Monday, October 05, 2009

To the Batcave!

Last week the family and I went on an expedition to Bracken Cave, home of the largest bat colony in North America. It's also the largest concentration of mammals in the world, with 20 million Mexican free-tail bats concentrated in a cavern approximately 100 yards long (the underground complex itself is thought to be somewhat larger, but a wall of guano has blocked any further exploration). There is an old vertical mine shaft at the back end of the cave through which the army used to harvest guano for gunpower manufacture. The pungent stench of ammonia coming out of that hole is staggering, easily burning nostrils and making eyes water uncontrollably.

The main opening to the cave is at the bottom of a sinkhole maybe 50 yards across. The angles of the sinkhole are such that the opening (above) appears to sit on more or less level ground. Not true. The sink hole is pretty deep, and obviously wide. The bats spiral out in a vortex that is very, very impressive. They create their own wind as they emerge. And do they emerge...

The Bracken Cave is a breeding colony. The males arrive from Mexico in the spring. As soon as the females arrive, the males depart for scattered bachelor colonies--one is located under the I-35 overpass at Walnut Street in New Braunfels. The females give birth to their pups in the cave. The temperature in the cave would normally be a steady 70-something degrees, like most Texas caves, but because of 20 million warm bodies packed in tightly and tons of guano decomposing on the floor, temperatures hover above 80 degrees at the floor of the cave and rise to 110 or so at the roof.

Ammonia and CO2 concentrations in the cave are lethal to humans. Doesn't bother the bats, obviously. The floor of the cave is also crawling with carnivorous, flesh-eating beetles, which brings to mind a potential setting for a new Indiana Jones film. With 20 million bats in the colony, it takes nearly four hours for all of them to exit each night. For comparison, the famous Congress Street Bridge bat colony in Austin is home to about 2 million bats. Carlsbad Caverns is home to about 400,000. When the bats exit Bracken Cave, the plume shows up very clearly on Doppler radar. Amazing stuff.

Because days are getting shorter, the bats don't exit until a good bit after sundown, so lighting was problematic for photography. We hope to return at the height of summer, when they emerge before sunset, and attempt some more shots under better lighting conditions. But for not knowing what we were doing, I think our shots are better than could be expected. All in all, it's an awesome sight.

Now Playing: Aerosmith Pandora's Box

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