African elephant vocal communication II: rumble variation reflects the individual identity and emotional state of callers

The most common vocalization of the African elephant, Loxodonta africana, is the rumble, but there is no consensus as to how many rumble subtypes exist. From the standpoint of social function, many types of rumble have been proposed. From a structural standpoint, however, few studies have examined detailed acoustic measurements of a large number of calls. We analysed 270 rumbles from six adult female African elephants housed at Disney's Animal Kingdom (Lake Buena Vista, Florida, U.S.A.). Subjects wore collars outfitted with microphones and radiotransmitters that allowed recording of vocalizations from identified individuals. Rumble vocalizations were digitized and both source and filter features were measured for each call. Behavioural and endocrine data were collected so that acoustical data could be placed into the context of ongoing social behaviour and reproductive state. Multidimensional scaling analysis revealed that, from a structural standpoint, rumbles from this captive setting could not be divided into distinct subtypes, but there was extensive acoustic variation across rumbles. Discriminant function analysis and MANOVA were employed to further explore this variation. First, acoustic characteristics varied according to the individual identity of the caller. Second, rumbles varied as a function of negative emotional arousal. When associating with dominant animals, subordinate females produced rumbles with lower cepstral coefficients, suggesting low tonality and unstable pitch in the voice, compared to rumbles produced outside of the presence of dominant animals. Rumbles as a whole did not cluster into distinct acoustic types, but structural variation in rumbles reflected the individual identity and emotional state of callers.