles; all of these organisms are important for keeping the cave from filling completely with guano.

The guano beetle, also called the lesser mealworm, is the major processor of the guano. Its larvae look similar to mealworms for sale in the pet store. In the summer, when guano increases dramatically, so does the beetle population, causing the floor of the cave to seethe with movement.

The flesh-eating beetles, as their name suggests, do not eat the guano, but debris and unlucky young bats. With millions of bats living in the cave, some are bound to fall to the floor and die; they are consumed in minutes.

So why do researchers need special protection? The beetles create toxic waste products including ammonia, and carbon dioxide from bat breath is often trapped near the floor—two gases incompatible with human lungs. And you’ll want to be covered head to toe to avoid attacks by those flesh-eating beetles.


5. Guano also meant gunpowder.

Bracken Cave has had a significant role in Texas and US history. In the 1800s, chemists discovered that the same concentrated nitrates in guano that make it a gardener’s dream could also be used to manufacture gunpowder. This doubled its value, and bat caves across Texas were raided for their guano stores to manufacture black gunpowder. “Guano was the biggest mineral export in Texas before oil was discovered,” Hutchins says. “In the 1860s, during the Civil War, Bracken Cave was mined extensively to make gunpowder.”

Bonus! Bat Weather

Those who have seen the bats’ nightly emergence have described the sound as similar to steadily falling rain. They might not be rain, but they can sure look like it on Doppler weather radar scans—check out the animation below.

 

 

 

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