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Federal Highway Administration > Publications > Public Roads > Vol. 59· No. 3 > "Attention Motorists... The Bats Have Landed on our Bridge!"

Winter 1996
Vol. 59· No. 3

"Attention Motorists... The Bats Have Landed on our Bridge!"

by Paul Garrett

Thousands of bats leave their roost under the Congress Avenue Bridge each summer evening to hunt for insects.Although highway agencies always consider wildlife when designing and building highways, we don't often associate impacts on bats with highway structures or construction.

This is in spite of the fact that bats are among the most diverse group of mammals and, at one time, the most numerous and widely distributed. And, if we are to believe new scientific information on the activities of bats, they are important and beneficial. Yet bats are often misunderstood sometimes even feared because of a lack of knowledge about their life history and habits. These misunderstandings are perpetuated by the connection of bats with such unsavory mythical characters as Dracula.

When it was first discovered in the early 1980s that Mexican freetail bats had established a roosting and breeding colony between the beams of the rehabilitated Congress Avenue Bridge in downtown Austin, Texas, there was immediate public concern about public health and safety. Many people were worried about the spread of rabies and other diseases by bats to other animals and to humans.

Through the educational efforts of Bat Conservation International, a private conservation and educational organization whose headquarters are located in Austin, the initial concern has been turned into civic pride, and the bat colony has become a local tourist attraction. The city of Austin has adopted the bat colony and publicizes this unique ecological and educational resource as an attraction to be enjoyed and appreciated by everyone. Some enterprising Austin entrepreneurs have even printed up bat t-shirts!

Families come to Town Lake to picnic and observe the bats.Largely through the work of Dr. Merlin Tuttle, world-renowned bat biologist and the head of Bat Conservation International, the Congress Avenue Bridge bat colony has been identified as the largest urban colony of bats in the world with a population estimated at one and a half million. The preservation of the colony is an outstanding example of melding transportation facilities, public recreation, and ecological needs.

Families often come to the shores of Town Lake, adjacent to the bridge, to picnic and observe the bats on summer evenings when the bats become active and fly out from beneath the bridge to disperse in the surrounding countryside and feed on insects. Children especially enjoy the sight of thousands of bats leaving the bridge in the evening, flying in such close formation it would seem that collisions are inevitable.

Mark Bloschock, a bridge engineer with Texas Department of Transportation (TxDOT), became interested in bat ecology when he found out that bats were using the narrow crevices between the bridge beams as places to roost and rear young. Mark's initial concern was much the same as that of the public: were the bats posing a hazard to public health? He was also concerned about the potentially corrosive and, therefore, detrimental effects that bat guano (accumulated bat feces) would have on the bridge structure itself.

Mark first established a research program to determine the impacts of bat colonies on bridge structures. These investigations have shown that, so far at least, there have been no ill effects on concrete or other structural elements from bat colonies.

In fact, as a result of Mark's collaboration with Bat Conservation International, TxDOT now has a research program studying the roosting preferences of different species of bats found in Texas. The results of the research will allow TxDOT and Bat Conservation International to develop roosting sites for bats in other highway structures, such as culverts and bridges, where it is desirable or needed to enhance bat habitat or to exclude them where their presence is unwanted.

Dr. Tuttle and other biologists working on the life histories and habits of these creatures have shown that bats are rarely responsible for transmitting rabies and other infectious diseases to humans, and bats are not generally a cause for concern about human health and well-being except when they are not present! Bats are exceptionally resistant to many animal diseases, and although they, just like other mammals, are capable of contracting and transmitting rabies, they are remarkably non-aggressive. To avoid contracting rabies or other diseases from bats, just don't pick them up or otherwise handle them. They are also remarkably clean and free of external parasites, probably because they groom themselves carefully to make maximum use of their flying ability.

The precision and accuracy of a bat's navigation system is so good that collisions between flying bats rarely occur, even in very tight quarters. I observed this amazing ability in the attic of a historic church in Pennsylvania, where I stood among a colony of 10,000 little brown bats. I watched them fly around the small space, and they never collided with one another, the walls or columns, or my hair.

