The researchers' models show that these signals can be transmitted through the ground at distances up to 10 miles (16 kilometers) for rumbles, and 20 miles (32 kilometers) for mock charges. Other research shows that airborne sounds travel only about 6 miles (10 kilometers) under ideal conditions.
If the elephants could indeed detect these seismic signals, they would be able to warn others of a predator in the vicinity from a far greater distance than previously believed, O'Connell-Rodwell and colleagues suggested.
To determine whether elephants can detect the seismic signals they produce, the team played recordings of vocalizations to captive elephants in Zimbabwe and India, as well as to wild elephants in Ethosa National Park. Seismic transmitters convert the audible sounds into underground seismic waves.
"We got mixed results in these preliminary studies as different elephants responded differently to the playback," O'Connell-Rodwell said. "Some responded dramatically and some not at allthe bulls in particular showed little interest."
When the researchers conduct playback experiments in Namibia this June, they will look for behavior that characterizes seismic signal detection: Orienting toward the source, freezing, sniffing with their trunks in the direction of the playback, and lifting a foot off the ground.
O'Connell-Rodwell and colleagues are also dissecting the feet and ears of elephants to see if they have certain properties that would allow for detection of seismic signals. The researchers expect to find special sensors in elephant feet, called pascinian and meissner corpuscles, that are also found in the trunk of the elephant. The sensors are specially adapted to detect vibrations.
"The pascinian corpuscle is like an onion and when the layers shift in response to a vibration, this shift stimulates a nerve to fire, thus sending a message to the brain," explained O'Connell-Rodwell.
Drumming Not Unusual
Jan Randall, a biologist at San Francisco State University in California, says that it is fairly common for animals to communicate by drumming a part of their body on the ground to create both seismic and airborne vibrations.
"In mammals, the behavior has been described in different lineages, including deer, many different rodents from all continents except Antarctica, skunks, rabbits, and wallabies," she said.
Randall has studied foot-drumming in rodents for 20 years. Her research on the banner-tailed kangaroo rat, a rodent that lives in the desert southwest of the United States, shows evidence of foot-drumming used to communicate territorial ownership.
According to O'Connell-Rodwell, humans too have the ability to communicate via vibes. She says that traditional instruments such as the didgeridoo of Australia, talking drums of West Africa, and the stomping dances of Native Americans all produce signals that have the potential to be carried through the ground over long distances.
"These instruments could have been important communication mechanisms similar to smoke signals, but at much greater distances than smoke signals could be detected," she said.
Human communication over long distances via seismic waves, however, has been replaced by modern transportation, telephones, and the Internet. In addition, said O'Connell-Rodwell, the ground is now polluted with human-generated seismic noises.
As a result, humans adapt.
Research shows that the human brain is malleable; an area of the brain previously dedicated to one sense can be taken over by another if it becomes irrelevant.
For most humans, this means their brain dedicates less room to detecting seismic signals and more to other senses, such as hearing and seeing. Deaf people, however, devote more of the part of the brain reserved for hearing to sensing vibrations.
"The more we can understand about the vibrational sensitivity of animals that dedicate their sensory system to seismic communication, the more we might be able to help deaf people feel their world better, perhaps from a younger age," O'Connell-Rodwell said.
O'Connel-Rodwell's research is made possible, in part, by a grant from the National Geographic Society's Committee for Research and Exploration.
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