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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.


The botanical images on this site were produced by The Photon Hunt.

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Copyright at Common Law by Manuel Flores