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Calls in the Wild

Midday sun scorches the plains of northern Namibia, and a female elephant just coming into estrus is making a racket. With a rumble loud enough to rival a jackhammer, she broadcasts her availability for mating. But gusty winds and turbulence created by heat shimmering through the atmosphere tend to break up the sound waves she generates, so her signal carries little more than half a mile. The odds are slim that a mature male in his own period of reproductive readiness will be close enough to hear her call.
 
Hours pass. Dusk settles over the flat sand veld. Winds calm, and air near the ground becomes cooler than the warm air above it, effectively creating a channel for sound. Now the low-frequency end of the eager female's cries can carry six miles (10 kilometers) in all directions, reaching the ears of elephants spread over a range of more than 110 square miles (280 square kilometers). The chance that a tall, strong mate will come swaggering her way is now a pretty sure thing.
 
To understand how elephants make the switch from local to long-distance calling, it helps to know something about weather. Wind and heat—along with rough terrain and dense vegetation—limit animal calling ranges because they disrupt sound waves. Wind can also be a problem for the receiver: Gusts blustering over elephants' massive heads and floppy ears can make it difficult for these distinctly non-aerodynamic animals to hear any signal clearly. High frequencies (think bird whistles and chimp chatter) are even more easily disturbed than low frequencies (elephant rumbles and lion roars).
 
The link between weather and sound has long intrigued Michael Garstang, a University of Virginia meteorologist with a tool kit full of weather instruments, a detailed grasp of how atmospheric patterns can affect acoustics—and a fondness for elephants.
 
Though Garstang has worked in the U.S. for decades, he was born in South Africa and retains an enduring fascination with the wild creatures of his youth. He began to pore over scholarly reports on elephant communication, work based on research in zoos and in the wild. The papers showed that long-distance elephant calling relies on ultralow tones classified as infrasound—frequencies below the bass threshold of human hearing.
 
Garstang became curious about whether elephants waste energy bellowing against the wind, or whether they take advantage of daily atmospheric changes that can boost bass tones like the woofers in stereo speakers. To find answers, he assembled a team of researchers and headed to Etosha National Park in Namibia, home to a healthy and growing elephant population. Hills and forests aren't part of Etosha's savanna habitat, so Garstang knew that weather, rather than terrain, would be the most significant environmental factor affecting the elephants' lowest pitched communications.
 
In the dry season large numbers of elephants gather to drink and bathe at water holes along the eastern end of Etosha. Near the park's Mushara water hole, the team installed an array of infrasound-sensitive microphones and a suite of weather instruments. Devices mounted on a 24-foot-tall (7-meter-tall) tower collected near-ground data about heat and wind. The same gear also gathered data on air pressure and humidity, but those factors had little effect on infrasound transmission. A tethered balloon lifted sensors into the air. And an acoustic sounder measured temperature and wind velocity from the surface to a height of about 650 feet (200 meters). Over a period of nearly three weeks, the team recorded more than 1,300 low-frequency elephant calls and the atmospheric conditions in which they were made.
 
Elephants can talk in a broad range of frequencies—sounds as high as the top note of a clarinet and lower than the lowest note on a concert piano. At the bass end of the scale, they can produce some of the lowest frequencies in the animal kingdom. Human ears can detect frequencies down to 20 hertz (Hz), but elephants can produce calls as low as 15 Hz. Most of the conversations recorded at Mushara were held in voices so deep that people couldn't hear them at all.
 
Daytime temperatures at the site rose to over 110°F (40°C), and the wind regularly blew at more than 20 miles (30 kilometers) an hour. In the evening, temperatures dropped below 40°F (4°C), and for hours the air was almost still. Ninety-six percent of the infrasonic signals Garstang's team recorded occurred in the cool calm between dusk and dawn. The greatest number of calls happened from an hour before sunset to three hours after, with another talkative period in the first two hours after sunrise.
 
Those early morning and early evening peaks in calls support the idea that elephants have indeed adapted their behavior to match patterns of change in the atmosphere. Garstang succinctly sums up his findings: "Elephants talk most when conditions are best."
 
That adaptation is crucial, since calling range helps determine the size of the area used by elephants at any given time. Long-distance vocalizations keep group members bonded as they trade information about resources and dangers and seek breeding partners across pachyderm-scale areas. Knowledge about how elephants communicate, Garstang says, can help us protect their habitat—and their future. 
 
—Lynne Warren


Web Links

Savanna Elephant Vocalization Project
www.elephantvoices.org/
Learn more about how, why, and what elephants communicate on this informative Web site.

Amboseli Elephant Research Project
www.elephanttrust.org/
Read reports from the field, get news updates, and more from the world's longest running elephant study.

Elephant Listening Project
www.birds.cornell.edu/brp/elephant/
Discover how researchers hope to conserve forest elephants and their habitat by learning more about their vocalizations.


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Bibliography

Garstang, Michael. "Long Distance, Low-frequency Elephant Communication: A Review," Journal of Comparative Physiology (publication forthcoming).

Garstang, Michael, and others. "The Daily Cycle of Low-frequency Elephant Calls and Near Surface Atmospheric Conditions," Journal of Experimental Biology (publication forthcoming).

Larom, David, and others. "The Influence of Surface Atmospheric conditions on the Range and Area Reached by Animal Vocalizations," Journal of Experimental Biology (1997), 421-31.
 
Morton, Eugene S., and Jake Page. Animal Talk: Science and the Voices of Nature. Random House, 1992.
 
Moss, Cynthia. Elephant Memories: Thirteen Years in the Life of an Elephant Family. William Morrow and Co., 1988.
 
Payne, Katy. Silent Thunder: In the Presence of Elephants. Simon and Schuster, 1998.




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