The American Physical Society (APS), in the latest podcast of ‘Life Lines’, has explained how elephant vocalizations travel through the ground for great distances, and how other elephants can understand them, just as they understand acoustic sound, which travels through the air.
Research that led to the development of the content of the podcast was done by Dr Caitlin O’Connell-Rodwell, who is the author of ‘The Elephant’s Secret Sense’.
Early in her research, Dr. O’Connell-Rodwell noticed behavior that indicates elephants are listening to acoustic (airborne) sounds by putting their ears out and orienting toward the sound’s source.
At other times, she also noticed a more puzzling behavior: Several elephants would freeze simultaneously, sometimes in mid-stride, and would press their front feet into the ground.
They might also roll a foot forward so that only their toes touched the ground. At other times, they would lift a front leg.
The behavior reminded the researcher of the behavior she saw in insects that communicate seismically.
She began a series of experiments that eventually found that:
Low-frequency elephant vocalizations, which are below the threshold of human hearing, travel through the ground in the same waveform as they do in the air.
The ground vocalization can travel faster or more slowly than acoustic sound, depending on soil conditions, but has the potential of travelling further as there is no outer limit to how far sounds can travel through the earth.
When she played a recorded elephant vocalization through the ground only, other elephants detected the vocalization.
Elephants understood the ground-borne vocalizations.
For example, they responded appropriately to an alarm call from another elephant by assuming their defensive posture of bunching and freezing.
They also responded only to alarm calls of elephants living in the area rather than those made from elephants elsewhere.
Elephants also have anatomical adaptations to help them ‘hear’ these ground-borne vocalizations.
They have an enlarged malleus, a middle ear bone that plays an important role in hearing.
Animals that communicate seismically often have an enlarged malleus as it also facilitates bone conducted detection of vibrations.
Elephants can close their middle ear canal, forming a closed acoustic tube which enhances bone conduction and blocks out acoustic sound, helping the elephant focus on the vibration pathway.
They have an acoustically designed foot, with a thick fat pad that perhaps helps in the transmission or conduction of vibrations.