Seismic communication describes communication through seismic vibrations of the earth. Seismic cues are generated through percussion on the earth substrate or acoustical waves that couple with the earth, called rayleigh waves.
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The haptic sense is employed by a range of small mammals as a mechanism of prey detection, predator warning and avoidance, and habitat sensing. Though recently, many studies have indicated intentional use of ground vibrations as a means of communication among the Cape mole rat[1] and the Asian elephant.[2]
Airborne sound waves spread spherically rather than cylindrically, attenuating more rapidly (losing 6 dB for every doubling of distance) than ground surface waves such as Rayleigh waves (3 dB loss for every doubling of distance), and thus these ground surface waves maintain integrity longer.[3]
Pioneering research in elephant infrasound communication was done by Katy Payne, of the Elephant Listening Project,[4] and is detailed in her book Silent Thunder. Though this research is still in its infancy, it is helping to solve many mysteries, such as how elephants can find distant potential mates, and how social groups are able to coordinate their movements over extensive range.[5] Joyce Poole has also begun decoding elephant utterances that have been recorded over many years of observation, hoping to create a lexicon based on a systematic catalogue of elephant sounds.[6] In the late 90's, Caitlin O'Connell-Rodwell first argued that elephants communicate over long distances using low-pitched sounds that are barely audible to humans.[7] Her insights generated international media attention after the 2004 Boxing day tsunami in Asia, following reports that trained elephants in Thailand had become agitated and fled to higher ground before the devastating wave struck, thus saving their own lives and those of the tourists riding on their backs. Because earthquakes and tsunamis generate low-frequency waves, O'Connell-Rodwell and other elephant experts have begun to explore the possibility that the Thai elephants were responding to these events.[7]
Seismic energy transmits most efficiently between the 10 and 40 Hz - in the same range as the fundamental frequency and 2nd harmonic of an elephant rumble.[8] It seems that when an elephant rumbles a replica of the airborne sound is also transmitted through the ground. Thus, African Bush Elephants are able to use seismic vibrations at infrasound frequencies for communication.[9]
These calls range in frequency from 15–35 Hz and can be as loud as 117 dB, allowing communication for many kilometres, with a possible maximum range of around 10 km.[5] This sound can be felt by the sensitive skin of an elephant's feet and trunk, which pick up the resonant vibrations much as the flat skin on the head of a drum. To listen attentively, every member of the herd will lift one foreleg from the ground, and face the source of the sound, or often lay its trunk on the ground. The lifting presumably increases the ground contact and sensitivity of the remaining legs. This ability is thought also to aid their navigation by use of external sources of infrasound.
The waves, or vibrations, travel up the leg bone of another elephant, through the shoulder and to the middle ear bones, where they are processed in the auditory cortex region of the brain.[10]