|
Bats and
Plants
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 and kidneys.
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 helices
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 as well as the National Weather Service radar in nearby
New Braunfels
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 dark chamber.
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 placed within an open-bottomed box.
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 (do not look up with mouth agape) 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 the 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.
,
|
