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Geo-Joint: Biggest Bat Bedroom

Posted on August 03 2016

If you’re lucky, you can still go out on an early summer evening and as twilight comes on, watch little figures flit about the air like night birds. They’re bats, of course, and while some folks would dash for the indoors at the sight, they shouldn’t. Bats are a wonder, living in dark recesses all day, emerging at dusk to eat prodigious numbers of insects that bug us much more than bats ever will. Bats come in many shapes and sizes, and with a range of food preferences. In the U.S. we have 40 species, but worldwide there are 900 known species, and possibly as many as 1,200. Their individual numbers are also impressive; it is estimated that bats represent one fifth of the world’s mammal population. The bats in this country are small, and almost all are part of the 70% of bats that eat insects. There are also tropical species which eat only fruit, and which are larger. Much larger. Some have wingspans up to six feet! Nectar-eating bats slurp flower juices and in so doing provide the same kind of pollen transfer that bees do. Other bats dine on frogs, fish, mice or birds, but the much-feared vampire bat, of which there are only three species, do not feed on blood from humans.

Bats that eat winged insects are probably best known for their uncanny abilities of echolocation, many orders of magnitude more sophisticated than the best human-devised systems. Without it they could not hunt at night, nor fly in caves or the other dark places they live in during the daylight hours. Bats are commonly found in these quiet, unlit spaces which can also include attics, tunnels and tree hollows. As with everything else, there is a world’s-biggest bat home, and that honor goes to Bracken Cave in Texas. Fewer than 20 miles northeast of the heart of San Antonio, this large cave houses between 15 and 20 million Mexican free-tailed bats which come north to Texas for the warmer months. As they issue forth from their home each night, a 60-foot wide hole in a low hill seems to erupt with flying life. It can take nearly four hours for them all to pass into the open sky, and in order to have enough time to get a good night’s meal, the emergence can begin up to three hours before sunset. Flying with tailwind-assisted speeds up to 60 mph to heights of 10,000 feet, they are extraordinary aviators. They can also range up to 150 miles in their search of food.

The bats in Bracken Cave are all female. The males gather elsewhere in smaller groups. One of the functions of the cave is as a nursery. After arriving at the cave pregnant in early spring, the bats give birth in late June. When going out at night to feed, they leave their tiny, hairless offspring huddled all together, as many as 500 to the square foot, in a special area of the cave exclusively for the babies. Amazingly, upon their return, each mother bat finds her own baby by sound and scent and nurses it twice daily. As harrowing as a baby bird’s first flight is, a novice bat has to accomplish its leap of faith in darkness, and in the company of millions of other first-timers. Their echolocation skills must work flawlessly from the start, as crashing into a cave wall or another bat can be deadly. Life is tough for a young bat and only half live beyond their first birthday.

Female Mexican free-tailed bats must eat their own weight in flying insects each night in order to produce enough milk for their offspring. Given the number of bats in Bracken Cave, this amounts to around 100 tons of insects per night. The service provided to humankind for this massive consumption of mosquitoes and other troublesome six-legged airborne creatures is immense. Losing bats to cave and mine closures, encroaching development, and disease can create acute environmental disruption. Many areas in the eastern half of the U.S. are feeling this loss as a relatively new threat has emerged in recent years, a disease called white-nose syndrome. It is a fungus that grows on bats’ muzzles as they hibernate and causes them to act unnaturally, flying around in the daytime. The fungus sickens and kills a very high percentage of the bats in caves and mines where it has appeared. First discovered in 2006/7, it has spread around the East rapidly, and in 2015 was suddenly found far to the west, in Washington state. The disease has killed millions of bats, wiping out some colonies, and no one knows how much damage it will eventually do. Luckily for the denizens of Bracken Cave, however, white-nose syndrome does not appear to affect Mexican free-tailed bats.

Bracken Cave is renowned for its nightly summertime spectacle of waking bats, but it took some foresight to keep it safe from the pressures of human development. Bat-minded citizens and organizations purchased the land upon which it sits in 1992 but areas nearby were slated for development a couple of decades later, as San Antonio sprawled to the northeast. Bats don’t do well in close proximity to modern civilization because the profusion of electric light at night draws them in to feed on the insects also drawn to that light. Having large numbers of bats hanging around people’s porch lights and streetlights and coming into close contact with people and pets wouldn’t be safe for them. Conversely, if rabies were to get into the bat population, the effects on humans would not be good. By 2014, luckily, bat awareness was high enough that money was raised to purchase the land contiguous to Bracken Cave and threats to both bats and people were kept widely separated. This kind of human advocacy for bats is something of a new attitude. For centuries bats have been vilified as evil, ugly, dangerous creatures best avoided or killed. Fortunately, their contribution to nature’s balance is becoming more widely understood and appreciated.

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