Each female African elephant roams with her mother, sisters, and other female relatives. The oldest female leads. In Kenya, such a group typically encounters 25 other families, or about 175 other elephants, each year. Strangers might spark disputes or harass calves, so unfamiliar calls send a herd into a defensive huddle and inspire much trunk waving, presumably for sniffing the air, explains McComb. In contrast, a herd of familiar neighbors doesn't pose much of a risk.
Elephants can sort their friends from strangers by identifying so-called contact calls.
The calls include both sounds audible to humans and lower-frequency rumbles that carry great distances. According to earlier research, a female typically recognizes about 100 peers by their calls. The researchers used responses to playback calls from elephants of various degrees of familiarity to predict responsiveness for a family. Matriarchs aged 55 and older proved highly sensitive to the difference between calls from a familiar elephant and those from a stranger. Bands with these older leaders rarely huddled in response to calls from elephants familiar to the leader.