Elephants may use vibrations in the ground created by their low-frequency "rumbles" to communicate, according to scientists who have recorded seismic waves made by elephant calls for the first time.
Biologists have known for decades that elephants talk to each other with infrasonic calls below the range of human hearing. Field biologists often see groups of wild elephants freeze in unison and spread their ears to scan for these infrasonic rumbles. While working in Namibia, biologist Caitlin O'Connell-Rodwell noticed something else: elephants occasionally freezing and lifting a foot without scanning with their ears. This behavior was often followed by the arrival of another group of elephants, and she suspected they might be shifting their weight onto three feet to get a better feel for something in the ground.
O'Connell-Rodwell, now at Stanford University, teamed up with Stanford geophysicist Simon Klemperer to find out if the rumbles really do create ground vibrations. The researchers placed geophones, devices that measure seismic waves, in the ground in a field near Salinas, California. They then reunited a trained male African elephant with his female companions after a 3-hour separation and recorded ground motion as he called out welcome rumbles. The calls created surface waves, known as Rayleigh waves, that can travel about 2 kilometers along the ground surface like water waves on the ocean, the pair reports in the June issue of Geophysical Research Letters.
O'Connell-Rodwell's previous research had suggested that the vibrations might travel as far as 16 kilometers, much farther than the 4 kilometers that rumbles are likely to travel through air. The current study shows that airborne waves travel farther under most conditions, but O'Connell-Rodwell suspects that ground waves may be more effective in some cases.
That seems plausible, says acoustic biologist Katy Payne of Cornell University in Ithaca, New York. "On a very windy day when it's very noisy, it might be that seismic communication would work better." The next step, says Payne, is to figure out if elephants actually use these signals to communicate. The Stanford team is currently in Namibia trying to answer this question by playing artificial seismic rumbles to wild elephants to see how they react. They also have an ongoing experiment with a trained elephant at the Oakland Zoo to see how sensitive elephants are to ground vibrations.