These conditions are most evident under times of atmospheric drought, when long distance communication would be most advantageous. Long distance communication during times of drought would possibly reduce competition for diminishing resources and also aid family groups in sourcing water. It would be advantageous in reducing the energy lost in sourcing food. Laws et al, (1975) observed that elephants at Etosha National Park migrate towards rain approaching from the north up to 2 weeks before other species of herd animals, which could be a reaction to low-frequency sound produced by the approaching thunderstorms (Garstang et al, 1995). .
5.0 Seismic Communication.
5.1 Propagation of Seismic Waves.
The use of seismic waves as a means of communication in elephants was first studied by O'Connell-Rodwell et al, (2000). From studies of seismic communication in insects, behavioral similarities were observed between the stance adopted by insects when detecting seismic cues and the stances elephants adopted when they were thought to be listening to sounds undetectable to humans. O'Connell-Rodwell et al, (2000) suggests that the use of acoustic and seismic communication offer the elephant a greater range of communication as certain environmental conditions can alter the efficiency of either mode of communication. .
To determine the ability of elephants to generate seismic vibrations, O'Connell-Rodwell et al, (2000) measured both the acoustic and seismic waves produced by an elephant "foot stomp" and also the low-frequency "rumble" vocalisation. The "foot stomp" was not detectable acoustically at 40m but had a substantial seismic reading, which may indicate that it may have a more important role in seismic communication. The "rumble" produced both acoustic and seismic waves indicating that the elephant has the ability to couple its vocalisations with the ground. The results also showed that the acoustic and seismic waves travelled at different velocities.
