Things that go chirp in the night

They might seem scary if you don't know what they are, but most of those strange howls and hoots in the dark are just animals and insects talking to one another. Some really pick up the chatter in the fall, during mating season. Others have been talking all summer, but we've been too busy to stop and listen. When you listen closely, you can learn to identify which species is doing the talking.

Identifying the sound of a particular species is quite important to crickets. During the autumn, males chirp their love songs to attract females, and each species has its own special call. Only the males chirp. They do it by rubbing their wing covers together to create the familiar chirping sound. The front area of the wing cover has a scraper that rubs against the rough area of the other wing. This is called stridulation (strihd-you-LAY-shun). You could make a similar sound if you rubbed a file against the edge of a tin can.

Crickets aren't the only creatures talking in the dark these days. Cicadas also chirp at dusk and dawn, but not during the night. Katydids also chirp by rubbing their wings together, but crickets normally put their right wing on top and katydids their left. Grasshoppers make noise by rubbing a leg against a wing. No known insect rubs its legs together to talk.

Each type of insect produces its own sound, and each species has its own unique mating call, to ensure that females find males of the same species. That fact has been very helpful to scientists who want to learn more about crickets. "Sometimes different species of crickets are very hard to tell apart," explains Dr. Thomas J. Walker of the University of Florida in Gainesville. Professor Walker teaches entomology, the study of insects, and has studied the noises of crickets and other insects around the world.

"In 1957," Walker says, "we thought there was just one species of field cricket. Now we know there are at least 10 east of the Mississippi River, four right here in Gainesville." Scientists learned to listen to the crickets' chirps.

"If you have 100 crickets in a box and they all look alike," Walker says, "it's hard to find the differences between the species. But if 50 of those crickets are making one sound and 50 are making another, you put them in two different boxes based on their chirps. Then you can look at them more closely to see what is different about the crickets in each box." This method has helped to identify a wide variety of species.

Insects aren't the only creatures you can hear at night. Other mating calls will be coming from tree frogs. They make their calls by pushing air from their lungs into an expanding pouch in their throats. Then they force the air back down into their lungs over their vocal cords. So while we sing by pushing air out of our lungs, tree frogs push the air in to make their songs. Their calls can be heard for a mile or more. They also call to tell other males to stay out of their territory and have a "rain call" that is often heard on rainy days. (To find out the types of frogs common to your area, visit the National Wildlife Federation's Frogwatch USA site: www.nwf.org/frogwatchusa/frogs_state.cfm.

If you hear unusual noises coming from above, it might be bats busy at work hunting for dinner. Many bats send out a call and listen for its echoes to locate insects to eat. They also make these "echolocation" calls to avoid obstacles and fly safely. Often these calls are at frequencies too high for humans to hear, but not always. Bats use other calls to talk to each other at frequencies we can hear. Bats use vocal cords just as we do to make their calls. The spotted bat makes the lowest-frequency bat calls, at 9 kilohertz. M. Brock Fenton of York University in Toronto, describes it as a high-pitched chirp. It sounds like a stone bouncing off a high-tension wire. Humans can hear sounds of up to 20 kilohertz, but some North American bats have calls in the 100 kilohertz range. Elsewhere, bat calls are above 200 kilohertz.

Bats may use different calls when hunting. A changing-frequency call will help them determine the location of an insect, while a constant-frequency call tells them how fast an insect is moving. Conditions in the atmosphere limit how well echolocation works. A few bats can "echolocate" prey as far away as 50 feet. Big brown bats can echolocate only to about 16 feet.

Another call you may hear at night is that of the barred owl. It sounds like a cross between a dog barking and a rooster crowing. Barred owls sometimes call in the daytime as well as at night. If you think you hear a barred owl, start counting the number of calls. They usually call eight times, then wait to hear a reply from other owls.

In some areas you might also hear the call of a coyote. They learn to call when they are old enough to join a hunt, and their calls can be heard up to three miles away. You might hear two short barks and a long, wavering yodel, a howl too high for dogs that size to produce. If you learn to imitate that call, you might find yourself talking to a faraway coyote some evening.

To hear some animal and insect calls, log on to: www.naturepark.com/sound1.htm. Play a sound and see if you can guess the creature that made it. Click on the answer to learn more about the creatures you may hear on an autumn night.

Ask an insect to tell you the temperature

You may already know that you can tell the temperature by the speed at which a cricket chirps. That's true, but it helps to know the crickets in your area. Crickets are cold-blooded, so their body temperature changes as the temperature around them does. As the temperature drops, the cricket cools, and it moves its wings more slowly. The rate at which it moves its wings is directly related to the temperature. You couldn't count fast enough for most crickets, though: They rub their wings together at from 10 to 180 times per second!

You can, however, count the chirps of the snowy tree cricket, which is found everywhere in the United States except the Southeast. These are the crickets you often hear in the background on TV and in films.

You also have to know the formula for the crickets in your area: In the Eastern US, count the number of chirps in 13 seconds and add 41 to get the number of degrees Fahrenheit. In Oregon, it's more accurate to count the chirps in 12 seconds and add 38. People in Utah count the number in 20 seconds and add 35. Other areas might use 15 seconds, plus 40. But even if your method isn't completely accurate, any of these methods will get you close to the right temperature.

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