Bats usually live in large, centralized colonies, in some cases numbering in the millions of individuals. The size of the bat colony at Eagle Creek Cave, Ariz., was estimated at over 30 million bats at its peak in the 1960s. This "colonial lifestyle" is caused, in part, because suitable roosting and breeding sites, such as large caves, are not widely distributed, and bats are capable of traveling great distances to feed and still return back to a roost.

Another advantage for the bats is that during cold seasons they can share body heat, and by staying close together, they can regulate their environment and use their fat supplies more efficiently. They are very tolerant of each other. Through unique behavior and adaptation, they can successfully use very small spaces and crowd a lot of bats into a small area. Most bats are very small animals although a few species, such as the "flying fox" of Madagascar, have a wingspan of up almost two meters.

Bats have varied feeding habits. Some are insectivorous, consuming several times their own weight per day in insects. The Eagle Creek Cave bat colony was estimated to consume almost 200 metric tons of insect crop pests nightly. In Texas, the Bracken Creek Cave colony of over 20 million Mexican freetail bats are estimated to consume almost 250 metric tons of insects, mostly crop pests and mosquitoes, nightly.

Other species are nectarivorous, feeding on flower pollen and nectar. These bats are important in the cross pollination and fertilization of plants over a large part of the world. They are especially important in the Sonoran desert of the Southwestern United States, where they are critical to the pollination and seed dispersion of many species of cactus and other desert plants. In rain forests, they are the primary source of seed dispersion and revegetation. More than 400 species of commercially important plants are known to depend on bats for pollination and seed dispersal.

A few of the larger species are carnivorous, feeding on small mammals and even fish. Bats feed on insects and larger prey through unique adaptations of their well-known echo location system, which allows them to locate prey and objects by projecting high frequency sound waves and locating the source of the echo. The accuracy and speed of this system of navigation and location is such that bats can locate and capture flying insects, small mammals, and even fish near the surface of small ponds by "reading" the patterns of ripples in the water.

Some, but not all, bats are true hibernators, their body temperature dropping to nearly that of their surroundings, but they are easily aroused by warming. When disturbed too frequently during hibernation, they use up the critical supplies of body fat they require to live through this period and are unable to survive until food is available.

North American bat species are almost completely beneficial. In addition to controlling insects and pollinating plants, they may be useful for medical reasons. Chemical analysis of their saliva has resulted in the discovery of a new anticoagulant drug that is now being tested as a treatment for human heart disease.. In many parts of the world, including the United States, bat populations are in precipitous decline. Part of this downturn is due to losses of roosting sites and feeding habitat. Also, in some parts of the world, bats are considered culinary delicacies, and they are hunted and captured for food.

Of the 44 species native to North America, six are officially listed as endangered, and eighteen more are candidates for listing under the federal Endangered Species Act. The major cause of the declines in our bat populations is thought to be the careless destruction of their habitat by humans, including deforestation and cave and mine closures.

In addition to the research program with TXDOT, Bat Conservation International is working with federal agencies such as the Bureau of Land Management, Bureau of Mines, Fish and Wildlife Service, and others to enhance bat habitat wherever possible and often by installing protective "gates" over caves or mines used as roosting and breeding sites.

TxDOT deserves our support and praise for initiating a thoughtful and unique research program, which should benefit bats throughout their natural range and at the same time benefit the ecosystems with which they share a mutual dependency. Mark Bloschock and Bat Conservation International hope that their research and habitat improvement program will benefit both bats and people.

For more information on the ecology of bats, their importance in ecosystems around the world, and measures that can be employed to conserve and enhance habitat for these beneficial animals (especially if you need some insect control!), contact Bat Conservation International, P.O. Box 162603, Austin, TX 78716.

Dr. Paul Garrett is the staff ecologist for the Federal Highway Administration's Office of Environment and Planning. He coordinates environmental policy, teaches the National Highway Institute courses on wetlands assessment and ecological impacts of highway projects, and directs ecological research for FHWA. He has worked as an ecologist and resource manager for 20 years in both the public and private sectors. He recently served on the National Academy of Science Wetlands Characterization Committee, which was directed by Congress to evaluate wetlands characteristics, functions, and management programs. He obtained a bachelor's degree from Memphis State University and received his master's degree in zoology and his doctorate in aquatic ecology from Montana State University.

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