"Blind as a bat," "bats in the belfry," "like a bat out of hell," these
and other colorful expressions of the contempt many humans have for bats
are indicators of fear and ignorance. True, bats are creatures of
the night, but we no longer believe darkness to be intrinsically evil.
Bats are marvels of adaptation, having evolved the ability to fly and also
to "see" by hearing the echoes of high-pitched sounds they emit.
They are pollinators of numerous species of plants, inadvertent architects
of the distribution of trees in tropical rain forests, prodigious consumers
of night-flying insects and models of extreme social cohesiveness.
Bats are reputed to be filthy creatures; a repository of every dreaded
disease. But if that were true, how could they exist unimaginably
crowded, their colonies numbering in the tens of millions, without those
same devastating diseases wiping them out? Some even claimed bats
had a special immunity to rabies, yet could still infect other animals.
Actually, they can die of rabies and their infection rate is no greater
than it is for other species. According to one source, only 9 cases
of humans contracting rabies from bats occurred in the United States in
the thirty years following World War II. Most of those were a result
of being bitten while disturbing a sick bat.
Vampires, of the Transylvanian type, do not exist. Natural selection
has, however, produced three bat species of tropical and subtropical America
which consume the blood of their prey. These species perform sanguivory
aided by exquisite modifications to their teeth, tongue, saliva, stomach
Their teeth evolved to cut skin with surgical precision, inflicting no
pain to alert the sleeping blood donor (usually cattle). The tongue
bears several grooves which aid in transporting blood into the mouth through
capillary action. Vampire bat saliva is a potent brew of anticoagulants,
designed to maintain the free flow of blood.
Their stomach is quite different from that of other bats. It is a
relatively long, tubular structure which can enlarge to hold the volume
of blood ingested and is richly endowed with an extensive tracery of capillaries
to hasten the movement of water to the kidneys. Their kidneys function
first to eliminate quickly the large fraction of water which constitutes
blood and later switch to a water-conserving mode to handle the toxic by-products
from the very high protein meal.
Chiropterophiles, bat lovers, can be proud of the local history of accommodating
bats by providing roosts for their colonies. Those unusual, elevated
buildings were called hygeiostatic bat towers. One particularly attractive
one near Comfort is recognized as a historical structure. They were
erected at a time when bats were thought to consume mosquitoes. The
public health benefit of those beautiful bat towers was exaggerated because
the locally abundant free-tailed bats eat moths almost exclusively.
The Mexican free-tailed bat, Tadarida brasiliensis subspecies mexicana,
also called the "guano bat", is a small animal whose wingspread can reach
12.5 inches. It is called the free-tailed bat because its tail is
not connected to the membrane which extends between the thighs. Unlike
the other species of Tadarida found in the U. S., the free-tailed
bat's ears are not attached at the mid-line. But the one characteristic
by which it is widely known is social, not anatomical.
The local free-tailed bat is the zoological antonym of solitary.
They are the most gregarious of mammals. One of their colonies, at
the Bracken Bat Cave just north of San Antonio, is the largest grouping
of mammals on earth. The Bracken Cave is a temporary residence of
the free-tailed bat. Their annual migration takes them to southern
and central Mexico for the winter and back to the banks of the Cibolo Creek
for most of the frost-free season.
Other free-tailed bats travel to caves throughout Texas, New Mexico and
Oklahoma. Though migration is not an absolute characteristic of this
species. The populations of the southeastern U.S. and also those
of California, Nevada and the northwest do not migrate, but instead seek
warm winter roosts in which to hibernate.
The migrating colonies of free-tailed bats which reach the large caves
of the south-central U. S. during the spring are composed almost exclusively
of pregnant females. The caves house nurse colonies which are teeming with
baby bats after the middle of June. Since infant mortality rates
have been described as being very low, the already huge populations almost
double. In the Bracken Bat Cave, the count of mothers and young is
estimated to exceed forty million individuals.
Ordinarily, such numbers would be incomprehensible. In the case of
free-tailed bats, these numbers are exhibited vividly most late-summer
evenings. The spectacular flights, lasting for hours, of the bats
swirling in ever-expanding, upward helixes as they leave the funnel-shaped
cave entrance, are among the grandest of natural displays one can witness.
Once the rapidly-rotating swarms, seemingly chaotic, but resulting in very
few collisions, have attained the level of the tree-tops, they break up
into large, undulating cloud-like flocks. These huge masses of bats
are detected by radar at nearby Randolph Air Force Base.
The tonnage of insects, mostly small moths, consumed by the Bracken Cave
colony is impressive. The best estimate is that the bats can eat
250,000 pounds of insects in one night. The intricate maneuvers of
flying and feeding can be accomplished in darkness because bats evolved
the ability to produce high-pitched chirps and use the echoes of those
sounds to create in their brains a three-dimensional representation of
the world. That ability, called echolocation, permits them to avoid
hitting a string the diameter of a human hair while flying in a completely
While most of the females roost in the huge nurse colonies, where are the
males? They congregate in smaller groupings in other caves, in buildings,
under bridges, etc. One can attract these male nocturnal aviators
by placing wooden roosts on walls or trees. These backyard bat houses can
be bought or made. Several designs are available and most consist
of vertical planks of rough-hewn cedar planks placed within an open-bottomed
The bat house should be suspended 10 to 15 feet above the ground and receive
morning sun. Use a minimum/maximum thermometer to make sure the temperature
range in the box is between 80º and 105º F. Invite the
bats by "scenting" the house with a paste of guano. A source of drinking
water, such as a water garden, should be nearby.
Intelligent human behavior in the vicinity of bats includes: 1) avoiding
any bat seen in daytime outside its roost, 2) staying out of bat caves
while bats are in residence, 3) wearing a respirator when exploring bat
caves during the winter, 4) keeping quiet while observing a bat flight
and 5) maintaining the vegetation and rocks intact for a 100 yard radius
around the entrance of a bat cave.
The tropical, fruit-eating bats reveal another facet of the intricate connection
between these flying mammals and their environment. They are prime
dispersers of seeds of trees. The species mix and distribution of
tropical trees result from bats randomly sowing seeds as they relieve themselves
on the wing. Other bats, those which consume pollen and nectar, play
important roles in the pollination of species as diverse as baobabs, century
plants and saguaro cactus.
The final beneficial aspect of bats to be discussed is their guano.
This highly concentrated natural fertilizer, with a very distinctive aroma,
enjoys wide usage by organic gardeners. Locally, the guano harvest
occurs during winter, when the free-tailed bats are hundreds of miles south
and the overpowering ammoniacal fumes are somewhat reduced. According
to scientists, humans can tolerate, for one hour, an atmosphere containing
up to 100 parts per million of ammonia. The free-tailed bat routinely
inhabits caves where the ammonia levels reach 5000 parts per million.
The toxic atmosphere also keeps out potential predators.
Once your nose becomes attuned to the disctinctive scent of their guano,
you will be pleasantly surprised to detect it in parking garages, abandoned
buildings and under bridges. The bats are there, eating moths at
night, thereby reducing the numbers of leaf-eating caterpillars and consequently
reducing our need to apply insecticides.