Bats
Eptesicus fuscus: the big brown bat. Photo by Dr. Robert Thomas and Margaret Orr, California Academy of Sciences.
JB: This is Earth and Sky for Tuesday, August 6 – and today is our 3,500th show. Nearly all bats in the United States feed on night-flying insects . . .
DB: Bats have high metabolisms and expend lots of energy in flight, so they eat up to 70 percent of their body weight each night. That’s like an adult man eating a nightly meal of 55 kilograms – about 120 pounds of food. A single brown bat can catch 1,200 insects in just an hour.
JB: Though bats can see, they use “echolocation” to track flying insects in the dark. They emit short sound pulses as they fly, and the sound waves spread out in front of the bat, strike objects in its path and bounce back. By interpreting these echoes, bats can tell the direction, distance, speed and size of objects around them. This the same principle by which ultrasound imaging works. In an ultrasound test, high frequency sound energy is bounced off tissues in the body, and transformed into a visual image.
DB: Bats help keep insect populations in check. But bat populations are declining at a rapid rate. For links on how to help bats, come to today’s show at our website at earthsky.org. Special thanks today to the U.S. Forest Service and to the National Fish and Wildlife Foundation – supporting the conservation of native fish, wildlife, plants and their habitats. We’re Block and Byrd for Earth and Sky.
The following individuals were interviewed for today’s show. Our thanks to:
Bob Benson
Bat Conservation International
Austin, TX
Gary McCracken
University of Tennessee
Knoxville, TN
The following books, articles and web sites were used in preparing this script:
Bat Conservation International
Potential Effects of Global Change on Bats
Author’s Notes:
Recent research shows that the bats in Bracken Cave in Texas are eating enormous numbers of agricultural pest, particularly the corm earworm (also known as the cotton bollworm) The corn earworm is the most destructive insect pest in America in terms of damage to crops and costs of control. The bats eat many of these insects at altitudes of several thousand meters above the ground as the insects migrate into America’s agricultural areas from Mexico and points farther south.
Bats are particularly vulnerable to extinction because they reproduce slowly – most produce only one young per year. Individual wild bats have been known to live as long as 34 years in the wild. So if their populations are decreased by hum activities, it takes a long time for numbers to increase.
It’s not just what the bat takes in, but also what it puts out that’s beneficial. Guano is the collective term used for bat feces. People all over the world use guano to fertilize their crops. Bat droppings in caves support whole ecosystems of unique organisms, including bacteria useful in detoxifying wastes and producing gasohol, detergents and antibiotics
A colony of 150 big brown bats can protect local farmers from up to 33 million or more rootworms each summer.
Humans can hear sounds up to 20 kilohertz but bats echo locate in the 9 – 200 kilohertz range.
Echolocation works this way: when a sound wave (which is a mechanical displacement of air particles that creates alternating regions of high air pressure and low air pressure like waves on a pond in which there are regions of “high” water (the wave) and “low” water (the trough) travels through the atmosphere it typically bounces off of objects in its path, or it’s path is bent (diffracted) by the object. The effect is maximized only IF the objects in the sound’s path have a diameter that is larger than the sound’s wavelength. Therefore, bats emit high-frequency sounds (which have very small wavelengths), and detect the sound that bounces off objects in the path of their emission. Small insects have small diameters, but the wavelengths are even smaller, so it is an effective way to “locate” food through the action of having a sound wave “echo” off objects in the bats path. We use the same strategy to locate sound sources, particularly high-frequency sounds, because our head and body reflect sound energy creating a difference in sound levels reaching our two eardrums. This process is less efficient for low frequency sounds, which have long wavelengths, hence the difficulty humans have in localizing things like elephant calls and the Taos hum. This is also the principle by which ultrasound imaging works. Here, high frequency energy is bounced off of tissues in the body, even very small tissues, and is transformed into a visual image of light (tissue) and dark (nontissue) areas.
Additional Teacher Resources
The Smithsonian Institution, Encyclopedia Smithsonian: Bat Facts
This site is a great resource as an introduction to bats. Information or research assistance regarding bats is frequently requested from the Smithsonian Institution and this site is a compilation of that information in a “frequently asked question” format.
U.S. Fish & Wildlife Service, Division of Endangered Species: Common Misconceptions About Bats
This site provides a list of interesting facts about bats. Links to sections on common myths, bat biology, hibernation and migration, reasons for population decline, and more information are also included.
U.S. Geological Society, Impacts of Climate Change on Life and Ecosystems in the Southwestern United States: Potential Effects of Global Change on Bats
It is generally believed that bat populations have declined worldwide in recent decades. Because bats have low reproductive rates, populations are very susceptible to elevated mortality. There is developing concern about the conservation status of bats as many species of bats are increasingly affected by the actions of humans. This article discusses their status and the conservation efforts under way.
U.S. Department of Agriculture, Natural Resources Conservation Service: Beneficial Bats
While myths have turned bats into monsters and “scary” creature of the night, they are, in fact, important, useful members of the ecosystem. This site explains why. There are also suggestions for ways students can practice bat conservation and provide shelter.
