for National Geographic News
African elephants have perfected the art of calling long-distance. They make most of their calls at night, when it is cooler and sound travels farthest.
Elephants are highly social animals, and they snort, scream, trumpet, roar, and rumble to communicate with one another. Low-level conversations among family members continue throughout the daya calf may rumble to its mother to stop so it can suckle, for instance. Close to 70 different sounds or "messages" used by elephants to keep in touch have been identified.
"Messages include both long and shorter-distance calls," said Michael Garstang, a meteorologist at the University of Virginia, Charlottesville.
"Many of the rumbling calls occur at the level of infrasound, a very low-frequency rumble below the audible hearing range of humans," he said. "Humans can hear the upper end as a rumble, although you're not hearing it in your ears, it's more like feeling the vibrations in your diaphragm."
The frequency of sound is measured in hertz (Hz). The infrasonic range is roughly between 1 and 20 Hz. Infrasound can travel long distances; however, atmospheric conditions can severely limit the distance.
Garstang and his team collected and analyzed more than 1,300 calls made by elephants on the Namibian savanna, where daytime temperatures frequently reach 120 degrees Fahrenheit (49 degrees Celsius), and winds blow at up to 20 miles an hour (32 kilometers an hour).
"Calls made when atmospheric conditions are optimum can be heard over at least 110 square miles [285 square kilometers]," Garstang said. "Wind gusts and shimmering heat waves can break up sound waves and reduce that distance to less than one square mile [2.6 square kilometers]."
Garstang found that 96 percent of the infrasound calls designed to travel long-distance occurred during the hours of dusk and dawn. Ideal atmospheric conditions occur near sunset and sunrise, when temperatures drop and the winds are quiet. Calls made during these optimum times could be heard up to 6 miles (10 kilometers) away.
Listening to Elephants
The ability to make themselves heard over long distances is crucial to elephant survival.
Elephants live in family groups of closely related females and their young, led by a matriarch. The males wander alone or in very small groups of other males. From an evolutionary point of view, one of the most important long-distance calls signals a female's readiness to mate.
"When the female is in estrus, breeding bulls that are 35 to 55 years old and in their prime can be many miles away," Garstang said. "The female wants the biggest and strongest male she can find. If her call went less than a mile [1.6 kilometers], the chance of her finding a really good male would be close to nonexistent."
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