If you've ever had the opportunity to watch a bat chasing insect prey at night, you probably noticed that they are amazingly quick and agile fliers. Darting, rolling, changing direction on a dime, avoiding obstacles and (usually) intercepting their insect prey all without the benefit of sight. Contrary to popular belief, bats actually have pretty good eyesight (similar to that of humans), but once it is dark, it isn’t their primary means of navigating or locating and catching prey.
So, how do bats catch quick-moving, flying insects in the dark? They use echolocation, emitting very high pitched sounds (higher than human hearing range) that bounce off obstacles in their path (trees, other bats, buildings, insects…). Bats use those reflected sounds to identify what an object is, how big it is, and what direction it is moving.
And if you have been lucky enough to go on a bat walk or talk with someone with a bat detector, you can actually listen to the echolocation calls, hearing the "terminal buzz" as the bat closes in on the insect, using superfast muscles to emit calls at a rate of over 160 per second. It's sort of like the scene in the movie Top Gun when the fighter pilot gets "radar lock" on the other plane... the bug stands no chance.
This pop culture reference isn't the only crossover between bats and things in the military world.
Bats have taken advantage of abandoned military bunkers in the U.S. and abroad for years, and recently biologists fighting white-nose syndrome have even been working to enhance bunkers on the east coast as potential safe hibernacula for bats in the disease-stricken region.
The U.S. military has been studying bats for years. During WWII, the Marines explored the idea of using bats to carry small incendiary bombs, and more recently, the Air Force has been studying bat flight to develop micro aircraft. Turns out, these incredible acrobats have been inspiring military technology for years.
This summer, I joined my friend and colleague, Nate Fuller, and his crew from the Bat Lab at Boston University on a bat-inspired military research trip to Texas. This research, using high-tech thermal infrared cameras to record 3-dimensional video of huge colonies Mexican free-tailed bats flying, was funded by the Office of Naval Research.
But before I talk more about that trip, a little more on bats' incredible flying skills and unique life history. The bats we were studying in Texas are Mexican free-tailed bats. These bats are some of the "Top Guns" of the bat world, flying up to 60 miles per hour with tail winds, and at altitudes of over 10,000 feet. They also live in huge colonies, some with millions of bats. Some of the Texas colonies are the largest Mexican free-tailed colonies, not to mention the largest congregations of mammals in the world. In the evening, shortly before sunset in the longest days of summer, the bats start to emerge to feed. The emergences are huge and impressive: thousands of bats pouring out of the roost, swirling around in a dense column, and heading off into the central Texas fields for a night of feasting on flying insects. The emergence is so massive from some of the larger roosts that it can even be seen on Doppler radar. Oh, and it’s estimated the Mexican free-tailed bats in the region eat hundreds of tons of insects each night, including pests for important crops like cotton and alfalfa. No wonder they are sometimes called “the farmer’s friend.”
It’s really amazing to watch, thousands of bats flying together as a group, seemingly coordinated but with no leader. How to the bats know what their neighbors are doing? How do they all fly so fluidly together, not slamming into each other as they swerve to miss trees and then gracefully merge back into their column?
Military engineers are interested in collecting information on how the bats manage to navigate flying in a column to develop unmanned aircraft. Even the fastest and best human fighter pilots (sorry, Maverick, that includes you) can't respond as quickly and agilely as the bats.
Back to my nights in Texas. We visited two sites during the time I was there. The first, an artificial roost called the “Chirptorium” on a private ranch. I spoke to the ranch owner, David Bamberger, about the fake cave and its inhabitants. David had been volunteering for years at Bracken Cave (home to the largest known Mexican free-tailed bat colony), and wanted to attract a colony to his property for pest control and guano for fertilizer. David told me that at first, people thought he was crazy. But within a few years, bats began using the cave, which is now home to over 10,000 Mexican free-tailed bats, and a small population of another species, the cave myotis.
The other site we visited was Davis Cave, home to about 2 million free-tails. This cave was up a short, steep hill overlooking the Texas hill country. The view was beautiful, but schlepping approximately 700 pounds of gear up there… whew.
In order to capture the 3-D video of the bats, we needed 3 thermal imaging cameras, each with their own computer to capture the data. We also needed electricity, so generators, gas, power cords also had to be brought to the site. Plus tape measures, calibration equipment, and our personal packs with headlamps, personal gear, food and water. This was all much easier at the Chiroptorium, where we could drive right up to the opening of the roost.
The cameras are set up so that they all have a slightly different vantage point of the expected column of bats (oh yeah, did I mention the bats “decide” on their exit flight route based on conditions when they emerge? We could guess all wrong and our data collection for the night could be useless…) so that we get a 3-dimentional image of the column and the individual bats in the column. Once the cameras are set up and calibrated, we waited on the bats to emerge.
And emerge they did! Coming in waves, thousands of bats at a time, it still takes hours for all the bats to get out of the cave. A few minutes before they emerge, they start “staging up” inside the entrance of the cave. Swirling around and chirping just inside the entrance, until some unknown cue lets them know it is time to go! Standing just outside the entrance, the breeze from all those thousands of wing beats rustles the vegetation and blows your hair. Even though I’ve been lucky enough to witness the huge emergence at Bracken Bat Cave, each time I watch thousands of bats fly out to feed for the night, I get a little overwhelmed by the sight.
The data collected at these sites (as well as a few others) will be analyzed using software developed to identify individual bats in the column, and look at how they are flying in respect to their neighboring bats in the same space- what are the “rules” for flight that help them avoid collisions, responding to obstacles and each other in 3-dimensional space. The Navy hopes to use this to develop unmanned aircraft that can fly as nimbly and efficiently as bats.
For more photos from this trip, check out our Flickr set.