Female African elephants use "contact calls" to communicate with other elephants in their bands (usually a family group). These infrasonic calls, with a frequency of about 21 Hz and a normal duration of 4-5 seconds, carry for long distances (several kilometers), and help elephants to determine the location of other indivduals. Calls vary among individual elephants, so that others respond differently to familiar calls than to unfamiliar calls. Perhaps elephants can recognize the identity of the caller. African elephants have a social structure best described as fluid; animals move freely over wide areas, sometimes affiliating with other animals. Female members of a family tend to stay together, and of course their juveniles travel with them. These female-centered groups may merge with other such groups periodically. Adult males are less likely to join groups.