Elephants in the wild seem to coordinate their movements even when widely separated. Male elephants seem to be able to find females in estrous even over long distances.
Bioacoustics researcher Katy Payne and her colleagues have found that elephants use low frequency sounds to communicate. These sounds are mostly below the range of human hearing but we feel them as "pulsations" in the air. In Africa these sounds may travel as far as 10 km and serve to coordinate elephant herds.
Payne is co-founder of the Elephant Listening Project, which uses acoustic methods to study and aid in the conservation of forest elephants in Central Africa.